GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

Multi-Port Fuel Injection (MFI) and Sequential Fuel Injection (SFI)


Both the MFI and SFI systems are controlled by a Computer Control Module (ECM or PCM) which monitors engine operations and generates output signals to provide the correct air/fuel mixture, ignition timing and engine idle speed control. Input to the control unit is provided by an oxygen sensor, coolant temperature sensor, detonation sensor, hot film mass sensor and throttle position sensor. The ECM/PCM also receives information concerning engine rpm, road speed, transaxle gear position, power steering and air conditioning.

The injectors are located, one at each intake port, rather than the single injector found on the earlier throttle body system. The injectors are mounted on a fuel rail and are activated by a signal from the computer control module. The injector is a solenoid-operated valve which remains open depending on the width of the electronic pulses (length of the signal) from the ECM/PCM. The longer the open time, the more fuel is injected. In this manner, the air/fuel mixture can be precisely controlled for maximum performance with minimum emissions.

Fuel is pumped from the tank by a high pressure fuel pump, located inside the fuel tank. It is a positive displacement roller vane pump. The impeller serves as a vapor separator and pre-charges the high pressure assembly. A pressure regulator maintains 41-47 psi (282-324 kPa) in the fuel line to the injectors and the excess fuel is fed back to the tank. A fuel accumulator is used to dampen the hydraulic line hammer in the system created when all injectors open simultaneously.

The Mass Air Flow (MAF) Sensor is used to measure the mass of air that is drawn into the engine cylinders. It is located just ahead of the air throttle in the intake system and consists of a heated film which measures the mass of air, rather than just the volume. A resistor is used to measure the temperature of the film at 75°F (24°C) above ambient temperature. As the ambient (outside) air temperature rises, more energy is required to maintain the heated film at the higher temperature and the control unit used this difference in required energy to calculate the mass of the incoming air. The control unit uses this information to determine the duration of fuel injection pulse, timing and EGR.

The throttle body incorporates an Idle Air Control (IAC) that provides for a bypass channel through which air can flow. It consists of an orifice and pintle which is controlled by the ECM/PCM through a step motor. The IAC provides air flow for idle and allows additional air during cold start until the engine reaches operating temperature. As the engine temperature rises, the opening through which air passes is slowly closed.

The Throttle Position Sensor (TPS) provides the control unit with information on throttle position, in order to determine injector pulse width and hence correct mixture. The TPS is connected to the throttle shaft on the throttle body and consists of as potentiometer with on end connected to a 5 volt source from the ECM/PCM and the other to ground. A third wire is connected to the ECM/PCM to measure the voltage output from the TPS which changes as the throttle valve angle is changed (accelerator pedal moves). At the closed throttle position, the output is low (approximately 0.4 volts). As the throttle valve opens, the output increases to a maximum 5 volts at Wide Open Throttle (WOT). The TPS can be misadjusted open, shorted, or loose and if it is out of adjustment, the idle quality or WOT performance may be poor. A loose TPS can cause intermittent bursts of fuel from the injectors and an unstable idle because the ECM/PCM thinks the throttle is moving. This should cause a trouble code to be set. Once a trouble code is set, the ECM/PCM will use a preset value for the TPS and some vehicle performance may return. A small amount of engine coolant is routed through the throttle assembly to prevent freezing inside the throttle bore during cold operation.


The fuel injection system is controlled by an on-board computer, the Electronic Control Module/Powertrain Control Module (ECM/PCM), usually located in the passenger compartment. The ECM/PCM monitors engine operations and environmental conditions (ambient temperature, barometric pressure, etc.) needed to calculate the fuel delivery time (pulse width/injector on-time) of the fuel injector. The fuel pulse may be modified by the ECM/PCM to account for special operating conditions, such as cranking, cold starting, altitude, acceleration and deceleration.

The ECM/PCM controls the exhaust emissions by modifying fuel delivery to achieve, as nearly as possible an air/fuel ratio of 14.7:1. The injector on-time is determined by the various sensor inputs to the ECM/PCM. By increasing the injector pulse, more fuel is delivered, enriching the air/fuel ratio. Pulses are sent to the injector in 2 different modes, synchronized and non-synchronized.

In synchronized mode operation, the injector is pulsed once for each distributor reference pulse. In non-synchronized mode operation, the injector is pulsed once every 12.5 milliseconds or 6.25 milliseconds depending on calibration. This pulse time is totally independent of distributor reference.

The ECM/PCM constantly monitors the input information, processes this information from various sensors, and generates output commands to the various systems that affect vehicle performance.

The ability of the ECM/PCM to recognize and adjust for vehicle variations (engine, transaxle, vehicle weight, axle ratio, etc.) is provided by a removable calibration unit (PROM) that is programmed to tailor the ECM/PCM for the particular vehicle. There is a specific ECM/PCM - PROM combination for each specific vehicle, and the combinations are not interchangeable with those of other vehicles.

The ECM/PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the SERVICE ENGINE SOON light, and store a code or codes which identify the problem areas to aid the in making repairs.

Instead of an edgeboard connector, newer ECM/PCM's have a header connector which attaches solidly to the ECM/PCM case. Like the edgeboard connectors, the header connectors have a different pinout identification for different engine designs.

The ECM/PCM consists of 3 parts, a Controller (the ECM/PCM without a PROM), a Calibrator called a PROM (Programmable Read Only Memory) and a Cal-Pak.


To allow 1 model of the ECM/PCM to be used for many different vehicles, a device called a Calibrator (or PROM) is used. The PROM is located inside the ECM/PCM and has information on the vehicle's weight, engine, transaxle, axle ratio and other components.

While one ECM/PCM part number can be used by many different vehicles, a PROM is very specific and must be used for the right vehicle. For this reason, it is very important to check the latest parts book and or service bulletin information for the correct PROM part number when replacing the PROM.

An ECM/PCM used for service (called a controller) comes without a PROM. The PROM from the old ECM/PCM must be carefully removed and installed in the new ECM/PCM.


See Figure 1

A device called a Cal-Pak is added to allow fuel delivery if other parts of the ECM/PCM are damaged. It has an access door in the ECM/PCM, and removal and replacement procedures are the same as with the PROM. If the Cal-Pak is missing, a Code 52 will be set. Not all vehicles with MFI/SFI are equipped with a Cal-Pak.

Click image to see an enlarged view

Fig. Fig. 1: How to remove the Cal-Pak


This assembly contains the functions of the PROM Cal-Pak and the ESC module used on other GM applications. Like the PROM, it contains the calibrations needed for a specific vehicle as well as the back-up fuel control circuitry required if the rest of the ECM becomes damaged or faulty.

Synchronized Mode

In synchronized mode operation, the injector is pulsed once for each distributor reference pulse.

Non-synchronized Mode

In non-synchronized mode operation, the injector is pulsed once every 12.5 milliseconds or 6.25 milliseconds depending on calibration. This pulse time is totally independent of distributor reference pulses. Non-synchronized mode results only under the following conditions:

  1. The fuel pulse width is too small to be delivered accurately by the injector (approximately 1.5 milliseconds)
  3. During the delivery of prime pulses (prime pulses charge the intake manifold with fuel during or just prior to engine starting)
  5. During acceleration enrichment
  7. During deceleration leanout

Starting Mode

When the engine is first turned ON , the ECM/PCM will turn on the fuel pump relay for 2 seconds and the fuel pump will build up pressure. The ECM/PCM then checks the coolant temperature sensor (ECT), throttle position sensor (TPS) and crank sensor, then the ECM/PCM determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 201°F (94°C).

The ECM/PCM controls the amount of fuel that is delivered in the Starting Mode by changing how long the injectors are turned on and off. This is done by pulsing the injectors for very short times.

Clear Flood Mode

If for some reason the engine should become flooded, provisions have been made to clear this condition. To clear the flood, the driver must depress the accelerator pedal enough to position the throttle in its wide-open position. The ECM/PCM then issues a command to completely turn off the fuel flow. The ECM/PCM holds this operational mode as long as the throttle stays in the wide-open position and the engine rpm is below 600. If the throttle position becomes less than 62% (2.9mv) on MFI vehicles or 80% on SFI vehicles, the ECM/PCM returns to the starting mode.

Run Mode

There are 2 different run modes. When the engine is first started and the rpm is above 400, the system goes into Open Loop operation. In open loop operation, the ECM/PCM will ignore the signal from the oxygen sensor (O 2 S) and calculate the injector on-time based upon inputs from the coolant sensor (ECT), mass air flow (MAF) and mass air temperature (MAT) sensors.

