With Powertrain Control Module (PCM)

PCM DESCRIPTION
The Powertrain Control Module (PCM), located under the right front fender, 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 Powertrain Control Module (PCM) 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 either a surface mounted quad driver module, which can independently control up to 4 outputs (PCM terminals), or output driver modules, which 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 the Crankshaft Position System Variation Procedure.

CLASS II SERIAL DATA
U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

PASSWORD LEARN PROCEDURE
In order for a theft deterrent vehicle to run, a password is communicated between the Instrument Panel Cluster (IPC) 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 initial ignition ON. If the IPC is replaced, the PCM needs to learn the new password from the IPC. The password learn procedure is as follows:

1. Attempt to start vehicle, then leave the ignition ON. The THEFT SYSTEM telltale will flash for 10 minutes.
2. When the THEFT SYSTEM telltale stops flashing, start the vehicle. Once the vehicle is running, the password is learned.