Isuzu Cars and Trucks 1981-1991

General Information


All Except V6 Engine

The fuel injection control system constantly monitors and controls engine operation, which in turn helps lower emissions while maintaining the fuel economy and driveability. The Electronic Control Unit (ECU) controls the fuel injection system, the ignition system and the turbocharger control system (if so equipped).

The ignition system consists of the following components; the crank angle sensor, the throttle valve switch, the vehicle speed sensor, the coolant temperature sensor, air flow sensor, knock sensor, transistorized ignition coil and the ECU.

In general, the fuel injection system consists of the following components; the crank angle sensor, the throttle valve switch, the vehicle speed sensor, the coolant temperature sensor, air flow sensor, oxygen sensor, fuel injectors and the ECU.

If equipped, the turbocharger system consists of the following components; the throttle valve switch, the throttle position sensor, the knock sensor, the coolant temperature sensor, air flow sensor, oxygen sensor, the stepping motor, the turbocharger controller and the ECU.

V6 Engine

The electronic throttle body fuel injection system used on the V6 engine 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). The ECM 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 to account for special operating conditions, such as cranking, cold starting, altitude, acceleration, and deceleration.

The ECM controls the exhaust emissions by modifying fuel delivery to achieve, as near as possible, an air/fuel ratio of 14.7:1. The injector on-time is determined by various inputs to the ECM. 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 nonsynchronized.


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.


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.

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.

Each fuel injector is a solenoid-operated device controlled by the ECM. 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 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. The ECM 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 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.

Air Conditioner Request Signal

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

Air Flow Sensor

The air flow sensor is usually located in the air cleaner housing assembly. The purpose of the air flow sensor is to measure the volume (rate) of air that is coming into the engine.

Back-Up Control System

This system is used in case there is a malfunction with themicrocomputer within the ECU, the back-up control system works to maintain the necessary functions of the control unit to permit continuous operation of the vehicle.

Coolant Temperature Sensor

This sensor is usually located on the engine block, under the intake manifold. It sends the coolant temperature information back to the ECU. The ECU then uses this information to determine the engine temperature for calculating the required air/fuel mixture.


The coolant sensor is a thermister (a resistor which changes value based on temperature) mounted in 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 40and#x00B0;F/40and#x00B0;C), while high temperature causes low resistance (70 ohms at 266and#x00B0;F/130and#x00B0;C).

The ECM supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ECM 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.

Crank Angle Sensor

The crank angle sensor is usually located inside the distributor housing, it is used to detect the engine speed and relative position of each piston in its cylinder. Using these parameters, the ECU calculates the proper ignition timing and dwell angle. The ECU then sends a signal to the transistorized ignition coil to create a spark.

Crankshaft and Camshaft Sensor

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.

The engine uses a 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 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.

Since this terminal is connected 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 uses this voltage input to help determine when to close and open the ignition coil primary circuit and fire the spark plug.

Direct Ignition System (DIS)

Components of the Direct Ignition System (DIS) are a coil pack, ignition module, crankshaft reluctor ring, magnetic sensor and the ECM. The coil pack consists of 2 separate, interchangeable, ignition coils. These coils operate in the same manner as previous coils. 2 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 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 bypass circuit becomes open or grounded.

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. 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, as with the distributor systems. The ECM controls the timing using crankshaft position, engine rpm, engine temperature and manifold absolute pressure sensing.

Electronic Control Unit

The ECU is usually located under the instrument panel. The ECU analyzes all electrical data signals from the sensors. It controls the fuel injection system, the ignition system and the turbocharger control system (is so equipped). The ECU has a built in back-up, diagnostic and fail safe control systems.

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. The ECM 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 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)

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 engine knock. The ESC controller signals the ECM which reduces spark advance until no more signals are received from the knock sensor.

Fuel Injector

The fuel injector is controlled by the ECU and injects fuel when ever it is energized by the ECU.

Knock Sensor

This sensor is usually located in the cylinder head. The purpose of this sensor is to send electrical impulses to the ECU when ever engine "knocking" occurs. The ECU uses these impulses to retard the ignition timing.