During Open Loop operation, the ECM/PCM analyzes the following items to determine when the system is ready to go to the Closed Loop mode.

  1. The oxygen sensor (O 2 S) varying voltage output showing that is hot enough to operate properly. This is dependent on temperature.
  3. The coolant sensor (ECT) must be above specified temperature.
  5. A specific amount of time must elapse after starting the engine. These values are stored in the PROM.
  7. The engine speed is above 800 rpm since start up.

When these conditions have been met, the system goes into Closed Loop operation In Closed Loop operation, the ECM/PCM will calculate the air/fuel ratio (injector on time) based upon the signal from the oxygen sensor. The ECM/PCM will decrease the on-time if the air/fuel ratio is too rich, and will increase the on-time if the air/fuel ratio is too lean.

Acceleration Mode

When the engine is required to accelerate, the opening of the throttle valve(s) causes a rapid increase in Manifold Absolute Pressure (MAP). This rapid increase in MAP causes fuel to condense on the manifold walls. The ECM/PCM senses this increase in throttle angle and MAP, and supplies additional fuel for a short period of time. This prevents the engine from stumbling due to too lean a mixture.

Deceleration Mode

Upon deceleration, a leaner fuel mixture is required to reduce emission of hydrocarbons (HC) and carbon monoxide (CO). To adjust the injection on-time, the ECM/PCM uses the decrease in MAP and the decrease in throttle position to calculate a decrease in injector on time. To maintain an idle fuel ratio of 14.7:1, fuel output is momentarily reduced. This is done because of the fuel remaining in the intake manifold. The ECM/PCM can cut off the fuel completely for short periods of time.

Battery Voltage Correction Mode

The purpose of battery voltage correction is to compensate for variations in battery voltage to fuel pump and injector response. The ECM/PCM compensates by increasing the engine idle rpm.

Battery voltage correction takes place in all operating modes. When battery voltage is low, the spark delivered by the distributor may be low. To correct this low battery voltage problem, the ECM/PCM can do any or all of the following:

Increase injector on time (increase fuel)
Increase idle rpm
Increase ignition dwell time

Fuel Cut-off Mode

When the ignition is OFF , no fuel will be delivered by the injectors. Fuel will also be cut off if the ECM/PCM does not receive a reference pulse from the distributor. To prevent dieseling, fuel delivery is completely stopped as soon as the engine is stopped. The ECM/PCM will not allow any fuel supply until it receives distributor reference pulses which prevents flooding.

Converter Protection Mode

In this mode the ECM/PCM estimates the temperature of the catalytic converter and then modifies fuel delivery to protect the converter from high temperatures. When the ECM/PCM has determined that the converter may overheat, it will cause open loop operation and will enrich the fuel delivery. A slightly richer mixture will then cause the converter temperature to be reduced.

Fuel Backup Mode

The ECM/PCM functions in the fuel backup circuit mode if any one, or any combination, of the following exist:

The ECM/PCM voltage is lower than 9 volts.
The cranking voltage is below 9 volts.
The PROM is missing or not functioning.
The ECM/PCM circuit fails to insure the computer operating pulse. The computer operating pulse (COP) is an internal ECM/PCM feature designed to inform the fuel backup circuit that the ECM/PCM is able to function.

Some engines run erratically in the fuel backup mode, while others seemed to run very well. Code 52 will be set to indicate a missing Cal-Pak. The fuel backup circuit is ignition fed and senses Throttle Position Sensor (TPS), Engine Coolant Temperature Sensor (ECT) and rpm. The fuel backup circuit controls the fuel pump relay and the pulse width of the injectors. Some vehicles have this mode, not all.

Data Link Connector (DLC/ALDL)

The Data Link Connector (DLC) or Assembly Line Data Link (ALDL), is a diagnostic connector located in the passenger compartment, usually under the instrument panel.

The assembly plant were the vehicles originate use these connectors to check the engine for proper operation before it leaves the plant. Vehicles with the ALDL system, enter the Diagnostic mode or the Field Service Mode by connecting or jumping terminal B, the diagnostic TEST terminal to terminal A, or ground circuit. Vehicles with the check connector, connect or jump terminal TE1, the diagnostic TEST terminal to terminal E1, or ground circuit. This connector is a very useful tool in diagnosing fuel injected engines. Important information from the ECM/PCM is available at this terminal and can be read with one of the many popular scanner tools.


The fuel control system is made up of the following components:

Fuel supply system
Throttle body assembly
Fuel injectors
Fuel rail
Fuel pressure regulator
Idle Air Control (IAC)
Fuel pump
Fuel pump relay
Inline fuel filter

The fuel control system starts with the fuel in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel gauge sending unit, pumps fuel to the fuel rail through an in-line fuel filter. The pump is designed to provide fuel at a pressure above the pressure needed by the injectors. A pressure regulator in the fuel rail keeps fuel available to the injectors at a constant pressure. Unused fuel is returned to the fuel tank by a separate line.

In order for the fuel injectors to supply a precise amount of fuel at the command of the ECM/PCM, the fuel supply system maintains a constant pressure drop of approximately 41-47 psi (284-325 kPa) across the injectors. As manifold vacuum changes, the fuel system pressure regulator controls the fuel supply pressure to compensate. On the MFI vehicles the fuel is supplied at the same time in a pulse injection timing, on the SFI vehicles the fuel is supplied in the order of the timing. The fuel pressure accumulator used on select models, isolates fuel line noise. The fuel rail is bolted rigidly to the engine and it provides the upper mount for the fuel injectors. It also contains a spring loaded pressure tap for testing the fuel system or relieving the fuel system pressure.

The injectors are controlled by the ECM/PCM. They deliver fuel in one of several modes as previously described. In order to properly control the fuel supply, the fuel pump is operated by the ECM/PCM through the fuel pump relay and oil pressure switch.

Throttle Body Unit

See Figure 2

The throttle body unit has a throttle valve to control the amount of air delivered to the engine. The TPS and IAC valve are also mounted onto the throttle body. The throttle body contains vacuum ports located at, above or below the throttle valve. These vacuum ports generate the vacuum signals needed by various components.

On some vehicles, the engine coolant is directed through the coolant cavity at the bottom of the throttle body to warm the throttle valve and prevent icing.

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Fig. Fig. 2: Throttle body unit on an SFI vehicle

Fuel Injector

See Figure 3

A fuel injector is installed in the intake manifold at each cylinder. Mounting is approximately 3-4 in. (7.5-10mm) from the center line of the intake valve on the V6 and the V8 engine applications. The nozzle spray pattern is on a 25 degree angle. The fuel injector is a solenoid operated device controlled by the ECM/PCM. The ECM/PCM turns on the solenoid, which opens the valve which allows fuel delivery. The fuel, under pressure, is injected in a conical spray pattern at the opening of the intake valve. The fuel, which is not used by the injectors, passes through the pressure regulator before returning to the fuel tank.

An injector that is partly open, will cause loss of fuel pressure after the engine is shut down, so long crank time would be noticed on some engines. Also dieseling could occur because some fuel could be delivered after the ignition is turned to OFF position.

There are 2 O-ring seals used. The lower O-ring seals the injector at the intake manifold. The O-rings are lubricated and should be replaced whenever the injector is removed from the intake manifold. The O-rings provide thermal insulation, thus preventing the formation of vapor bubbles and promoting good hot start characteristics. The O-rings also prevent excess injector vibration.

Air leakage at the injector/intake area would create a lean cylinder and a possible driveability problem. A second seal is used to seal the fuel injector at the fuel rail connection. The injectors are identified with an ID number cast on the injector near the top side. Injectors manufactured by Rochester® Products have an RP positioned near the top side in addition to the ID number.

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Fig. Fig. 3: The injectors are identified with an ID number cast on the injector near the top side. Injectors manufactured by Rochester Products® have an RP positioned near the top side in addition to the ID number

Fuel Rail

See Figure 4

The fuel rail is bolted rigidly to the engine and it provides the upper mount for the fuel injectors. It distributes fuel to the individual injectors. Fuel is delivered to the input end of the fuel rail by the fuel lines, goes through the rail, then to the fuel pressure regulator. The regulator keeps the fuel pressure to the injectors at a constant pressure. The remaining fuel is then returned to the fuel tank. The fuel rail also contains a spring loaded pressure tap for testing the fuel system or relieving the fuel system pressure.

