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

Throttle Body Fuel Injection (TBI)

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See Figure 1

The electronic throttle body fuel injection system is a fuel metering system with the amount of fuel delivered by the throttle body injector(s) (TBI) determined by an electronic signal supplied by the Electronic Control Module (ECM) or Powertrain Control Module (PCM). The ECM/PCM monitors various engine and vehicle conditions to calculate the fuel delivery time (pulse width) of the injector(s). 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.



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Fig. Fig. 1: TBI engine component location on the 2.5L (VIN R) engine

The Throttle Body Injection (TBI) system provides a means of fuel distribution for controlling exhaust emissions within legislated limits. The TBI system, by precisely controlling the air/fuel mixture under all operating conditions, provides as near as possible complete combustion.

This is accomplished by using a Computer Control Module (ECM/PCM), a small on-board microcomputer that receives electrical inputs from various sensors about engine operating conditions. An oxygen sensor in the main exhaust stream functions to provide feedback information to the ECM/PCM as to the oxygen content, lean or rich, in the exhaust. The ECM/PCM uses this information from the oxygen sensor, and other sensors, to modify fuel delivery to achieve, as near as possible, an ideal air/fuel ratio of 14.7:1. This air/fuel ratio allows the 3-way catalytic converter to be more efficient in the conversion process of reducing exhaust emissions while at the same time providing acceptable levels of driveability and fuel economy.

COMPUTER CONTROL MODULE



The ECM/PCM program electronically signals the fuel injector in the TBI assembly to provide the correct quantity of fuel for a wide range of operating conditions. Several sensors are used to determine existing operating conditions and the ECM/PCM then signals the injector to provide the precise amount of fuel required.

The ECM/PCM used on TBI vehicles has a learning capability. If the battery is disconnected to clear diagnostic codes, or for repair, the learning process has to begin all over again. A change may be noted in vehicle performance. To teach the vehicle, make sure the vehicle is at operating temperature and drive at part throttle, under moderate acceleration and idle conditions, until performance returns.

The TBI assembly is centrally located on the intake manifold where air and fuel are distributed through a single bore in the throttle body, similar to a carbureted engine. Air for combustion is controlled by a single throttle valve which is connected to the accelerator pedal linkage by a throttle shaft and lever assembly. A special plate is located directly beneath the throttle valve to aid in mixture distribution.

Fuel for combustion is supplied by 1 or 2 fuel injector(s), mounted on the TBI assembly, whose metering tip is located directly above the throttle valve. The injector is pulsed or timed open or closed by an electronic output signal received from the ECM/PCM. The ECM/PCM receives inputs concerning engine operating conditions from the various sensors (coolant temperature sensor, oxygen sensor, etc.). The ECM/PCM, using this information, performs high speed calculations of engine fuel requirements and pulses or times the injector, open or closed, thereby controlling fuel and air mixtures to achieve, as near as possible, ideal air/fuel mixture ratios.

When the ignition key is turned ON , the ECM/PCM will initialize (start program running) and energize the fuel pump relay. The fuel pump pressurizes the system to approximately 10 psi. If the ECM/PCM does not receive a distributor reference pulse (telling the ECM/PCM the engine is turning) within 2 seconds, the ECM/PCM will then de-energize the fuel pump relay, turning off the fuel pump. If a distributor reference pulse is later received, the ECM/PCM will turn the fuel pump back on.

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

Synchronized Mode

In synchronized mode operation, the injector is pulsed once for each distributor reference pulse. In dual injector throttle body systems, the injectors are pulse alternately.

Non-synchronized Mode

See Figures 2 and 3

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:



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Fig. Fig. 2: This non-synchronized mode of operation is for a single throttle body injection system



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Fig. Fig. 3: This non-synchronized mode of operation is for a dual throttle body injection system



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

The basic TBI unit is made up of 2 major casting assemblies: (1) a throttle body with a valve to control airflow and (2) a fuel body assembly with an integral pressure regulator and fuel injector to supply the required fuel. An electronically operated device to control the idle speed and a device to provide information regarding throttle valve position are included as part of the TBI unit.