Manifold Absolute Pressure Sensor

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 2035 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 to adjust automatically for different altitude.

The ECM 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 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 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 Sensor

The Manifold Air Temperature (MAT) 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 40and#x00B0;F/40and#x00B0;C), while high temperature cause low resistance (70 ohms at 266and#x00B0;F/130and#x00B0;C).

The ECM supplies a 5 volt signal to the MAT sensor through a resistor in the ECM 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 circuit should set either a Code 23 or Code 25.

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.

Oxygen Sensor

This sensor is usually threaded into the exhaust manifold. The oxygen sensor measures and produces an electrical signal proportional to the amount of the oxygen present in the exhaust gases.


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 will determine what fuel mixture command to give to the injector (lean mixture - low voltage - rich command, rich mixture - high voltage - lean command).

Remember that the oxygen sensor indicates to the ECM 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 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.

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 when the transmission 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.

Power Steering Pressure Switch

The power steering pressure switch is used so that the power steering pump load will not effect the engine idle. Turning the steering wheel increases 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.

Stepper Motor

The stepper motor is used on turbocharged vehicles only. The stepper motor is rotated by a voltage pulse sent to it by the turbocharger control unit. The stepper motor is able to rotate 90and#x00B0;, depending on the number of pulses it receives from the turbocharger control unit. A microswitch inside the stepper motor unit tells the turbocharger control unit when the stepper motor is at 0and#x00B0; angle. 250 pulses from the turbocharger control unit are required to move the stepper motor 90and#x00B0;

Throttle Position Sensor

The throttle position sensor (TPS) is used on turbocharged models only. The sensor controls the fuel cut system.


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. 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 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.

By monitoring the output voltage from the TPS, the ECM 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 that the throttle is always closed, resulting in poor performance. This usually sets a Code 22. A shorted TPS gives the ECM 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 too high or too low a voltage signal.

Throttle Valve Position Switch

The throttle position switch is usually located on the throttle body. It is used to detect the throttle valve position at engine idle, part throttle and wide open throttle.

Turbocharger Control Unit

This control unit is usually located in the rear of the vehicle, behind the driver's side tail light cover in the luggage compartment. The turbocharger control unit controls the operation of the turbocharger system.

Vacuum Switching Valve

The vacuum switching valve is controlled by a signal from the ECU. This valve controls the fuel pressure according to the vacuum developed in the intake manifold.

Vehicle Speed Sensor

The vehicle speed sensor is incorporated into the speedometer. The ECU receives electrical impulses from the vehicle speed sensor.


A vehicle equipped with a speed sensor, should not be driven without 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 transmission/speedometer drive gear. It provides electrical pulses to the ECM from the speedometer head. The pulses indicate the road speed. The ECM 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 transmission 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.


Be careful not to get water on any fuel injection system component. Pay close attention to the relay box and throttle valve switch connector. The connector is not water proofed and will be damaged by water.

When charging the battery be sure to remove it from the vehicle first. Never disconnect the battery cable from the battery when the engine is running. The generation of surge voltage may damage the control unit and other electrical parts such as the multi-drive monitor.

When replacing parts or checking the system, make sure to set the starter switch to the OFF position. When measuring voltage at the control unit harness connector, disconnect all the control unit harnesses first, then set the starter switch to the ON position.

When checking the electrical terminals of the control unit with a tester, do not apply the probe to terminal directly but insert a pin into the terminal from the harness side and perform the measurement through the pin. If the tester probe is held against the terminal directly, the terminal will be deformed, causing poor contact. Connect each harness correctly and firmly to insure a good contact.

The wiring connectors for the fuel injector, throttle valve switch, air regulator and water temperature sensor are provided with locked wires. To unlock the connector, pull and shake it gently.

System cables must be placed at least 100mm (4 in.) away from the tension cables. Be careful not to apply any shock to the system components such as the air flow sensor, crank angle sensor, and control unit. Component parts of the fuel injection system are precisely set. Even a slight distortion or dent will seriously affect performance.

The fuel pump must not be operated without fuel. Since fuel lubricates the pump, noise or other serious problems such as parts seizure will result. It is also prohibited to use any fuel other than gasoline.