Engine Control Module: Description and Operation

Powertrain Control Module:

PCM PROM Location:

DESCRIPTION
The Powertrain Control Module (PCM), is located under the instrument panel on the passenger side. It is the control center of the engine and transaxle systems. It constantly monitors information from various sensors, and controls the systems that affect vehicle performance. The PCM contains a Programmable Read Only Memory (PROM) unit that is serviced separately.

The PCM is connected to the engine wiring harness by three 32-pin connectors. Each connector is color coded, red, white, or blue, and each row of pins in these three connectors has been given an unique alphabetic designation, "A" or "B" (red connector), "C" or "D" (white connector), "E" or "F" (blue connector).

The PCM performs the diagnostic function of the engine and transaxle control systems. It can recognize operational problems, alert the driver through the "SERVICE ENGINE SOON" MIL, and store a DTC or DTCs which identify the problem areas to aid the technician in making repairs.

PCM EEPROM
The PCM contains a data storage unit which can save critical data even when battery power is removed. This unit, the Electrically Erasable Programmable Read Only Memory (EEPROM) is used by the PCM to store Throttle Position (TP) sensor / idle learned values, transaxle shift adapt values, transaxle oil life index, and cruise control lash/learn.

These values are stored in the EEPROM during the first 30 seconds after the key is turned "OFF." Therefore, you should wait at least 30 seconds after key "OFF" before disconnecting the battery to allow these values to be stored in the EEPROM.

EEPROM DATA FOLLOWING PCM REPLACEMENT
Since the data contained in the EEPROM is specific to the vehicle that the PCM is in, the EEPROM should reset if the PCM is installed in another vehicle. The PCM does this automatically by comparing the season odometer value it has saved to the season odometer value stored in the IPC. If these two values are different, the PCM assumes it is in a new vehicle and resets EEPROM data. This reset may also occur if the IPC is replaced.

When this reset occurs, the following data will be erased:

- TP sensor/Idle learn.
- Transaxle adapts (shift, steady state, and garage shifts).
- Transaxle oil life.
- Cruise lash learn.

Resetting TP sensor/Idle learn may trigger DTC 80 and resetting transaxle oil life may trigger the "CHANGE TRANS FLUID" message. The TP sensor/Idle learn can be relearned by following the DTC 80 chart. The transaxle oil life may be reset to its previous value using the Tech 1. The transaxle adapts will be relearned during normal operation.

OPERATION
The PCM supplies either 5 or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are so high in value that a test light will not light 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.
Therefore, a 10 megohm input impedance digital voltmeter is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injector(s), Idle Speed Control, cooling fan relays, etc. by controlling the ground circuit through transistors or a device called a "quad-driver."

PCM SNAPSHOT DIAGNOSIS
The PCM snapshot feature is designed to assist in diagnosis of intermittent or unrepeatable problems. When a PCM snapshot is taken, all or some of the PCM's data and input parameters from that moment in time are stored and can be retrieved from the PCM with the Tech 1 diagnostic tool.

When a PCM DTC sets, the PCM does several things. Among them is to save useful data and input parameter information for service diagnosis. You will see references to Snapshot on DTCs set or snapshot data and input parameters in many PCM DTC charts. This information can be useful in pinpointing a problem even if the problem is not present when the vehicle is in the service bay.