Powertrain Control Module (PCM)

Description
The Powertrain Control Module (PCM), located under the instrument panel, is the control center of the fuel injection system. It constantly looks at the information from various sensors, and controls the systems that affect emission or engine performance.

The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL), and store a DTC or DTCs which identify the problem areas to aid the technician in making repairs. See Introduction in Powertrain Control Module for more information on using the diagnostic functions of the PCM.

EEPROM
This assembly contains the functions of the Electrically Erasable Programmable Read Only Memory (EEPROM) and is a permanent part of the PCM. The EEPROM contains the calibrations needed for a specific vehicle applications and is serviced only through a re-programming procedure.

PCM Function
The Powertrain Control Module (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 assure accurate voltage readings.

The Control Module supplies a buffered voltage to various sensors and switches. The input and output devices in the control module include an analog to digital converters, signal buffers, counters, and special drivers. The Control Module controls most components with electronic switches which complete a ground circuit when turned ON. These switches are arranged in groups of 4 and 7 called one of the following:
^ Quad Driver Module
^ Output Driver Modules

The surface mounted Quad Driver Module can independently control up to 4 outputs (Control Module) terminals. The Output Driver Modules can independently control up to 7 outputs. Not all outputs are always used.

PCM Re-Programming
The replacement PCM must be reprogrammed and the crankshaft position system variation procedure must be performed. Refer to the latest Techline information on PCM re-programming and this section for the Crankshaft Position System Variation Procedure. Testing and Inspection

Password Learn Procedure
In order for a theft deterrent vehicle to run, a password is communicated between the Body Function Control (BFC) and the PCM. If a PCM is replaced, the new PCM needs to learn the correct password of the vehicle. When the new PCM is installed, the EEPROM calibration is flashed into the new PCM and the vehicle will learn the new password upon the initial ignition ON cycle. if the Body Function Control (BFC) is replaced, the PCM needs to learn the new password from the Body Function Control (BFC). The password learn procedure is as follows:

1. Attempt to the start vehicle. The vehicle will stall.
2. Leave the ignition ON until the Theft System telltale turns OFF (approx.10 minutes).
3. Turn the ignition switch OFF.
4. Attempt to the start vehicle. The vehicle will start on the this attempt. The password is now learned.

Attempting to start the vehicle by bypassing the Passlock II system or substituting parts without performing the password learn procedure may set a DTC P1631 and P1632. If no password is received a DTC P1610, P1626 and P1632 may be set. Refer to the Applicable DTC Table if set. Testing and Inspection