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Fig. Fig. 4: Fuel rails, like injectors, have an identification number stamped on them

Pressure Regulator

See Figure 5

The fuel pressure regulator contains a pressure chamber separated by a diaphragm relief valve assembly with a calibrated spring in the vacuum chamber side. The fuel pressure is regulated when the pump pressure acting on the bottom spring of the diaphragm overcomes the force of the spring action on the top side.

The diaphragm relief valve moves, opening or closing an orifice in the fuel chamber to control the amount of fuel returned to the fuel tank. Vacuum acting on the top side of the diaphragm along with spring pressure controls the fuel pressure. A decrease in vacuum creates an increase in the fuel pressure. An increase in vacuum creates a decrease in fuel pressure.

An example of this is under heavy load conditions the engine requires more fuel flow. The vacuum decreases under a heavy load condition because of the throttle opening. A decrease in the vacuum allows more fuel pressure to the top side of the pressure relief valve, thus increasing the fuel pressure.

The pressure regulator is mounted on the fuel rail and serviced separately. If the pressure is too low, poor performance could result. If the pressure is too high, excessive odor and a Code 45 may result.

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Fig. Fig. 5: The pressure regulator is mounted on the fuel rail and is serviced separately

Idle Air Control (IAC)

The purpose of the Idle Air Control (IAC) system is to control engine idle speeds while preventing stalls due to changes in engine load. The IAC assembly, mounted on the throttle body, controls bypass air around the throttle plate. By extending or retracting a conical valve, a controlled amount of air can move around the throttle plate. If rpm is too low, more air is diverted around the throttle plate to increase rpm.

During idle, the proper position of the IAC valve is calculated by the ECM/PCM based on battery voltage, coolant temperature, engine load, and engine rpm. If the rpm drops below a specified rate, the throttle plate is closed. The ECM/PCM will then calculate a new valve position.

Three different designs are used for the IAC conical valve. The first design used is single taper while the second design used is a dual taper. The third design is a blunt valve. Care should be taken to insure use of the correct design when service replacement is required.

The IAC motor has 255 different positions or steps. The zero, or reference position, is the fully extended position at which the pintle is seated in the air bypass seat and no air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate.

The ECM/PCM always monitors how many steps it has extended or retracted the pintle from the zero or reference position thus, it always calculates the exact position of the motor. Once the engine has started and the vehicle has reached approximately 40 mph, the ECM/PCM will extend the motor 255 steps from whatever position it is in. This will bottom out the pintle against the seat. The ECM/PCM will call this position 0 and thus keep its zero reference updated.

The IAC only affects the engine's idle characteristics. If it is stuck fully open, idle speed is too high (too much air enters the throttle bore) If it is stuck closed, idle speed is too low (not enough air entering). If it is stuck somewhere in the middle, idle may be rough, and the engine won't respond to load changes.

Fuel Pump

The fuel is supplied to the system from an in-tank positive displacement roller vane pump. The pump supplies fuel through the in-line fuel filter to the fuel rail assembly. The pump is removed for service along with the fuel gauge sending unit. Once they are removed from the fuel tank, they pump and sending unit can be serviced separately.

Fuel pressure is achieved by rotation of the armature driving the roller vane components. The impeller at the inlet end serves as a vapor separator and a precharger for the roller vane assembly.

The fuel pump delivers more fuel than the engine can consume even under the most extreme conditions. Excess fuel flows through the pressure regulator and back to the tank via the return line. The constant flow of fuel means that the fuel system is always supplied with cool fuel, thereby preventing the formation of fuel-vapor bubbles (vapor lock).

Fuel Pump Relay Circuit

The fuel pump relay is usually located on the right or left front inner fender (or shock tower) or on the engine side of the firewall (center cowl). The fuel pump electrical system consists of the fuel pump relay, ignition circuit and the ECM/PCM circuits are protected by a fuse. The fuel pump relay contact switch is in the normally open (NO) position.

When the ignition is turned ON the ECM/PCM will for 2 seconds, supply voltage to the fuel pump relay coil, closing the open contact switch. The ignition circuit fuse can now supply ignition voltage to the circuit which feeds the relay contact switch. With the relay contacts closed, ignition voltage is supplied to the fuel pump. The ECM/PCM will continue to supply voltage to the relay coil circuit as long as the ECM/PCM receives the rpm reference pulses from the ignition module.

The fuel pump control circuit also includes an engine oil pressure switch with a set of normally open contacts. The switch closes at approximately 4 lbs. of oil pressure and provides a secondary battery feed path to the fuel pump. If the relay fails, the pump will continue to run using the battery feed supplied by the closed oil pressure switch. A failed fuel pump relay will result in extended engine crank times in order to build up enough oil pressure to close the switch and turn on the fuel pump.

In-Line Fuel Filter

The fuel filter is of a 10-20 micron size and is serviced only as a complete unit. O-rings are used at all threaded connections to prevent fuel leakage. The threaded flex hoses connect the filter to the fuel tank feed line.


A variety of sensors provide information to the ECM/PCM regarding engine operating characteristics. These sensors and their functions are described below. Be sure to take note that not every sensor described is used with every GM engine application.

Electronic Spark Timing (EST)

Electronic spark timing (EST) is used on all engines equipped with HEI distributors and direct ignition systems. The EST distributor contains no vacuum or centrifugal advance and uses a 7-terminal distributor module. It also has 4 wires going to a 4-terminal connector in addition to the connectors normally found on HEI distributors. A reference pulse, indicating both engine rpm and crankshaft position, is sent to the ECM/PCM. The ECM/PCM determines the proper spark advance for the engine operating conditions and sends an EST pulse to the distributor.

The EST system is designed to optimize spark timing for better control of exhaust emissions and for fuel economy improvements. The ECM/PCM monitors information from various engine sensors, computes the desired spark timing and changes the timing accordingly. A backup spark advance system is incorporated in the module in case of EST failure.

The basic function of the fuel control system is to control the fuel delivery to the engine. The fuel is delivered to the engine by individual fuel injectors mounted on the intake manifold near each cylinder.

The main control sensor is the oxygen sensor which is located in the exhaust manifold. The oxygen sensor tells the ECM/PCM how much oxygen is in the exhaust gas and the ECM/PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture (ratio) to minimize exhaust emissions is 14.7:1 which allows the catalytic converter to operate the most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a CLOSED LOOP system.

Whenever the term Electronic Control Module (ECM) is used in this manual, it refers to the engine control computer, regardless of whether it is a Powertrain Control Module (PCM) or Electronic Control Module (ECM).

Electronic Spark Control (ESC)

When engines are equipped with ESC in conjunction with EST, ESC is used to reduce spark advance under conditions of detonation. A knock sensor signals a separate ESC controller to retard the timing when it senses knock. The ESC controller signals the ECM/PCM which reduces spark advance until no more signals are received from the knock sensor.

Engine Coolant Temperature (ECT)

See Figure 6

The coolant sensor is a thermister (a resistor which changes value based on temperature) mounted on the engine coolant stream. As the temperature of the engine coolant changes, the resistance of the coolant sensor changes. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while high temperature causes low resistance (70 ohms at 130°C/266°F).

The ECM/PCM supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ECM/PCM determines the engine coolant temperature. The voltage will be high when the engine is cold and low when the engine is hot. This information is used to control fuel management, IAC, spark timing, EGR, canister purge and other engine operating conditions.

Click image to see an enlarged view

Fig. Fig. 6: The ECT sensor and connector

A failure in the coolant sensor circuit should either set a Code 14 or 15 on MFI and P0117 or P0118 on SFI. These codes indicate a failure in the coolant temperature sensor circuit. Once the trouble code is set, the ECM/PCM will use a default valve for engine coolant temperature.

Intake Air Temperature (IAT)

The Intake Air Temperature Sensor (IAT) is a thermister which changes the value based on the temperature of air entering the engine. A low temperature produces a high resistance, while a low temperature causes a low resistance. The ECM/PCM supplies a 5 volt signal to the sensor through a resister in the ECM/PCM, then measures the voltage. A failure in the IAT sensor could result in a Code of P0112 or P0113 in the SFI and Codes 23 or 25 in the MFI. The Manifold Air Temperature (MAT) Sensor is another name for the IAT sensor.

