GM Corsica/Beretta 1988-1996 Repair Guide

Electronic Control Module (ECM)



When the term Electronic Control Module (ECM) is used in this guide it will refer to the engine control computer regardless that it may be a Powertrain Control Module (PCM) or Electronic Control Module (ECM).

The heart of the electronic control system, which is found on the vehicles covered by this information, is a computer control module. The module gathers information from various sensors, then controls fuel supply and engine emission systems. Most early model vehicles are equipped with an Engine Control Module (ECM) which, as its name implies, controls the engine and related emissions systems. Some ECMs may also control the Torque Converter Clutch (TCC) on automatic transaxle vehicles or the manual upshift light on manual transmission vehicles. Later model vehicles may be equipped with a Powertrain Control Module (PCM). This is similar to the original ECMs, but is designed to control additional systems as well. The PCM may control the manual transmission shift lamp or the shift functions of the electronically controlled automatic transmission.

Regardless of the name, all computer control modules are serviced in a similar manner. Care must be taken when handling these expensive components in order to protect them from damage. Carefully follow all instructions included with the replacement part. Avoid touching pins or connectors to prevent damage from static electricity.

All of these computer control modules contain a Programmable Read Only Memory (PROM) chip, MEM-CAL or EEPROM that contains calibration information which is particular to the vehicle application. For all applications except those equipped with an EEPROM, this chip is not supplied with a replacement module, and must be transferred to the new module before installation. If equipped with an Electronically Erasable Programmable Read Only Memory (EEPROM), it must be reprogrammed after installation. Some later model vehicles have a Knock Sensor (KS) module, mounted in the PCM. The KS module contains the circuitry that allows the PCM to utilize the Knock Sensor signal to diagnose the circuitry.

To prevent the possibility of permanent control module damage, the ignition switch MUST always be OFF when disconnecting power from or reconnecting power to the module. This includes unplugging the module connector, disconnecting the negative battery cable, removing the module fuse or even attempting to jump your dead battery using jumper cables.

In the event of an ECM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although at a reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the ECM enters this mode.


See Figures 1, 2 and 3

For most applications, the computer control module is mounted inside the passenger compartment, attached to a bracket located in the right side upper instrument panel.

  1. Make sure the ignition switch is turned OFF , then disconnect the negative battery cable.

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