The fuel injector(s) is a solenoid-operated device controlled by the ECM/PCM. The incoming fuel is directed to the lower end of the injector assembly which has a fine screen filter surrounding the injector inlet. The ECM/PCM actuates the solenoid, which lifts a normally closed ball valve off a seat. The fuel under pressure is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve. The excess fuel passes through a pressure regulator before being returned to the vehicle's fuel tank.

The pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and air cleaner pressure on the other. The function of the regulator is to maintain a constant pressure drop across the injector throughout the operating load and speed range of the engine.

The throttle body portion of the TBI may contain ports located at, above, or below the throttle valve. These ports generate the vacuum signals for the EGR valve, MAP sensor, and the canister purge system.

The Throttle Position Sensor (TPS) is a variable resistor used to convert the degree of throttle plate opening to an electrical signal to the ECM/PCM. The ECM/PCM uses this signal as a reference point of throttle valve position. In addition, an Idle Air Control (IAC) assembly, mounted in the throttle body is used to control idle speeds. A cone-shaped valve in the IAC assembly is located in an air passage in the throttle body that leads from the point beneath the air cleaner to below the throttle valve. The ECM/PCM monitors idle speeds and, depending on engine load, moves the IAC cone in the air passage to increase or decrease air bypassing the throttle valve to the intake manifold for control of idle speeds.

Cranking Mode

See Figure 4

During engine crank, for each distributor reference pulse the ECM/PCM will deliver an injector pulse (synchronized). The crank air/fuel ratio will be used if the throttle position is less than 80% open. Crank air fuel is determined by the ECM/PCM and ranges from 1.5:1 at 33°F (36°C) to 14.7:1 at 201°F (94°C).

The lower the coolant temperature, the longer the pulse width (injector on-time) or richer the air/fuel ratio. The higher the coolant temperature, the less pulse width (injector on-time) or the leaner the air/fuel ratio.



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Fig. Fig. 4: During engine cranking, the ECM/PCM will deliver a synchronized injector pulse for each distributor reference pulse

Clear Flood Mode

See Figure 5

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 open to wide-open throttle position. The ECM/PCM then issues injector pulses at a rate that would be equal to an air/fuel ratio of 20:1. The ECM/PCM maintains this injector rate as long as the throttle remains wide open and the engine rpm is below 600. If the throttle position becomes less than 80%, the ECM/PCM then would immediately start issuing crank pulses to the injector calculated by the ECM/PCM based on the coolant temperature.



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Fig. Fig. 5: The clear flood mode is run through the ECM/PCM and distributor pulses

Run Mode

See Figures 6 and 7

There are 2 different run modes. When the engine 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 (0 2 ) sensor and calculate the injector on-time based upon inputs from the coolant and manifold absolute pressure sensors.



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Fig. Fig. 6: Run mode with the engine cold in open loop

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 varying voltage output. (This is dependent on temperature.)
  2.  
  3. The coolant sensor must be above the specified temperature.
  4.  
  5. A specific amount of time must elapse after starting the engine. These values are stored in the PROM.
  6.  

When these conditions have been met, the system goes into closed loop operation In closed loop operation, the ECM/PCM will modify the pulse width (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.



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Fig. Fig. 7: Run mode with the engine cold in closed loop

The pulse width, thus the amount of enrichment, is determined by manifold pressure change, throttle angle change, and coolant temperature. The higher the manifold pressure and the wider the throttle opening, the wider the pulse width. The acceleration enrichment pulses are delivered non-synchronized. Any reduction in throttle angle will cancel the enrichment pulses. This way, quick movements of the accelerator will not over-enrich the mixture.

Acceleration Enrichment Mode

See Figure 8

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 the manifold pressure causes fuel to condense on the manifold walls. The ECM/PCM senses this increase in throttle angle and MAP, then supplies additional fuel for a short period of time. This prevents the engine from stumbling due to too lean a mixture.



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Fig. Fig. 8: Schematic of the acceleration enrichment mode

Deceleration Leanout Mode

See Figure 9

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 manifold pressure and the decrease in throttle position to calculate a decrease in pulse width. 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 during deceleration.



