The Powertrain Control Module (PCM) is responsible for the operation of the emission control devices, cooling fans, ignition and advance, and automatic transmission functions. Because the PCM oversees both the ignition timing and the fuel injector operation, a precise air/fuel ratio will be maintained under all operating conditions. The PCM contains a microprocessor, or small computer, which receives electrical inputs from several sensors, switches, and relays on and around the engine.
Based on combinations of these inputs, the PCM controls outputs to various devices concerned with engine operation and emissions. The control module relies on the signals to form a correct picture of current vehicle operation. If any of the input signals is incorrect, the PCM reacts to whatever picture is painted for it. For example, if the coolant temperature sensor is inaccurate and reads too low, the PCM may see a picture of the engine never warming up. Consequently, the engine settings will be maintained as if the engine were cold. Because so many inputs can affect one output, correct diagnostic procedures are essential on these systems.
Additionally, the system employs adaptive fuel logic. This process is used to compensate for normal wear and variability within the fuel system. Once the engine enters steady-state operation, the control module watches the oxygen sensor signal for a bias, or tendency, to run slightly rich or lean. If such a bias is detected, the adaptive logic corrects the fuel delivery to bring the air/fuel mixture towards a centered ratio of 14.7:1. This compensating shift is stored in the Keep Alive Memory (KAM), which is retained by battery power even with the ignition switched OFF . The correction factor is then available the next time the vehicle is operated.
There is also a fail-safe function designed into the PCM. If the microprocessor, the EEPROM (read only memory), or related circuits malfunction, the fail-safe circuit prevents the vehicle from becoming completely disabled. Fuel injection volume and ignition timing values are fixed, allowing minimal engine operation.
Removal & Installation
- Disconnect the negative battery cable.
- Slide the ashtray and cupholder assembly out. Release the two locking tabs and disengage the assembly from the bracket. Remove the assembly.
- Remove the four lower center console cover plastic pin clips and remove the cover.
- Remove the two screws from the center instrument panel bezel. There are four metal retaining clips which snap into the instrument panel to hold the bezel in place. Pull the bezel evenly away from the instrument panel to release the clips, being careful not to damage the bezel, and remove it.
- Open the glove compartment door. Release the stops on either side of the door and allow the door to hang fully open.
- Remove the passenger-s side air bag access panel in the top of the glove compartment. Disconnect the electrical connector.
- Remove the four outer glove compartment screws. Remove the four inner glove compartment bolts. Slide the glove compartment out, disconnect the electrical connector from the latch, and remove the glove compartment.
- Remove the two passenger-s side air bag bracket nuts and the bracket bolt. Remove the bracket.
- Unscrew the PCM electrical connector bolt. Disconnect the connector carefully and position it aside.
- Remove the two PCM bolts. Slide the PCM out enough to disengage the rear mounting tab from the clip.
- Tilt the PCM down slightly and unclip the two electrical connectors from the top of the PCM.
- Slide the relay off of the bracket on top of the PCM.
- Remove the PCM from the vehicle.
- The installation is the reverse of removal, but please note the following important steps.
- When installing the PCM, tighten the nuts to 44 inch lbs. (5 Nm).
- When installing the PCM electrical connector, tighten the bolt to 53 inch lbs. (6 Nm). There should be no gap between the orange indicators.
When installing the air bag bracket, tighten the nuts to 44 inch lbs. (5 Nm).