Oxygen (O2S)

See Figures 7 and 8

The exhaust oxygen sensor is mounted in the exhaust system where it can monitor the oxygen content of the exhaust gas stream. The oxygen content in the exhaust reacts with the oxygen sensor to produce a voltage output. This voltage ranges from approximately 100 millivolts (high oxygen - lean mixture) to 900 millivolts (low oxygen - rich mixture).

By monitoring the voltage output of the oxygen sensor, the ECM/PCM will determine what fuel mixture command to give to the injector (lean mixture = low voltage and a rich command, rich mixture = high voltage and a lean command).

Click image to see an enlarged view

Fig. Fig. 7: A cross-sectional view of the oxygen sensor

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Fig. Fig. 8: The heated oxygen sensor looks the same as the non-heated variety

Remember that the oxygen sensor only indicates to the ECM/PCM what is happening in the exhaust. It does not cause things to happen. It is a type of gauge: high oxygen content = lean mixture; low oxygen content = rich mixture. The ECM/PCM adjusts fuel to keep the system working.

The oxygen sensor, if open, should set a Code 13. A constant low voltage in the sensor circuit should set a Code 44 in MFI and P0131 in SFI, while a constant high voltage in the circuit should set a Code 45 in MFI and P0132 or P0131 in SFI. Codes 44, 45, P0132 and P0131 could also be set as a result of fuel system problems. The oxygen sensor on MFI vehicles is not heated, but it is heated on SFI vehicles.

Manifold Absolute Pressure (MAP)

The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). A closed throttle engine coastdown would produce a relatively low MAP value (approximately 20-35 kPa), while wide-open throttle would produce a high value (100 kPa). This high value is produced when the pressure inside the manifold is the same as outside the manifold, and 100% of outside air (or 100 kPa) is being measured. This MAP output is the opposite of what you would measure on a vacuum gauge. The use of this sensor also allows the ECM/PCM to adjust automatically for different altitude.

The ECM/PCM sends a 5 volt reference signal to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage the ECM/PCM can determine the manifold pressure. A higher pressure, lower vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel. The ECM/PCM uses the MAP sensor to control fuel delivery and ignition timing. A failure in the MAP sensor circuit should set a Code 33 or Code 34.

Manifold Air Temperature (MAT)

The Manifold Air Temperature (MAT) sensor also known as the IAT sensor, is a thermistor mounted in the intake manifold. A thermistor is a resistor which changes resistance based on temperature. Low manifold air temperature produces a high resistance (100,000 ohms at &ndash40°F/&ndash4°C), while high temperature cause low resistance (70 ohms at 266°F/130°C).

The ECM/PCM supplies a 5 volt signal to the MAT sensor through a resistor in the ECM/PCM and monitors the voltage. The voltage will be high when the manifold air is cold and low when the air is hot. By monitoring the voltage, the ECM calculates the air temperature and uses this data to help determine the fuel delivery and spark advance. A failure in the MAT or IAT sensor could result in Codes P0112 or P0113 for SFI vehicles and Codes 23 or 25 for MFI vehicles. Once the trouble code is set, the ECM/PCM will use an artificial default value for the MAT and some vehicle performance will return.

Mass Air Flow (MAF)

The Mass Air Flow (MAF) sensor measures the amount of air which passes through it. The ECM/PCM uses this information to determine the operating condition of the engine, to control fuel delivery. A large quantity of air indicates acceleration, while a small quantity indicates deceleration or idle.

This sensor produces a frequency output between 32 and 150 hertz. A scan tool will display air flow in terms of grams of air per second (gm/sec), with a range from 3gm/sec to 150 gm/sec. A failure in the MAF sensor could produce a Code of P0101 for SFI vehicles.

Vehicle Speed

A vehicle equipped with a speed sensor, should not be driven without a the speed sensor connected, as idle quality may be affected.

The Vehicle Speed Sensor (VSS) is mounted behind the speedometer in the instrument cluster or on the transaxle/speedometer drive gear. It provides electrical pulses to the ECM/PCM from the speedometer head. The pulses indicate the road speed. The ECM/PCM uses this information to operate the IAC, canister purge, and TCC.

Some vehicles equipped with digital instrument clusters use a Permanent Magnet (PM) generator to provide the VSS signal. The PM generator is located in the transaxle and replaces the speedometer cable. The signal from the PM generator drives a stepper motor which drives the odometer. A failure in the VSS circuit should set a Code 24 for MFI vehicles and Code P0502 for SFI vehicles.

Throttle Position (TPS)

The Throttle Position Sensor (TPS) is connected to the throttle shaft and is controlled by the throttle mechanism. A 5 volt reference signal is sent to the TPS from the ECM/PCM. As the throttle valve angle is changed (accelerator pedal moved), the resistance of the TPS also changes. At a closed throttle position, the resistance of the TPS is high, so the output voltage to the ECM/PCM will be low (approximately 0.5 volt). As the throttle plate opens, the resistance decreases so that, at wide open throttle, the output voltage should be approximately 5 volts. At closed throttle position, the voltage at the TPS should be less than 1.25 volts.

By monitoring the output voltage from the TPS, the ECM/PCM can determine fuel delivery based on throttle valve angle (driver demand). The TPS can either be misadjusted, shorted, open or loose. Misadjustment might result in poor idle or poor wide-open throttle performance. An open TPS signals the ECM/PCM that the throttle is always closed, resulting in poor performance. This usually sets a Code 22 for MFI vehicles. A shorted TPS gives the ECM/PCM a constant wide-open throttle signal and should set a Code 21 on MFI vehicles. A loose TPS on MFI vehicles indicates to the ECM/PCM that the throttle is moving. This causes intermittent bursts of fuel from the injector and an unstable idle. Once the trouble code is set, the ECM/PCM will use an artificial default value for the TBI and some vehicle performance will return.

A problem in the 5 volt circuit of SFI vehicles should set off Codes P0122 or P0123 and a problem with the TPS sensor ground circuit may set Codes P0123 and P0117. Once the DTC is set on SFI vehicles, the ECM/PCM will use an artificial default value based on the mass air flow for the TPS sensor and some vehicle performance will return. A high idle on SFI vehicles may result in Codes P0122 or P0123 to be set.


Some systems use a magnetic crankshaft sensor, mounted remotely from the ignition module, which protrudes into the block within approximately 0.050 in. (0.127mm) of the crankshaft reluctor. The reluctor is a special wheel cast into the crankshaft with 7 slots machined into it, 6 of which are equally spaced (60 degrees apart). A seventh slot is spaced approximately 10 degrees from one of the other slots and severs to generate a SYNC PULSE signal. As the reluctor rotates as part of the crankshaft, the slots change the magnetic field of the sensor, creating an induced voltage pulse.

Based on the crank sensor pulses, the ignition module sends 2X reference signals to the ECM/PCM which are used to indicate crankshaft position and engine speed. The ignition module continues to send these reference pulses to the ECM at a rate of 1 per each 180 degrees of the crankshaft rotation. This signal is called the 2X reference because it occurs 2 times per crankshaft revolution.

The ignition also sends a second, 1X reference signal to the ECM/PCM which occurs at the same time as the SYNC PULSE from the crankshaft sensor. This signal is called the 1X reference because it occurs 1 time per crankshaft revolution. The 1X reference and the 2X reference signals are necessary for the ECM/PCM to determine when to activate the fuel injectors. Also known as the 3X Crankshaft Position Sensor.

By comparing the time between pulses, the ignition module can recognize the pulse representing the seventh slot (sync pulse) which starts the calculation of the ignition coil sequencing. The second crank pulse following the SYNC PULSE signals the ignition module to fire the No. 2-3 ignition coil and the fifth crank pulse signals the module to fire the No. 1-4 ignition coil.

24X Crankshaft Position Sensor

The 24X sensor is used to improve the idle spark control at engine speeds up to approximately 1200 rpm. Four 24X pulses to be seen between two 3X pulses. If the sequence of these pulses is not correct for 50 engine revolutions, the ECM/PCM will set off a Code of P0321.

Dual Crank/Combination

The dual crank sensor is mounted in a pedestal on the front of the engine near the harmonic balancer. The sensor consists of 2 Hall Effect switches, which depend on 2 metal interrupter rings mounted on the balancer to activate them. Windows in the interrupters activate the hall effect switches as they provide a patch for the magnetic field between the switches transducers and magnets. When one of the hall effect switches is activated, it grounds the signal line to the C 3 I module, pulling that signal line's (Sync Pulse or Crank) applied voltage low, which is interpreted as a signal.

