Engine Control Module: Description and Operation

Fig 1 Powertrain Control Module:

POWERTRAIN CONTROL MODULE (PCM)
The PCM regulates the ignition system (Fig. 1). The PCM supplies battery voltage to the ignition coil through the Automatic Shutdown (ASD) Relay. The PCM also controls the ground circuit for the ignition coil. By switching the ground path for the coil on and off, the PCM adjusts ignition timing to meet changing engine operating conditions.

During the crank-start period the PCM advances ignition timing a set amount. During engine operation the following inputs determine the amount of spark advance provided by the PCM. During the run mode the coils are turned on and off by inputs from the following:
- Intake air temperature
- Coolant temperature
- Engine RPM
- Available manifold vacuum
- Knock sensor

The PCM also regulates the fuel injection system.

5 VOLT SUPPLY-PCM OUTPUT
The PCM supplies 5 volts to the following sensors:
- A/C pressure transducer
- Engine coolant temperature sensor
- Manifold absolute pressure sensor
- Throttle position sensor
- Linear EGR solenoid

8-VOLT SUPPLY-PCM OUTPUT
The PCM supplies 8 volts to the crankshaft position sensor, camshaft position sensor.

SENSOR RETURN-PCM INPUT
The sensor return circuit provides a low electrical noise ground reference for all of the systems sensors. The sensor return circuit connects to internal ground circuits within the powertrain control module.

SPEED CONTROL-PCM INPUT
The speed control system provides five separate voltages (inputs) to the Powertrain Control Module (PCM). The voltages correspond to the ON/OFF, SET, RESUME, CANCEL, and DECEL.

The speed control ON voltage informs the PCM that the speed control system has been activated.

The speed control SET voltage informs the PCM that a fixed vehicle speed has been selected. The speed control RESUME voltage indicates the previous fixed speed is requested. The speed control CANCEL voltage tells the PCM to deactivate but retain set speed in memory (same as depressing the brake pedal). The speed control DECEL voltage informs the PCM to coast down to a new desired speed. The speed control OFF voltage tells the PCM that the speed control system has deactivated.