Audi Cars 1999-05

Engine Control Module (ECM)


Description & Operation

The heart of an OBD compatible system is the Engine Control Module (ECM) that is used to monitor and store faults associated with all components/systems that can influence exhaust and evaporative emissions.

The CIS-E Motronic system used in the 1990-93 Passat models does allow adjustment of the idle mixture, however with the introduction of the 1993 Passat VR6 GLX and subsequent models, the fuel mixture, engine idle speed, and ignition timing are not adjustable and are controlled by the ECM. The fuel mixture is controlled by the ECM by changing the opening time (pulse width) of the fuel injectors, the idle speed is controlled by ECM via the Idle Air (IAC) Control valve, and the ignition timing is adaptable, and controlled by the ECM based on feed back from input sensors.

The terms Engine Control Module (ECM), Powertrain Control Module (PCM) and Digital Motor Electronics (DME) control unit are used interchangeably and refer to the vehicle's computer control module, regardless of the terminology used.

The Engine Control Module (ECM), also referred to as the Powertrain Control Module (PCM) or Digital Motor Electronics (DME) control unit, uses a variety of input sensors which send electrical signals which are interpreted by the ECM to control the fuel delivery, ignition timing and idle speed of the engine.

These input signals include the following:

Mass Airflow (MAF) sensor (OBD II)
Airflow meter (OBD I)
Throttle Position (TP) sensor
Air temperature sensor
Engine Coolant Temperature (ECT) sensor
Crankshaft speed sensor
Camshaft Position (CMP) sensor
If equipped, knock sensors
Pre Catalyst Oxygen sensor (OBD I)
Pre Catalyst and Post Catalyst Heated Oxygen sensor (OBD II)

In case of an ECM or sensor failure, the system will default to a pre-programmed set of values. These are compromise values that allow the engine to operate, although at a reduced efficiency. This is variously known as the default, limp or back-up mode. Driveability is usually affected when the ECM enters this mode and should trigger the Malfunction Indicator Lamp (MIL) indicator.

If the MIL indicator is flashing, the integrity of the catalytic converter may be compromised and the engine should not be operated at full throttle unit the cause can be diagnosed and resolved.

Removal & Installation

The ignition key must be in the OFF position and the negative battery cable must be disconnected before removal and/or disconnection of the Engine Control (ECM) Module and its electrical connector.

  1. Turn the ignition switch to the OFF position.
  3. Disconnect the negative battery cable.
  5. Detach all wiring harnesses from the Electronic Control Module (ECU).

Use caution when detaching the wiring harnesses from the ECU, old plastic harnesses are very fragile and may break upon removal if too much force is exerted.

  1. Remove the mounting fasteners from the ECU.
  3. Ground yourself at all times when handling the ECU. Ground straps are sold at most electronic stores and make a wise investment if you plan to do work on sensitive components such as the engine computer.
  5. Remove the ECU.

To install:

  1. Installation is the reverse of removal.