Powertrain Control Module
PCM is located on right front cowl/firewall. See Fig 1 . The PCM is the control center of the vehicle. It controls the following:
- Fuel metering system.
- Transmission shifting.
- Ignition timing.
- On-board diagnostics for powertrain functions.
- The evaporative emissions (EVAP) system.
- The generator.
- The A/C clutch control.
- The cooling fan control.
The PCM constantly looks at the information from various sensors and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic functions for those systems. It can recognize operational problems and will alert the driver through the Malfunction Indicator Lamp (MIL) when a malfunction has occurred. When the PCM detects a malfunction, it stores a DTC which will help identify problem areas. This is done to aid the technician in making repairs.
The PCM supplies either 5 or 12 volts to power various sensors and switches. This is done through transistors in the PCM. The circuits have very high impedance and typically will not illuminate a test lamp when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low in comparison with the input impedance of the circuit being probed. Therefore, a DMM with at least 10 megohms input impedance is required to ensure accurate voltage readings.
The PCM controls all ignition system functions, and constantly corrects the spark timing. The PCM monitors information from various sensor inputs that include the following:
- The Throttle Position (TP) sensor.
- The Engine Coolant Temperature (ECT) sensor.
- The Intake Air Temperature (IAT) sensor.
- The Vehicle Speed Sensor (VSS).
- The transmission gear position or range information sensors.
- The engine knock sensors.