The term Electronic Control Module (ECM) is used in this guide to refer to the engine control computer, whether it is a Powertrain Control Module (PCM) or an Engine Control Module (ECM).
The heart of the electronic engine management system, which is found on the vehicles covered by this guide, is the computer control module. On vehicles equipped with a manual transaxle, there is no need for a control unit to control shift points as the transaxle is shifted manually. On these models, the engine management system uses an ECM. Some models used a separate Transmission Control Module (TCM) that is linked with the ECM to control the automatic transaxle shift points.
Many models equipped with an automatic transaxle, the Transmission Control Module (TCM) and Engine Control Module (ECM) have been combined into one unit, called the Powertrain Control Module (PCM). The ECM/PCM processes input information from various sensors, compares the input with pre-programmed information and sends output signals to control the fuel supply, ignition timing, and the engine emission system. The PCM also controls the shift functions of the on vehicles equipped with an automatic transaxle.
Regardless of who the manufacturer may be, all computer control modules are serviced in a similar manner. Care must be taken when handling these expensive electronic 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 or contaminating the electrical connection. Some input voltages and resistance values are quite small and the readings very sensitive. A poor electrical connection could drastically affect the values recognized by the PCM and cause a poor running condition.
Since the PCM is a sensitive electronic component and must be kept away from areas of heat, debris and fluids, it is located in the interior.
All of the computer control modules contain a Programmable Read Only Memory (PROM) chip that contains calibration information specific to the vehicle application.
In case 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 or back-up mode. Drivability is usually affected when the ECM enters this mode and should trigger the Malfunction Indicator Lamp (MIL) indicator.