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Fig. Fig. 9: Schematic of the deceleration leanout mode

Deceleration Fuel Cut-Off Mode

The purpose of deceleration fuel cut-off is to remove fuel from the engine during extreme deceleration conditions. Deceleration fuel cut-off is based on values of manifold pressure, throttle position, and engine rpm stored in the calibration PROM. Deceleration fuel cut-off overrides the deceleration enleanment mode.

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 modifies the pulse width by a correction factor in the PROM. When battery voltage decreases, pulse width increases.

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 pulse width (increase fuel)
 
Increase idle rpm
 
Increase ignition dwell time
 

Fuel Cut-Off Mode

When the ignition is OFF , the ECM/PCM will not energize the injector. 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.

Backup Mode

When in this mode, the ECM/PCM is operating on the fuel backup logic calibrated by the Cal-Pak. The Cal-Pak is used to control the fuel delivery if the ECM/PCM fails. This mode verifies that the backup feature is working properly. The parameters that can be read on a scan tool in this mode are not much use for service.

Highway Mode

When driven at highway speeds the system may enter highway or semi-closed loop mode. This improves fuel economy by leaning out fuel mixture slightly. The ECM/PCM must see correct engine temperature, ignition timing, canister activity and a constant vehicle speed before if will enter this mode. The system will switch back to closed loop periodically to check all system functions.

A scan tool determines highway mode by looking at the integrator/block learn values and oxygen sensor voltage. Integrator and block learn will show very little change and the oxygen sensor voltage is be less than 100 millivolts.

DLC/ALCL/ALDL Connector

The Assembly Line Communication Link (ALCL) or Assembly Line Diagnostic Link (ALDL) is now called the Data Link Connector (DLC). The diagnostic connector is usually located in the passenger compartment. It has terminals which are used in the assembly plant to check that the engine is operating properly before it leaves the plant. 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.

FUEL INJECTION SUBSYSTEMS



Electronic Fuel Injection (EFI) is the name given to the entire fuel injection system. Various subsystems are combined to form the overall system. These subsystems are:



Fuel supply system
 
Throttle Body Injector (TBI) assembly
 
Idle Air Control (IAC)
 
Computer Control Module (ECM/PCM)
 
Data sensors
 
Electronic Spark Timing (EST)
 
Emission controls
 

Fuel Supply System

See Figures 10 and 11

Fuel, supplied by an electric fuel pump mounted in the fuel tank, passes through an in-line fuel filter to the TBI assembly. To control fuel pump operation, a fuel pump relay is used.



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Fig. Fig. 10: Fuel pump wiring circuit



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Fig. Fig. 11: Fuel pump supply system on a TBI vehicle

When the ignition switch is turned to the ON position, the fuel pump relay activates the electric fuel pump for 1.5-2.0 seconds to prime the injector. If the ECM/PCM does not receive reference pulses from the distributor after this time, the ECM/PCM signals the relay to turn the fuel pump off. The relay will once again activate the fuel pump when the ECM/PCM receives distributor reference pulses.

The oil pressure sender is the backup for the fuel pump relay. The sender has 2 circuits, 1 for the instrument cluster light or gauge, the other to activate the fuel pump if the relay fails. If the fuse relay has failed, the sender activates the fuel pump when oil pressure reaches 4 psi. Thus a failed fuel pump relay would cause a longer crank, especially in cold weather. If the fuel pump fails, a no start condition exists.

Throttle Body Injector (TBI) Assembly

The basic TBI model 700 is used on 4-cylinder engines, is made up of 2 major casting assemblies: (1) a throttle body with a valve to control airflow and (2) a fuel body assembly with an integral pressure regulator and fuel injector to supply the required fuel. A device to control idle speed (IAC) and a device to provide information about throttle valve position (TPS) are included as part of the TBI unit.

The model 220 is used on V6 and V8 engines, consists of 3 major castings. (1) fuel meter cover with pressure regulator, (2) Fuel meter body with injectors and (3) throttle body with IAC valve and TPS sensor.

