Circuit Operation

CIRCUIT OPERATION
The engine is controlled by the Engine Control Module (ECM). The ECM 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 ECM from the A/C thermostat unit via the A/C high and low pressure switches. The ECM 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 ECM.

Canister Purge Valve
With the engine running, the ECM 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 ECM when the following conditions prevail:

1. Engine is running at speeds above 1700 rpm and temperatures above 54°C (ECM will open the valve as necessary).

2. Engine speed is below 1700 rpm (ECM 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 ECM applies a voltage signal to the coolant temperature sensor. The ECM 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 ECM applies a voltage signal to the engine fuel temperature sensor. The ECM 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 ECM via the 2 wires that link them together.

Fuel Injectors
With the engine running, the ECM 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 ECM in groups of 4. The ECM grounds each group of fuel injectors alternately.

Fuel Pump
With the engine running, the ECM 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 ECM controls engine idle speed by adjusting the IACV to compensate for increased electrical and mechanical loads. The ECM controls the IACV by sending voltage signals to a pair of motor control windings inside the IACV. This allows the ECM to reverse the voltage signals to the IACV, moving it 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 ECM 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 ECM senses this voltage and adjusts fuel supply to the engine.

Mass Air Flow Sensor
With the ignition switch in position II, the ECM applies ground to the fuel injection load relay, energizing the relay and applying battery voltage to the mass air flow sensor. The ECM applies 5 volts to the mass air flow sensor terminal 6 and the sensor sends a voltage signal to the ECM terminal 35. The greater the volume of air passing through the mass air flow sensor, the greater the voltage signal to the ECM.

Starter Inhibit Reverse Switch
The starter inhibit reverse switch applies ground to the ECM with the transmission in P or N.

Throttle Position Sensor
With the ignition switch in position II, the ECM 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 ECM terminal 20. This signal varies between less than 1 volt at closed throttle to more than 4 volts at full open throttle.

Vehicle Speed Sensor Buffer
The vehicle speed sensor buffer sends signals to the ECM in the form of voltage pulses. The voltage varies between battery voltage and 0 volts 6 times per wheel revolution.