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Fault diagnosis schedule

Fault diagnosis schedule

To bear in mind when diagnosing faults

It is not possible to give any general rules for carrying out fault diagnosis in each individual case. Depending on the symptom of the fault and other information available, one method may be best under one set of circumstances but less suited under another set of circumstances.

On the other hand, the following points may act as a guide when conducting fault diagnosis on the theft alarm system.


1. Always start fault diagnosis by using the ISAT scan tool to obtain readouts of the diagnostic trouble codes, if any. Use the command "ALL SYSTEMS" to obtain readouts of all diagnostic trouble codes. Make a note of the diagnostic trouble codes or save them in the ISAT scan tool\s internal memory with the command "SAVE FAULT CODES".

Internally stored diagnostic trouble codes can be read using the command "RECALL FAULT CODES".

2. Certain fault diagnosis procedures involve unplugging connectors while the ignition is in the ON position. This can result in diagnostic trouble codes. Always therefore erase any diagnostic trouble codes when work is finished. Erase all diagnostic trouble codes by using the "CLEAR" command.

3. Check fuses 17, 23 and 36.

4. Check connectors, especially for oxidized contact pins, gaps, loose contacts or anything else that may negatively affect the connection. If connector problems are suspected, always use contact spray KONTAKT 61 (part no. 45-30 04 520) on the female contacts in the connectors.

5. Note whether the malfunction indicator (CHECK ENGINE) lamp lights or if any other warning lamp is lit.

6. Faults may be permanent or intermittent.

A permanent fault continuously meets fault criteria, that is to say that the fault exists.

An intermittent fault temporarily meets fault criteria, that is to say that the fault only exists under certain circumstances. Examples of such circumstances are:

• Faulty connectors (see point 4)

• Electromagnetic interference.

Interference may be both radiation emitted from the car's own equipment and radiated interference when the car passes particular places where there are strong transmitters, for example at airports, radio and TV masts or high voltage cables.

• Faulty components

The switch function in relays, solenoids etc. can cause interference due to sparking.

Intermittent faults require careful analysis so that a fault-free components are not changed.

7. In general:

• The BOB is connected with the ignition in the OFF position.

• Connectors are unplugged with the ignition in the OFF position.

• Resistance measurement is done with the ignition in the OFF position.

8. Measuring the resistance of valves/relays.

The nominal resistance is specified at 20 °C (68°F). Winding resistance is highly temperature-dependant, and increases with increasing temperature.

Be careful when assessing the result of such Ohm measurements.