Because of the way the signal is created by the dual crank sensor, the signal circuit is always either at a high or low voltage (square wave signal). Three crank signal pulses and one SYNC PULSE are created during each crankshaft revolution. The crank signal is used by the C 3 I module to create a reference signal which is also a square wave signal similar to the crank signal. The reference signal is used to calculate the engine rpm and crankshaft position by the ECM/PCM. The SYNC PULSE is used by the C 3 I module to begin the ignition coil firing sequence starting with No. 3-6 coil. The firing sequence begins with this coil because either piston No. 3 or piston No. 6 is now at the correct position in compression stroke for the spark plugs to be fired. Both the crank sensor and the SYNC PULSE signals must be received by the ignition module for the engine to start. A misadjusted sensor or bent interrupter ring could cause rubbing of the sensor resulting in potential driveability problems, such as rough idle, poor performance, or a nor start condition.

Failure to have the correct clearance will damage the crankshaft sensor.

The dual crank sensor is not adjustable for ignition timing but positioning of the interrupter ring is very important. A clearance of 0.025 in. (0.635mm) is required on either side of the interrupter ring. A dual crank sensor that is damaged, due to mispositioning or a bent interrupter ring, can result in a hesitation, sag stumble or dieseling condition.

To determine if the dual crank sensor could be at fault, scan the engine rpm with a suitable scan tool, while driving the vehicle. An erratic display indicates that a proper reference pulse has not been received by the ECM/PCM, which may be the result of a malfunctioning dual crank sensor.

Air Conditioning Pressure

The air conditioning (A/C) pressure sensor provides a signal to the computer control module (ECM/PCM) which indicates varying high side refrigerant pressure between approximately 0-450 psi (0-3102 kPa). The ECM/PCM used this input to the A/C compressor load on the engine to help control the idle speed with the IAC valve.

The A/C pressure sensor electrical circuit consists of a 5 volt reference line and a ground line, both provided by the ECM/PCM and a signal line to the ECM/PCM. The signal is a voltage that varies approximately 0.1 volt at 0 psi (0 kPa), to 4.9 volts at 450 psi (3102 kPa) or more. A problem in the A/C pressure circuits or sensor should set a Code 66 on MFI vehicles and Code P1530 on SFI vehicles, then will make the A/C compressor inoperative.

Non-Air Conditioning Program Input

Vehicles not equipped with air conditioning (A/C) have a circuit connecting the ECM/PCM terminal BC3 to ground, to program to operate without A/C related components connected to it. Vehicles with A/C do not have a wire in ECM BC3 terminal. If this circuit is open on a non-A/C vehicle it may cause false Codes 26 and/or Code 66. If terminal BC3 is grounded on A/C equipped vehicles, it will cause the compressor relay to be on whenever the ignition is in the ON position.

Detonation (Knock)

See Figure 9

This sensor is a piezoelectric sensor located near the back of the engine (transaxle end). It generates electrical impulses which are directly proportional to the frequency of the knock which is detected. A buffer then sorts these signals and eliminates all except for those frequency range of detonation. This information is passed to the ESC module and then to the ECM/PCM, so that the ignition timing advance can be retarded until the detonation stops.

Click image to see an enlarged view

Fig. Fig. 9: A piezoelectric sensor is located near the back of the engine (transaxle end). It generates electrical impulses which are directly proportional to the frequency of the knock which is detected


Park/Neutral Switch

Vehicle should not be driven with the park/neutral switch disconnected as idle quality may be affected in park or neutral and a Code 24 (VSS) may be set.

This switch indicates to the ECM/PCM when the transaxle is in P or N . the information is used by the ECM/PCM for control on the torque converter clutch, EGR, and the idle air control valve operation.

Air Conditioning Request Signal

This signal indicates to the ECM/PCM that an air conditioning mode is selected at the switch and that the A/C low pressure switch is closed. The ECM/PCM controls the A/C and adjusts the idle speed in response to this signal.

Torque Converter Clutch Solenoid

See Figure 10

The purpose of the torque converter clutch system is to eliminate power loss (slippage) by the converter and increase fuel economy. By locking the converter clutch, a more effective coupling to the flywheel is achieved. The converter clutch is operated by the ECM/PCM controlled torque converter clutch solenoid.

Click image to see an enlarged view

Fig. Fig. 10: The torque converter clutch solenoid operates in conjunction with the ECM/PCM to control the converter clutch

Power Steering Pressure Switch

The power steering pressure switch is used so that the power steering oil pressure pump load will not effect the engine idle. Turning the steering wheel increase the power steering oil pressure and pump load on the engine. The power steering pressure switch will close before the load can cause an idle problem. The ECM/PCM will also turn the A/C clutch off when high power steering pressure is detected.

Oil Pressure Switch

The oil pressure switch is usually mounted on the back of the engine, just below the intake manifold. Some vehicles use the oil pressure switch as a parallel power supply, with the fuel pump relay and will provide voltage to the fuel pump, after approximately 4 psi (28 kPa) of oil pressure is reached. This switch will also help prevent engine seizure by shutting off the power to the fuel pump and causing the engine to stop when the oil pressure is lower than 4 psi (28 kPa).


Various components are used to control exhaust emissions from a vehicle. These components are controlled by the ECM/PCM based on different engine operating conditions. These components are described in the following paragraphs. Not all components are used on all engines.

Exhaust Gas Recirculation (EGR) System

See Figure 11

EGR is a oxides of nitrogen (NOx) control which recycles exhaust gases through the combustion cycle by admitting exhaust gases into the intake manifold. The amount of exhaust gas admitted is adjusted by a vacuum controlled valve in response to engine operating conditions. If the valve is open, the recirculated exhaust gas is released into the intake manifold to be drawn into the combustion chamber.

The integral exhaust pressure modulated EGR valve uses a transducer responsive to exhaust pressure to modulate the vacuum signal to the EGR valve. The vacuum signal is provided by an EGR vacuum port in the throttle body valve. Under conditions when exhaust pressure is lower than the control pressure, the EGR signal is reduced by an air bleed within the transducer. Under conditions when exhaust pressure is higher than the control pressure, the air bleed is closed and the EGR valve responds to an unmodified vacuum signal. Physical arrangement of the valve components will vary depending on whether the control pressure is positive or negative.

The newer models use what is called a digital EGR valve. The digital EGR valve is designed to accurately supply EGR to an engine, independently of the intake manifold vacuum. The valve controls EGR flow from the exhaust to the intake manifold through three orifices which increment in size to produce seven different combinations. When a solenoid is energized, the armature, with the attached shaft and swivel pintle is lifted, this opens the orifice. The flow accuracy is dependent on the metering orifice size only, which results in improved control.

The digital EGR is opened by the ECM/PCM QDM (quad driver), grounding each respective solenoid circuit. The EGR valve usually opens under certain conditions such as, a warm engine operation, and an above idle speed condition.

Click image to see an enlarged view

Fig. Fig. 11: Newer models use a digital EGR valve, which is designed to accurately supply EGR to an engine, independent of intake manifold vacuum

Positive Crankcase Ventilation (PCV) System

See Figure 12

A closed Positive Crankcase Ventilation (PCV) system is used to provide more complete scavenging of crankcase vapors. Fresh air from the air cleaner is supplied to the crankcase, mixed with blow-by gases and then passed through a PCV valve into the induction system.

The primary mode of crankcase ventilation control is through the PCV valve, which meters the mixture of fresh air and blow-by gases into the induction system at a rate dependent upon manifold vacuum.

Click image to see an enlarged view

Fig. Fig. 12: The primary mode of crankcase ventilation control is through the PCV valve, which meters the mixture of fresh air and blow-by gases into the induction system at a rate dependent upon manifold vacuum

To maintain the idle quality, the PCV valve restricts the ventilation system flow whenever intake manifold vacuum is designed to allow excessive amounts of blow-by gases to backflow through the breather assembly into the air cleaner and through the throttle body to be consumed by normal combustion.

Evaporative Emission Control (EEC) Systems

The basic evaporative emission control system used on all vehicles uses the carbon canister storage method. This method transfers fuel vapor to an activated carbon storage device for retention when the vehicle is not operating. A ported vacuum signal is used for purging vapors stored in the canister.


