Volkswagen Golf/Jetta/Cabriole 1990-1999

Electronic Control Unit (ECU)



Motronic System

This unit is located above the firewall in the engine compartment. The ECU is supplied with an internal voltage regulator and operates at 8 volts to prevent control fluctuations when vehicle accessories are switched on. The ECU accepts the various input signals and calculates optimum fuel injection and ignition output control signals. The main output devices are the differential pressure regulator, ignition coil and the idle air stabilizer valve. The maximum engine speed is limited by the ECU through the differential pressure regulator. Above the tachometer ``red line``, the lower chamber pressure is increased relative to the system pressure and the fuel supply to the injectors is interrupted.

The ECU is equipped with an adaptive learning program that learns the normal operating point of the air/fuel mixture. This information comes from the oxygen sensor and is only read when the charcoal canister frequency valve is in the OFF cycle. As the engine wears or is driven differently or with changes in altitude, control signals that the ECU considers ``normal" are modified to account for the deviations.


The ECU controls all fuel injection, idle speed and ignition functions and also acts as a rev limiter. Above the maximum rpm rating, the ECU will intermittently shut off the fuel injectors to prevent over-revving the engine. If an input or output device fault is detected and it is serious enough to cause engine control problems, the ECU will switch into a "limp home" mode and the engine will be operated at a pre-set condition. The ECU is mounted in the tray above the fire wall.

Diesel and Turbo Diesel Injected (TDI) models

This fuel injection system is very similar to the Motronic system. It uses various sensors to monitor and adjust combustion as necessary. Sensors send inputs to the computer, based on preset programs adjustments are made, and the performance and emissions are controlled to obtain optimum levels.

removal & installation

  1. Disconnect the negative battery cable.
  3. 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.