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.
- Remove the four lower center console cover screws and remove the cover.
- Remove the plastic clip from the side of the glove compartment assembly. Remove the two lower screws, two center bolts, two upper screws, and the screw at the instrument panel bracket. Separate the glove compartment from the instrument panel, disconnect the electrical connector from the latch, and remove the glove compartment.
- Remove the two nuts and one bolt from the passenger-s side air bag bracket and remove the bracket.
- Remove the PCM electrical connector bolt. Disconnect the connector carefully and position it aside. `
- Remove the two PCM bolts and remove the PCM.
The installation is the reverse of removal. When installing the PCM electrical connector, tighten the bolt until the red projection appears next to the bolt.