The ECM/PCM controls a solenoid valve which controls vacuum to the purge valve in the charcoal canister. In open loop, before a specified time has expired and below a specified rpm, the solenoid valve is energized and blocks vacuum to the purge valve. When the system is in closed loop, after a specified time and above a specified rpm, the solenoid valve is de-energized and vacuum can be applied to the purge valve. This releases the collected vapors into the intake manifold. On systems not using an ECM/PCM controlled solenoid, a Thermo Vacuum Valve (TVV) is used to control purge. See the appropriate vehicle sections for checking procedures.

Air Management Control

The air management system aids in the reduction of exhaust emissions by supplying air to either the catalytic converter, engine exhaust manifold, or to the air cleaner. The ECM/PCM controls the air management system by energizing or de-energizing an air switching valve. Operation of the air switching valve is dependent upon such engine operating characteristics as coolant temperature, engine load, and acceleration (or deceleration), all of which are sensed by the ECM/PCM.


The Pulsair Injection Reactor (PAIR) system utilizes exhaust pressure pulsations to draw air into the exhaust system. Fresh air from the clean side of the air cleaner supplies filtered air to avoid dirt build-up on the check valve seat. The air cleaner also serves as a muffler for noise reduction. The internal mechanism of the Pulsair valve reacts to 3 distinct conditions.

The firing of the engine creates a pulsating flow of exhaust gases which are of positive (+) or negative (&ndash) pressure. This pressure or vacuum is transmitted through external tubes to the Pulsair valve.

  1. If the pressure is positive, the disc is forced to the closed position and no exhaust gas is allowed to flow past the valve and into the air supply line.
  3. If there is a negative pressure (vacuum) in the exhaust system at the valve, the disc will open, allowing fresh air to mix with the exhaust gases.
  5. Due to the inertia of the system, the disc ceases to follow the pressure pulsations at high engine rpm. At this point, the disc remains closed, preventing any further fresh air flow.

Catalytic Converter

Of all emission control devices available, the catalytic converter is the most effective in reducing tailpipe emissions. The major tailpipe pollutants are hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).


When working around any part of the fuel system, take precautionary steps to prevent fire and/or explosion:

Disconnect negative terminal from battery (except when testing with battery voltage is required).
When ever possible, use a flashlight instead of a drop light.
Keep all open flame and smoking material out of the area.
Use a shop cloth or similar to catch fuel when opening a fuel system.
Relieve fuel system pressure before servicing.
Use eye protection.
Always keep a dry chemical (class B) fire extinguisher near the area.

Due to the amount of fuel pressure in the fuel lines, before doing any work to the fuel system, the fuel system should be depressurized.

Electrostatic Discharge Damage

Electronic components used in the control system are often design to carry very low voltage and are very susceptible to damage caused by electrostatic discharge. It is possible for less than 100 volts of static electricity to cause damage to some electronic components. By comparison it takes as much as 4000 volts for a person to even feel the zap of a static discharge.

There are several ways for a person to become statically charged. The most common methods of charging are by friction and induction. An example of charging by friction is a person sliding across a car seat, in which a charge as much as 25,000 volts can build up. Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches ground. Charges of the same polarity are drained off, leaving the person highly charged with the opposite polarity. Static charges of either type can cause damage, therefore, it is important to use care when handling and testing electronic components.

To prevent possible electrostatic discharge damage to the ECM/PCM, do not touch the connector pins or soldered components on the circuit board. When handling a PROM, Mem-Cal or Cal-Pak, do not touch the component leads and remove the integrated circuit from the carrier.


The ECM/PCM has a learning capability. If the battery is disconnected, the learning process has to begin all over again. A change may be noted in the vehicle's performance. To teach the ECM/PCM, insure the vehicle is at operating temperature and drive at part throttle, with moderate acceleration and idle conditions, until performance returns.


See Figure 13

The Data Link Connector or Assembly Line Diagnostic Link (DLC/ALDL), is a diagnostic connector located usually under the instrument panel, it is sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR." The assembly plant were the vehicles originate use these connectors to check the engine for proper operation before it leaves the plant. The connector can also be used by a technician to identify stored codes using different procedures and to read the ECM/PCM data using a hand held scan tool, such as TECH 1. Although it is recommended to use as hand held scan tool to read diagnostic trouble codes, it may be possible to flash codes on certain vehicle. These vehicles have a 12 pin DLC.

Click image to see an enlarged view

Fig. Fig. 13: The Data Link Connector (DLC) or Assembly Line Diagnostic Link (ALDL) is a diagnostic connector located usually under the instrument panel. It is sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR"


This information is able to be read by putting the ECM/PCM into 1 of 4 different modes. These modes are entered by inserting a specific amount of resistance between the DLC/ALDL connector terminals A and B. The modes and resistances needed to enter these modes are as follows:

Diagnostic Modes - 0 Ohms

When 0 resistance is between terminals A and B of the DLC/ALDL connector, the diagnostic mode is entered. There are 2 positions to this mode. One with the engine OFF , but the ignition ON ; the other is when the engine is running called Field Service Mode.

If the diagnostic mode is entered with the engine in the OFF position, trouble codes will flash and the idle air control motor will pulsate in and out. Also, the relays and solenoids are energized with the exception of the fuel pump and injector.

As a bulb and system check, the SERVICE ENGINE SOON light will come on with the ignition switch ON and the engine not running. When the engine is started, the SERVICE ENGINE SOON light will turn off. If the SERVICE ENGINE SOON light remains on, the self-diagnostic system has detected a problem.

If the B terminal is then grounded with the ignition ON , engine not running, each trouble code will flash and repeat 3 times. If more than 1 problem has been detected, each trouble code will flash 3 times. Trouble codes will flash in numeric order (lowest number first). The trouble code series will repeat as long as the B terminal is grounded.

A trouble code indicates a problem in a given circuit (Code 14, for example, indicates a problem in the coolant sensor circuit, this includes the coolant sensor, connector harness, and ECM/PCM). The procedure for pinpointing the problem can be found in diagnosis. Similar charts are provided for each code.

Also in this mode all ECM/PCM controlled relays and solenoids except the fuel pump relay. This allows checking the circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions. The IAC valve will move to its fully extended position on most vehicles, block the idle air passage. This is useful in checking the minimum idle speed.

Field Service Mode - 0 Ohms

When the DLC/ALDL connector terminal B is grounded with the engine running, the ECM/PCM goes into the field service mode. In this mode, the SERVICE ENGINE SOON light flashes closed or open loop and indicates the rich/lean status of the engine. The ECM/PCM runs the engine at a fixed ignition timing advanced above the base setting.

The SERVICE ENGINE SOON light will show whether the system is in Open Loop or Closed Loop. In Open Loop the SERVICE ENGINE SOON light flashes 2 times and one half times per second. In Closed Loop the light flashes once per second. Also in Closed Loop, the light will stay OUT most of the time if the system is too lean. It will stay ON most of the time if the system is too rich. In either case the Field Service mode check, which is part of the Diagnostic circuit check, will lead you into choosing the correct diagnostic chart to refer to.

Back-up Mode - 3.9 Kilo-ohms

The backup mode is entered by applying 3.9 kilo-ohms resistance between terminals A and B of the DLC/ALDL connector with the ignition switch in the ON position. The DLC/ALDL scanner tool can now read 5 of the 20 parameters on the data stream. These parameters are as mode status, oxygen sensor voltage, rpm, block learn and idle air control. There are 2 ways to enter the backup mode. Using a scan tool is one way, putting a 3.9 kilo-ohms resistor across terminals A and B of the DLC/ALDL is another.

Special Mode - 10 Kilo-ohms

This special mode is entered by applying a 10K ohms resistor across terminals A and B. When this happens, the ECM/PCM does the following:

Allows all of the serial data to be read
Bypasses all timers
Adds a calibrated spark advance
Enables the canister purge solenoid on some engines
Idles at 1000 rpm fixed idle air control and fixed base pulse width on the injector
Forces the idle air control to reset at part throttle (approximately 2000 rpm)
Disables the Park/Neutral restriction functions

Open or Road Test Mode - 20 Kilo-ohms

The system is in this mode during normal operation and is used by a scan tool to extract data while driving the vehicle.

DLC/ALDL Scan Tester Information

An DLC/ALDL display unit (DLC/ALDL tester, scanner, monitor, etc.), allows a you to read the engine control system information from the DLC/ALDL connector under the instrument panel. It can provide information faster than a digital voltmeter or ohmmeter can. The scan tool does not diagnose the exact location of the problem. The tool supplies information about the ECM/PCM, the information that it is receiving and the commands that it is sending plus special information such as integrator and block learn. To use an DLC/ALDL display tool you should understand thoroughly how an engine control system operates.

