GM Century/Lumina/Grand Prix/Intrigue 1997-2000

Powertrain Control Module



The vehicle's Powertrain Control Module (PCM), is a powerful on-board computer that controls most powertrain related functions, hence its name. Earlier versions were often called the Engine Control Module (ECM) but as the computers have become more advanced and, as more and more demands have been placed on them to control more driveline functions, the computers have come to be called Powertrain Control Modules, more accurately reflecting its increased role in vehicle operations.

The PCM in your W-Body vehicle is located underhood, in front of the right strut tower near the engine coolant reservoir. It is the control center for the fuel injection system and constantly looks at the information from various sensors and it controls the systems that affect vehicle performance. It controls the fuel metering system, automatic transaxle shifting, the engine's ignition timing, and also contains a powerful on-board diagnostic system for powertrain functions.

The PCM constantly looks at the information from various sensors and controls the systems that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the MIL (Malfunction Indicator Lamp, also known as the Service Engine Soon light), and store Diagnostic Trouble Codes (DTCs) which identify the problem areas to aid the technician in making repairs. A qualified technician with a piece of equipment called a scan tool, can connect into the system and read the data stream, getting real-time information on the engine's operation. In addition, the technician can interrogate the system and retrieve any DTCs to aid in diagnosing many driveability problems.

For service (replacement parts), the PCM consists of two parts:

The controller (the PCM body without the Knock Sensor module)
The Knock Sensor (KS) module

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 digital voltmeter with at least 10 megaohms input resistance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the fuel injectors, Idle Air Control (IAC), cooling fan relays, etc., by controlling either the ground or the power feed circuit through transistors or a device called a Driver.

The Electrically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered within the PCM. The EEPROM contains program and calibration information that the PCM needs to control powertrain operation.

Unlike the Programmable Read Only Memory (PROM) used in certain past applications, the EEPROM is not replaceable. If the PCM is replaced, the new PCM will need to be programmed. This can only be done at a GM dealership. A scan tool such as GM's Tech 1®, or Tech 2®, containing the correct program and calibration for the vehicle is required to program the replacement PCM.


The service PCM EEPROM will not be programmed. This procedure must be done at an authorized dealership with the correct equipment. In most all cases, this is not a job for the non-professional.

To prevent possible Electrostatic Discharge damage to the PCM, DO NOT touch the connector pins or the soldered components on the circuit board. To prevent internal PCM damage, make sure the ignition switch is in the OFF position when installing or removing the PCM connectors and disconnecting or re