The throttle body portion of the TBI unit may contain ports located at, above, or below the throttle valve. These ports generate the vacuum signals for the EGR valve, MAP sensor, and the canister purge system.

The fuel injector is a solenoid-operated device controlled by the ECM/PCM. The incoming fuel is directed to the lower end of the injector assembly which has a fine screen filter surrounding the injector inlet. The ECM/PCM turns on the solenoid, which lifts a normally closed ball valve off a seat. The fuel, under pressure, is injected in a conical spray pattern at the walls of the throttle body bore above the throttle valve. The excess fuel passes through a pressure regulator before being returned to the vehicle fuel tank.

The pressure regulator is a diaphragm-operated relief valve with the injector pressure on one side, and the air cleaner pressure on the other. The function of the regulator is to maintain constant pressure (approximately 11 psi or 76 kPa) to the injector throughout the operating loads and speed ranges of the engine. If the regulator pressure is too low, below 9 psi (62 kPa), it can cause poor performance. Too high a pressure could cause detonation and a strong fuel odor.

Idle Air Control (IAC)

See Figure 12

The purpose of the Idle Air Control (IAC) system is to control engine idle speed 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 35 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.



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Fig. Fig. 12: The three different designs of the IAC valve

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.

Idle Speed Control (ISC)

Incorrect diagnosis and/or misunderstanding of the idle speed control systems used on fuel injected engines may lead to unnecessary replacement of the IAC valve. Engine idle speed is controlled by the ECM/PCM which changes the idle speed by moving the IAC valve. The ECM/PCM adjusts idle speed in response to fluctuations in engine load (A/C, power steering, electrical loads, etc.) to maintain acceptable idle quality and proper exhaust emission performance.

The following is provided to help you better understand the system. Asking yourself questions that a mechanic would ask will help you narrow down the problem area.

Rough Idle/Low Idle Speed

The ECM/PCM will respond to increases in engine load, which would cause a drop in idle speed, by moving the IAC valve to maintain proper idle speed. After the induced load is removed the ECM/PCM will return the idle speed to the proper level.

During A/C compressor operation (MAX, BI-LEVEL, NORM or DEFROST mode) the ECM/PCM will increase idle speed in response to an A/C-ON signal, thereby compensating for any drop in idle speed due to compressor load. On some vehicles, the ECM/PCM will also increase the idle speed in response to high power steering loads.

During periods of especially heavy loads (A/C-ON plus parking maneuvers) significant effects on idle quality may be experienced. These effects are more pronounced on 4-cylinder engines. Abnormally low idle, rough idle and idle shake may occur if the ECM/PCM does not receive the proper signals from the monitored systems.

High Idle Speed/Warm-Up Idle Speed (No Kickdown)

Engine idle speeds as high as 2100 rpm may be experienced during cold starts to quickly raise the catalytic converter to operating temperature for proper exhaust emissions performance. The idle speed attained after a cold start is ECM/PCM controlled and will not drop for 45 seconds regardless of driver attempts to kickdown.

It is important to note that fuel injected engines have no accelerator pump or choke. Idle speed during warm-up is entirely ECM/PCM controlled and cannot be changed by accelerator kickdown or pumping.

Abnormally low idle speeds are usually caused by an ECM/PCM system-controlled or monitored irregularity, while the most common cause for abnormally high idle speed is an induction (intake air) leak. The idle air control valve may occasionally lose its memory function, and it has an ECM/PCM programmed method of relearning the correct idle position. This reset, when required, will occur the next time the car exceeds 35 mph (56 km/h). At that time, the ECM/PCM seats the pintle of the IAC valve in the throttle body to determine a reference point. Then it backs out a fixed distance to maintain proper idle speed.

SENSORS



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 engine application.

Coolant Temperature (ECT)

The Engine Coolant Temperature Sensor (ECT) 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.

A failure in the coolant sensor circuit should either set a Code 14 or 15. These codes indicate a failure in the coolant temperature sensor circuit.

Oxygen

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).

Remember that oxygen sensor 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 while a constant high voltage in the circuit should set a Code 45. Codes 44 and 45 could also be set as a result of fuel system problems.