An DLC/ALDL scanner or monitor puts a fuel injection system into a special test mode. This mode commands an idle speed of 1000 rpm. The idle quality cannot be evaluated with a tester plugged in. When a scanner is in a special test mode it commands a fixed spark with no advance. On vehicles with Electronic Spark Control (ESC), there will be a fixed spark, but it will be advanced. On vehicles with ESC, there might be a serious spark knock, this spark knock could be bad enough so as not being able to road test the vehicle in the DLC/ALDL test mode. Be sure to check the tool manufacturer for instructions on special test modes which should overcome these limitations.

When a tester is used with a fuel injected engine, it bypasses the timer that keeps the system in Open Loop for a certain period of time. When all Closed Loop conditions are met, the engine will go into Closed Loop as soon as the vehicle is started. This means that the air management system will not function properly and air may go directly to the converter as soon as the engine is started.

These tools cannot diagnose everything. They do not tell where a problem is located in a circuit. The diagnostic charts to pinpoint the problems must still be used. These testers do not let you know if a solenoid or relay has been turned on. They only tell the you the ECM/PCM command. To find out if a solenoid has been turned on, check it with a suitable test light or digital voltmeter, or see if vacuum through the solenoid changes.

Scan Tools For Intermittents

In some scan tool applications, the data update rate may make the tool less effective than a voltmeter, such as when trying to detect an intermittent problem which lasts for a very short time. Some scan tools have a snapshot function which stores several seconds or even minutes of operation to located an intermittent problem. Scan tools allow one to manipulate the wiring harness or components under the hood with the engine not running while observing the scan tool's readout.

The scan tool can be plugged in and observed while driving the vehicle under the condition when the SERVICE ENGINE SOON light turns on momentarily or when the engine driveability is momentarily poor. If the problem seems to be related to certain parameters that can be checked on the scan tool, they should be checked while driving the vehicle. If there does not seem to be any correlation between the problem and any specific circuit, the scan tool can be checked on each position. Watching for a period of time to see if there is any change in the reading that indicates intermittent operation.

The scan tool is also an easy way to compare the operating parameters of a poorly operating engine with typical scan data for the vehicle being serviced or those of a known good engine. For example, a sensor may shift in value but not set a trouble code. Comparing the sensor's reading with those of a known good parameters may uncover the problem.

The scan tool has the ability to save time in diagnosis and prevent the replacement of good parts. The key to using the scan tool successfully for diagnosis lies in the ability to understand the system being diagnosed, as well as the scan tool's operation and limitations.


When the ECM/PCM detects a problem with the system, the SERVICE ENGINE SOON light will come on and a trouble code will be recorded in the ECM/PCM memory. If the problem is intermittent, the SERVICE ENGINE SOON light will go out after 10 seconds, when the fault goes away. However the trouble code will stay in the ECM/PCM memory until the battery voltage to the ECM/PCM is removed. Removing the battery voltage for 10 seconds will clear all trouble codes. Do this by disconnecting the ECM/PCM harness from the positive battery terminal pigtail for 10 seconds with the key in the OFF position, or by removing the ECM/PCM fuse for 10 seconds with the key OFF .

To prevent ECM/PCM damage, the key must be OFF when disconnecting and reconnecting ECM/PCM power.


The integrator and block learn functions of the ECM/PCM are responsible for making minor adjustments to the air/fuel ratio on the fuel injected GM vehicles. These small adjustments are necessary to compensate for pinpoint air leaks and normal wear.

The integrator and block learn are 2 separate ECM/PCM memory functions which control fuel delivery. The integrator makes a temporary change and the block learn makes a more permanent change. Both of these functions apply only while the engine is in Closed Loop. They represent the on-time of the injector. Also, integrator and block learn controls fuel delivery on the fuel injected engines as does the MC solenoid dwell on the CCC carbureted engines.


Integrator is the term applied to a means of temporary change in fuel delivery. Integrator is displayed through the DLC/ALDL data line and monitored with a scanner as a number between 0 and 255 with an average of 128. The integrator monitors the oxygen sensor output voltage and adds and subtracts fuel depending on the lean or rich condition of the oxygen sensor. When the integrator is displaying 128, it indicates a neutral condition. This means that the oxygen sensor is seeing results of the 14.7:1 air/fuel mixture burned in the cylinders.

An air leak in the system (a lean condition) would cause the oxygen sensor voltage to decrease while the integrator would increase (add more fuel) to temporarily correct for the lean condition. If this happened the injector pulse width would increase.

Block Learn

Although the integrator can correct fuel delivery over a wide range, it is only for a temporary correction. Therefore, another control called block learn was added. Although it cannot make as many corrections as the integrator, it does so for a longer period of time. It gets its name from the fact that the operating range of the engine for any given combinations of rpm and load is divided into 16 cell or blocks.

The computer has a given fuel delivery stored in each block. As the operating range gets into a given block the fuel delivery will be based on what value is stored in the memory in that block. Again, just like the integrator, the number represents the on-time of the injector. Also, just like the integrator, the number 128 represents no correction to the value that is stored in the cell or block. When the integrator increases or decreases, block learn which is also watching the integrator will make corrections in the same direction. As the block learn makes corrections, the integrator correction will be reduced until finally the integrator will return to 128 if the block learn has corrected the fuel delivery.

Block Learn Memory

Block learn operates on 1 of 2 types of memories depending on application: non-volatile or volatile. The non-volatile memories retain the value in the block learn cells even when the ignition switch is turned OFF . When the engine is restarted, the fuel delivery for a given block will be based on information stored in memory.

The volatile memories lose the numbers stored in the block learn cells when the ignition is turned to the OFF position. Upon restarting, the block learn starts at 128 in every block and corrects from that point as necessary.

Integrator/Block Learn Limits

Both the integrator and block learn have limits which will vary from engine to engine. If the mixture is off enough so that the block learn reaches the limit of its control and still cannot correct the condition, the integrator would also go to its limit of control in the same direction and the engine would then begin to run poorly. If the integrators and block learn are close to or at their limits of control, the engine hardware should be checked to determine the cause of the limits being reached, vacuum leaks, sticking injectors, etc.

If the integrator is lied to, for example, if the oxygen sensor lead was grounded (lean signal) the integrator and block learn would add fuel to the engine to cause it to run rich. However, with the oxygen sensor lead grounded, the ECM/PCM would continue seeing a lean condition eventually setting a Code 44 and the fuel control system would change to open loop operations.


The purpose of closed loop fuel control is to precisely maintain an air/fuel mixture 14.7:1. When the air/fuel mixture is maintained at 14.7:1, the catalytic converter is able to operate at maximum efficiency which results in lower emission levels.

Since the ECM/PCM controls the air/fuel mixture, it needs to check its output and correct the fuel mixture for deviations from the ideal ratio. The oxygen sensor feeds this output information back to the ECM/PCM.


Engine performance diagnostic procedures are guides that will lead to the most probable causes of engine performance complaints. They consider the components of the fuel, ignition, and mechanical systems that could cause a particular complaint, and then outline repairs in a logical sequence.

It is important to determine if the SERVICE ENGINE SOON light is on or has come on for a short interval while driving. If the SERVICE ENGINE SOON light has come on, the Computer Command Control System should be checked for stored TROUBLE CODES which may indicate the cause for the performance complaint.

All of the symptoms can be caused by worn out or defective parts such as spark plugs, ignition wiring, etc. If time and/or mileage indicate that parts should be replaced, it is recommended that it be done.

Before checking any system controlled by the Electronic Fuel Injection (EFI) system, the Diagnostic Circuit Check must be performed or misdiagnosis may occur. If the complaint involves the SERVICE ENGINE SOON light, go directly to the Diagnostic Circuit Check.

Basic Troubleshooting

The following explains how to activate the trouble code signal light in the instrument cluster and gives an explanation of what each code means. This is not a full system troubleshooting and isolation procedure.

Before suspecting the system or any of its components as faulty, check the ignition system including distributor, timing, spark plugs and wires. Check the engine compression, air cleaner, and emission control components not controlled by the ECM/PCM. Also check the intake manifold, vacuum hoses and hose connectors for leaks.

