Circuit Operation
CIRCUIT OPERATION
The engine is controlled by the electronic control unit (ECU). The ECU is a microprocessor-controlled device that uses the following electrical components to control engine operation:
A/C Thermostat Unit
When compressor clutch operation is requested, a voltage signal is sent to the ECU from the A/C thermostat unit via the A/C high and low pressure switches. The ECU uses this signal both to engage the compressor clutch and to compensate for the added load placed on the engine by the compressor clutch. If the refrigerant pressure is too high or too low, the high or low pressure switch opens, interrupting the voltage signal from the thermostat unit to the ECU.
Canister Purge Valve
With the engine running, the ECU provides a ground for the fuel pump relay. The relay applies battery voltage to the canister purge valve. Ground is applied to operate the canister purge valve by the ECU when the following conditions prevail:
1. Engine is running at speeds above 1700 rpm and temperatures above 54°C, ECU will open the valve as necessary.
2. Engine speed is below 1700 rpm, ECU will pulse valve open for short periods.
Engine Coolant Temperature Sensor
The engine coolant temperature sensor decreases its resistance as engine coolant temperature increases. The ECU applies a voltage signal to the coolant temperature sensor. The ECU detects a small current when the engine coolant is at a low temperature; it detects a larger current with a high temperature.
Engine Fuel Temperature Sensor
The engine fuel temperature sensor decreases its resistance with an increase in temperature. The ECU applies a voltage signal to the Engine Fuel Temperature Sensor. The ECU detects a small current when the fuel is at a low temperature; it detects a larger current with a high temperature.
Fuel Injection Fault Display Unit (USA Vehicles Only)
The fuel injection fault display unit receives battery voltage when the ignition switch is in position II. It is grounded at all times. Behind the dark exterior of this unit there is a 2-digit 7-segment digital display that is visible only when a fault code is set. The unit receives data from the ECU via the 2 wires that link them together.
Fuel Injectors
With the engine running, the ECU applies ground to the fuel injection load relay, energizing the relay and applying battery voltage to the fuel injectors. The fuel injectors are connected to the ECU in groups of 4. The ECU grounds each group of Fuel Injectors alternately.
Fuel Pump
With the engine running, the ECU provides a ground for the fuel pump relay, which applies battery voltage to the fuel pump via the inertia fuel shutoff switch. The switch opens to turn off the fuel pump when the vehicle experiences a sudden impact.
Idle Air Control Valve (IACV)
The ECU controls engine idle speed by adjusting the idle air control valve to compensate for increased electrical and mechanical loads. The ECU controls the control valve by sending voltage signals to a pair of motor control windings inside the control valve. This allows the ECU to reverse the voltage signals to the control valve, moving the IACV in or out to vary air flow to the plenum chamber.
Ignition Control Module
The ignition system consists of the ignition control module and distributor. The ECU receives a pulsing ground signal from the ignition control module via a 6.8K ohm resistor. The rate of these pulses corresponds to the engine speed.
Left and Right Heated Oxygen Sensors
A heating element and an oxygen sensor are inside each heated oxygen sensors. When the engine is running, battery voltage is applied to both heated oxygen sensors via the fuel pump relay. A small voltage is generated as exhaust gas passes the sensors. The ECU senses this voltage and adjusts fuel supply to the engine.
Mass Air Flow Sensor
With the ignition switch in position II, the ECU applies ground to the fuel injection load relay, energizing the relay and applying battery voltage to the mass air flow sensor. The ECU applies 5 volts to the Mass Air Flow Sensor terminal 6 and the sensor sends a voltage signal to the ECU terminal 35. The greater the volume of air passing through the mass air flow sensor, the greater the voltage signal to the ECU.
Starter Inhibit Reverse Switch
The starter inhibit reverse switch applies ground to the ECU with the transmission in P or N.
Throttle Position Sensor (TPS)
With the ignition switch in position II, the ECU provides a ground via terminal 25 and supplies 5 volts to the TPS terminal 3. When the throttle is moved, the TPS sends a voltage signal to ECU terminal 20. This signal varies between less than 1 volt at closed throttle to more than 4 volts at full open throttle.
Tune Select Resistor (Not Fitted To USA Vehicles after VIN 451518)
The tune select resistor receives a 5 volt signal from the ECU. The opposite terminal of the tune select resistor is grounded at all times. The tune select resistor causes the 5 volt signal read at the ECU terminal 5 to decrease. The ECU determines how the engine should perform based on the amount of voltage at terminal 5.
Vehicle Speed Sensor Buffer
The vehicle speed sensor buffer sends signals to the ECU in the form of voltage pulses. The voltage varies between battery voltage and 0 volts 6 times per wheel revolution.