NOTE: When the term Electronic Control Unit (ECU) is used in this manual, it will refer to the engine control computer regardless that it may be a Electronic Control Unit (ECU), Powertrain Control Module (PCM) or Engine Control Module (ECM).
The heart of the electronic control system, which is found on the vehicles covered by this manual, 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.
WARNING
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.