Manifold Absolute Pressure

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.

Intake Air Temperature

The Intake Air Temperature (IAT) and Manifold Air Temperature (MAT) are the same sensor. This sensor is a thermistor mounted in the intake manifold or air intake. A thermistor is a resistor which changes resistance based on temperature. Low manifold air temperature produces a high resistance (100,000 ohms at 40°F/40°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/IAT 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/PCM calculates the air temperature and uses this data to help determine the fuel delivery and spark advance. A failure in the MAT/IAT circuit should set either a Code 23 or Code 25.

Vehicle Speed

See Figure 13

A vehicle equipped with a speed sensor, should not be driven without a the speed sensor connected, as idle quality may be affected. Also extreme poor gas mileage and a code will be stored in the computers memory.

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.



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Fig. Fig. 13: The vehicle speed sensor is mounted behind the speedometer in the instrument panel or on the transaxle/speedometer drive gear

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.

Throttle Position

See Figure 14

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 volts). 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.



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Fig. Fig. 14: A 5 volt reference signal is sent to the TPS from the ECM/PCM

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. A shorted TPS gives the ECM/PCM a constant wide-open throttle signal and should set a Code 21. A loose TPS indicates to the ECM that the throttle is moving. This causes intermittent bursts of fuel from the injector and an unstable idle. On some vehicles, the TPS is adjustable and therefore can be adjusted to correct any complications caused by to high or to low of a voltage signal.

Crankshaft and Camshaft

See Figures 15 and 16

These sensors are mounted on the engine block, near the engine crankshaft, and also near the camshaft on some engines. The sensors are used to send a signal through the Direct Ignition System (DIS) module to the ECM. The ECM uses this reference signal to calculate engine speed and crankshaft position. There are several names for these sensors, 3X and 24X Crankshaft Position Sensor (CPS).

In a typical 4 cylinder engine application, a sensor is mounted with the ignition module and 2 ignition coils to comprise the direct ignition assembly. When mounted on the engine block, the sensor tip is very close to a metal disk wheel with slots which is mounted on the crankshaft.



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Fig. Fig. 15: 3X Crankshaft Position Sensor (CKP) mounting on 3.1L (VIN M) engine

The sensor tip contains a small magnet and a small coil of wire. As the metal disk wheel with the slots rotates past the sensor tip, the magnetic field of the permanent magnet is changed and a voltage is induced into the coil. This voltage signal is sent to the ignition module. The ignition module is able to determine engine speed from the frequency of the voltage curve, which changes with engine speed.

A 6-cylinder engine may use a different type of sensor called a hall effect switch. With the direct ignition connected to the vehicle electrical system, the system voltage is applied to the hall effect switch located near the tip of the sensor. A small permanent magnet creates a magnetic field in the hall effect switch circuit. As the disc wheel with the slots rotates past the sensor tip, the magnetic field in the hall effect switch changes and a change in the voltage occurs at the hall effect switch output terminal.



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Fig. Fig. 16: Camshaft sensor mounting location on 3.1L (VIN M) engine

Since this terminal is connect to the ignition module, the module senses this change in voltage and correlates the frequency of the voltage curve to determine the engine speed. The ignition module then uses this voltage input to help determine when to close and open the ignition coil primary circuit and fire the spark plug.

SWITCHES



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 for control on the torque converter clutch, EGR, and the idle air control valve operation.

Air Conditioner 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 (TCC) Solenoid

See Figure 17

The purpose of the Torque Converter Clutch (TCC) system is designed to eliminate power loss by the converter (slippage) to 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.



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Fig. Fig. 17: The converter clutch is operated by the ECM/PCM controlled torque converter clutch solenoid

Power Steering Pressure (PSPS)

The Power Steering Pressure Switch (PSPS) 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.

Oil Pressure

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).

IGNITION



Various components are used in conjunction with the ignition system. Below are the basics for the TBI.

Electronic Spark Timing (EST)

Electronic Spark Timing (EST) is used on all engines. 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.