The following symptoms could indicate a possible problem with the system:

Stalls or rough idle-cold
Stalls or rough idle-hot
Poor gasoline mileage
Sluggish or spongy performance
Hard starting-cold
Objectionable exhaust odors (that rotten egg smell)
Cuts out
Improper idle speed

As a bulb and system check, the SERVICE ENGINE SOON light will come on when the ignition switch is turned to the ON position but the engine is not running. The SERVICE ENGINE SOON light will also produce the trouble code or codes by a series of flashes which translate as follows. When the diagnostic test terminal under the dash is grounded, with the ignition in the ON position and the engine not running, the SERVICE ENGINE SOON light will flash once, pause, then flash twice in rapid succession. This is a Code 12, which indicates that the diagnostic system is working. After a long pause, the Code 12 will repeat itself 2 more times. The cycle will then repeat itself until the engine is started or the ignition is turned OFF .

When the engine is started, the SERVICE ENGINE SOON light will remain on for a few seconds, then turn off. If the SERVICE ENGINE SOON light remains on, the self-diagnostic system has detected a problem. If the test terminal is then grounded, the trouble code will flash 3 times. If more than 1 problem is found, each trouble code will flash 3 times. Trouble codes will flash in numerical order (lowest code number to highest). The trouble codes series will repeat as long as the test terminal is grounded.

A trouble code indicates a problem with a given circuit. For example, trouble Code 14 indicates a problem in the cooling sensor circuit. This includes the coolant sensor, its electrical harness, and the ECM/PCM. Since the self-diagnostic system cannot diagnose every possible fault in the system, the absence of a trouble code does not mean the system is trouble-free. To determine problems within the system which do not activate a trouble code, a system performance check must be made.

In the case of an intermittent fault in the system, the SERVICE ENGINE SOON light will go out when the fault goes away, but the trouble code will remain in the memory of the ECM/PCM. Therefore, it a trouble code can be obtained even though the SERVICE ENGINE SOON light is not on, the trouble code must be evaluated. It must be determined if the fault is intermittent or if the engine must be at certain operating conditions (under load, etc.) before the SERVICE ENGINE SOON light will come on. Some trouble codes will not be recorded in the ECM/PCM until the engine has been operated at part throttle for about 5-18 minutes.


An intermittent open in the ground circuit would cause loss of power through the ECM/PCM and intermittent SERVICE ENGINE SOON light operation. When the ECM/PCM loses ground, distributor ignition is lost. An intermittent open in the ground circuit would be described as an engine miss.

Therefore, an intermittent SERVICE ENGINE SOON light, no code stored and a driveability comment described as similar to a miss will require checking the grounding circuit and the Code 12 circuit as it originates at the ignition coil.


An undervoltage condition below 9 volts will cause the SERVICE ENGINE SOON light to come on as long as the condition exist.

Therefore, an intermittent SERVICE ENGINE SOON light, no code stored and a driveability comment described as similar to a miss will require checking the grounding circuit, Code 12 circuit and the ignition feed circuit to terminal C of the ECM/PCM. This does nor eliminate the necessity of checking the normal vehicle electrical system for possible cause such as a loose battery cable.


The ECM/PCM will also shut off when the power supply rises above 16 volts. The overvoltage condition will also cause the SERVICE ENGINE SOON to come on as long as this condition exists.

A momentary voltage surge in a vehicle's electrical system is a common occurrence. These voltage surges have never presented any problems because the entire electrical system acted as a shock absorber until the surge dissipated. Voltage surges or spikes in the vehicle's electrical system have been known, on occasion, to exceed 100 volts.

The system is a low voltage (between 9 and 16 volts) system and will not tolerate these surges. The ECM/PCM will be shut off by any surge in excess of 16 volts and will come back on, only after the surge has dissipated sufficiently to bring the voltage under 16 volts.

A surge will usually occur when an accessory requiring a high voltage supply is turned off or down. The voltage regulator in the vehicle's charging system cannot react to the changes in the voltage demands quickly enough and surge occurs. The driver should be questioned to determine which accessory circuit was turned off that caused the SERVICE ENGINE SOON light to come on.

Therefore, intermittent SERVICE ENGINE SOON light operation, with no trouble code stored, will require installation of a diode in the appropriate accessory circuit.


Due to the varied application of components, a general procedure is outlined. For the exact procedure for the vehicle being service use Code 21 or 22 chart the appropriate engine.

With Scan Tool
  1. Use a suitable scan tool to read the TPS voltage.
  3. With the ignition switch ON and the engine OFF , the TPS voltage should be less than 1.25 volts.
  5. If the voltage reading is higher than specified, replace the throttle position sensor.

Without Scan Tool
  1. Remove air cleaner. Disconnect the TPS harness from the TPS.
  3. Using suitable jumper wires, connect a digital voltmeter J-29125-A or equivalent to the correct TPS terminals A and B.
  5. With the ignition ON and the engine running, The TPS voltage should be 0.3-1.0 volts at base idle to approximately 4.5 volts at wide open throttle.
  7. If the reading on the TPS is out of specification, check the minimum idle speed before replacing the TPS.
  9. If the voltage reading is correct, remove the voltmeter and jumper wires and reconnect the TPS connector to the sensor.
  11. Reinstall the air cleaner.



When the ignition switch is turned ON , the in-tank fuel pump is energized for as long as the engine is cranking or running and the control unit is receiving signals from the HEI distributor or DIS. If there are no reference pulses, the control unit will shut off the fuel pump within 2 seconds. The pump will deliver fuel to the fuel rail and injectors and then to the pressure regulator, where the system pressure is controlled to maintain 26-46 psi (179-317 kPa). Each vehicle varies slightly in the amount of psi.

  1. Connect pressure gauge J-34730-1, or equivalent, to fuel pressure test point on the fuel rail. Wrap a rag around the pressure tap to absorb any leakage that may occur when installing the gauge.
  3. Turn the ignition ON and check that pump pressure is 24-40 psi (165-275 kPa). This pressure is controlled by spring pressure within the regulator assembly.
  5. Start the engine and allow it to idle. The fuel pressure should drop to 28-32 psi (193-220 kPa) due to the lower manifold pressure.

The idle pressure will vary somewhat depending on barometric pressure. Check for a drop in pressure indicating regulator control, rather than specific values.

  1. On turbocharged vehicles, use a low pressure air pump to apply air pressure to the regulator to simulate turbocharger boost pressure. Boost pressure should increase fuel pressure by 1 lb. for every lb. of boost. Again, look for changes rather than specific pressures. The maximum fuel pressure should not exceed 46 psi (317 kPa).
  3. If the fuel pressure drops, check the operation of the check valve, the pump coupling connection, fuel pressure regulator valve and the injectors. A restricted fuel line or filter may also cause a pressure drip. To check the fuel pump output, restrict the fuel return line and run 12 volts to the pump. The fuel pressure should rise to approximately 75 psi (517 kPa) with the return line restricted.

Before attempting to remove or service any fuel system component, it is necessary to relieve the fuel system pressure.


  1. Disconnect the negative battery cable.
  3. Rotate the harmonic balancer using a 28mm socket and pull on the handle until the interrupter ring fills the sensor slots and edge of the interrupter window is aligned with the edge of the deflector on the pedestal.
  5. Insert feeler gauge adjustment tool J-36179 or equivalent into the gap between the sensor and the interrupter on each side of the interrupter ring.
  7. If the gauge will not slide past the sensor on either side of the interrupter ring, the sensor is out of adjustment or the interrupter ring is bent.
  9. The clearance should be checked again, at 3 positions around the interrupter ring approximately 120° apart.
  11. If found out of adjustment, the sensor should be removed and inspected for potential damage.


To properly diagnose driveability problems, refer to the following trouble code charts. Make certain the charts cover the appropriate engine. If your check engine light is not lit, check for engine stored engine codes. If any codes are stored write them down for reference later. Clear the codes as described earlier. Road test the vehicle to see if any of the codes return. Never try to fix a code problem until you're sure that it comes back. It may have been an old code.

After clearing any codes and checking that they do not return. If the car drives fine you're finished. But if there are no codes and the car runs poorly you'll need to check the symptoms charts. The problem is most likely not the computer or the devices it controls but the ignition system or engine mechanical.

If you do have a code(s) that returns start with the lowest code and follow the proper chart. You must follow every step of the chart and not jump from test to test or you'll never be certain to find and fix the real problem.

Start with the lowest to highest code chart, making sure to use the charts for your engine. If you have a 50 series code, like code 54. Always check those out first. They are rare but usually indicate a problem with the computer itself or its ability to test itself properly.

Whereas component repair and replacement are covered in Section 5, this section only deals with testing the system for driveability and emission problems.