Fault Diagnosis Hints and Tips: M91 and Early M92

Fault diagnosis hints and tips: M91 and early M92

On these cars, some of the components manufactured before a certain date have not been of the specified standard of quality. These defective components, sometimes in conjunction with electrical contact or adjustment faults, can cause the system to go into Limp Home mode. This will result in complaints from customers or necessitate taking the car to a garage for repair and adjustment.

Two types of Limp Home may occur:
^ The system leaves Limp Home mode on being restarted
^ The system has to be remedied/reset with the ISAT

Since fault diagnosis and locating the faults may sometimes prove difficult, we have compiled the following list of causes and recommended remedies.

Malfunction causes
Faults may of course occur for other reasons besides those given below. However, we would point to certain special factors which have proved to be the root cause of most of the driveablity problems that have been encountered. This applies to the following:
^ 24-pin connector in the main wiring harness (front bulkhead partition). Contact problems due to corrosion caused by water leakage.
^ Earthing points G7 and G15. Excessively high contact resistance caused by insufficient contact.
^ Throttle wire. Position of wire incorrectly adjusted.
^ Control valve. Component defects (leakage) may occur up to and including manufacturing date 0391.
^ Non-return valve (leakage)
^ Pedal potentiometer. Component defects may occur up to and including manufacturing date 101 (w10 year 91).
^ Throttle housing (throttle potentiometer). Component defects may occur up to and including serial number 117885.

Remedies
If a fault occurs in the Traction Control System (in Limp Home mode), start fault diagnosis as follows:

1 Read and make a note of the fault codes that may be recorded in the ISAT.

2 Check and, if necessary, adjust the throttle wire so that maximum clearance is obtained. The clearance should be at east 1 mm. Calibration of TCS

3 Unplug the 24-pin connector in the main wiring harness and check whether there is any moisture or water inside it. If there is, fit a new rubber seal, part No.43 52 936.

Wipe the connector dry and apply type 5-55 rust-removing oil or the equivalent to the contact pins. Then plug the two connector halves together and separate them a few times to remove any coating of oxide that may have formed.

4 Check earthing points G7 (engine, 2) and G15 (at the TSI socket).

5 Check that the non-return valve is correctly positioned.

After carrying out this initial check and any necessary remedial action, clear all fault codes and drive the car to see whether it goes into Limp Home mode again or whether any of the warning lamps light up.

If the fault or faults persist, carry out fault diagnosis according to the test procedure described under the relevant fault code.