Electronic Spark Control (ESC)

See Figures 18, 19, 20, 21 and 22

When engines are equipped with Electronic Spark Control (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 engine knock. The ESC controller signals the ECM/PCM which reduces spark advance until no more signals are received from the knock sensor.



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Fig. Fig. 18: Electronic Spark Control (ECS) schematic



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Fig. Fig. 19: Electronic spark control timing inputs



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Fig. Fig. 20: Electronic spark control timing circuit during cranking mode



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Fig. Fig. 21: Electronic spark control timing circuit during running mode



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Fig. Fig. 22: Electronic spark control knock sensor circuit

Direct Ignition System (DIS)

See Figures 23 and 24

Components of the Direct Ignition System (DIS) are a coil pack, ignition module, crankshaft reluctor ring, magnetic sensor and the ECM/PCM. The coil pack consists of 2 separate, interchangeable, ignition coils. These coils operate in the same manner as previous coils. Two coils are needed because each coil fires for 2 cylinders. The ignition module is located under the coil pack and is connected to the ECM/PCM by a 6 pin connector. The ignition module controls the primary circuits to the coils, turning them on and off and controls spark timing below 400 rpm and if the ECM/PCM bypass circuit becomes open or grounded.



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Fig. Fig. 23: DIS misfire under load schematic - 2.5L (VIN R) engine



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Fig. Fig. 24: DIS misfire at idle schematic - 2.5L (VIN R) engine

The magnetic pickup sensor inserts through the engine block, just above the pan rail in proximity to the crankshaft reluctor ring. Notches in the crankshaft reluctor ring trigger the magnetic pickup sensor to provide timing information to the ECM/PCM. The magnetic pickup sensor provides a cam signal to identify correct firing sequence and crank signals to trigger each coil at the proper time.

This system uses EST and control wires from the ECM/PCM, as with the distributor systems. The ECM/PCM controls the timing using crankshaft position, engine rpm, engine temperature and manifold absolute pressure sensing.

EMISSION CONTROL



Various components are used to control exhaust emissions from a vehicle. These components are controlled by the ECM 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

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.

Positive Crankcase Ventilation (PCV) System

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.

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.

Thermostatic Air Cleaner (TAC) System

To assure optimum driveability under varying climatic conditions, a heated intake air system is used on engines. This system is designed to warm the air entering the TBI to insure uniform inlet air temperatures. Under this condition, the fuel injection system can be calibrated to efficiently reduce exhaust emission and to eliminate throttle blade icing. The Thermac system used on fuel injected vehicles operates identically to other Thermac systems.

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.

CONTROLLED CANISTER PURGE

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.

PULSAIR REACTOR SYSTEM

See Figure 25

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.



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.
 
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.
 
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.
 

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.



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Fig. Fig. 25: The system and operation of the pulsair injection system

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).

SERVICE PRECAUTIONS



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 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.

DIAGNOSTIC ENGINE COMPUTER CODES



Data Link Connector (DLC)

See Figure 26

The Data Link Connector (DLC) is also known as the Assembly Line Communication Link (ALCL) and also known as the Assembly Line Diagnostic Link (ALDL). It is a diagnostic connector located in the passenger compartment usually under the instrument panel, and is sometimes called the DIAGNOSTIC CONNECTOR. The assembly plant were the vehicles originate use the connector to check the engine for proper operation before it leaves the plant. Terminal B is the diagnostic TEST terminal (lead) and it can be connected to terminal A, or ground, to enter the Diagnostic mode or the Field Service Mode.



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Fig. Fig. 26: The Data Link Connector (DLC) is also known as the Assembly Line Communication Link (ALCL) and as the Assembly Line Diagnostic Link (ALDL)

Reading Codes and Diagnostic Modes

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 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 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 models, block the idle air passage. This is useful in checking the minimum idle speed.

FIELD SERVICE MODE - 0 OHMS

When the DLC 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.

BACKUP 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 connector with the ignition switch in the ON position. The DLC 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 of putting a 3.9 kilo-ohms resistor across terminals A and B of the DLC 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 does the following:



Allows all of the serial data to be read.
 
Bypasses all timers.
 
Add a calibrated spark advance.
 
Enables the canister purge solenoid on some engines.
 
Idles at 950-1050 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 restrict functions.
 

OPEN OR ROAD TEST MODE - 20 KILO-OHMS

On engines that can be monitored in the Open mode, certain parameters can be observed without changing the engine operating characteristics. The parameters capable of being read vary with the engine families. Most Scan tools are programmed so that the system will go directly into the DLC mode if the Open mode is not available.

DLC SCAN TESTER INFORMATION

See Figure 27

A DLC display unit (DLC tester, scanner, monitor, etc.), allows you to read the engine control system information from the DLC 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 a DLC display tool you should understand thoroughly how an engine control system operates.



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Fig. Fig. 27: The Data Link Connector (DLC) display unit (DLC tester, scanner, monitor, etc.) allows you to read the engine control system information from the DLC connector under the instrument panel. It can provide information faster than a digital voltmeter or ohmmeter

A DLC 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. Also the test mode commands a fixed spark with no advance. On vehicles with Electronic Spark Control (ECS) and its knock sensor, there will be a fixed spark, but it will be advanced. On vehicles with ESC, there might be a serious spark knock which is bad enough to prevent road testing the vehicle in the DLC 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 tester's do not let you know if a solenoid or relay has been turned on. They only tell 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 INTERMITTENT PROBLEMS

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.

CLEARING TROUBLE CODES



When the ECM/PCM finds 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.

INTEGRATOR AND BLOCK LEARN



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

Integrator is the term applied to a means of temporary change in fuel delivery. Integrator is displayed through the DLC data line and monitored with a scanner as a number range between 0 and 255 with an average of 128. The integrator monitors the oxygen sensor output voltage (usually below 450 mV) 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 it's 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.

Closed Loop Fuel Control

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 DIAGNOSIS



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:



Detonation
 
Stalls or idles rough when cold
 
Stalls or idles rough when hot
 
Missing
 
Hesitation
 
Surges
 
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 started. 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.

Fuel System Pressure Testing

Due to the varied application of components, a general procedure is outlined. For the exact procedure for the vehicle being service use Chart A7 for the appropriate engine. A fuel system pressure test is part of several of the diagnostic charts and symptom checks.

  1. Relieve the fuel pressure from the fuel system. Turn the ignition OFF and remove the air cleaner assembly (if necessary).
  2.  
  3. Plug the Thermac vacuum port if required on the TBI unit.
  4.  
  5. Uncouple the fuel supply flexible hose in the engine compartment and install fuel pressure gauge J-29658/BT-8205 or equivalent in the pressure line or install the fuel pressure gauge into the pressure line connector located near the left engine compartment frame rail. Connection of the fuel gauge will vary accordingly to all the different engine application.
  6.  
  7. Be sure to tighten the fuel line to the gauge to ensure that there no leaks during testing.
  8.  
  9. Start the engine and observe the fuel pressure reading. The fuel pressure should be 9-13 psi (62-90 kPa).
  10.  
  11. Relieve the fuel pressure. Remove the fuel pressure gauge and reinstall the fuel line. Be sure to install a new O-ring on the fuel feed line.
  12.  
  13. Start the engine and check for fuel leaks. Stop the engine and remove the plug covering the Thermac vacuum port on the TBI unit and install the air cleaner assembly.
  14.  

Some vehicles will use more sensors than others. Also, a complete general diagnostic section is outlined. The steps and procedures can be altered as necessary according to the specific model being diagnosed and the sensors it is equipped with. If the battery power is disconnected for any reason, the volatile memory resets and the learning process begins again. A change may be noted in the performance of the vehicle. To teach the vehicle, ensure that the engine is at normal operating temperature. Then, the vehicle should be driven at part throttle, with moderate acceleration and idle conditions until normal performance returns.

TBI SYSTEM DIAGNOSTIC CHARTS



To properly diagnose driveability problems, refer to the trouble code charts which appear later in this section. 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 from years ago that was never cleared or a code that was set do to a rain storm, battery jump, etc.

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 Fuel System , this section only deals with testing the system for driveability and emission problems.

 
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