Points to Note When Diagnosing Faults
IMPORTANT: Diagnostic trouble codes should be used only for purposes of diagnosing faults. It is not absolutely certain that they indicate a faulty component.
It is impossible to give any general rules for the procedure to follow when diagnosing faults in each individual case. Depending on the fault symptoms and other information available, one method may be the best in some situations and another method best in others.
However, the following points may serve as guidance when diagnosing faults in the Motronic system.
1. Always start fault diagnosis by obtaining readouts of any diagnostic trouble codes on the Scan Tool. Use the "ALL SYSTEMS" command to obtain readouts of all trouble codes. Make a note of the trouble codes or save them in the Scan Tool's internal memory by using the "SAVE TROUBLE CODES" command.
Readouts of trouble codes stored in the internal memory can be obtained by using the "RETRIEVE TROUBLE CODES" command.
2. In some fault diagnosis procedures, connectors are unplugged while the ignition switch is in the ON position. This can give rise to the generation of trouble codes. Accordingly, always clear any trouble codes when you are through with fault diagnosis. Clear all trouble codes by using the "CLEAR ALL" command.
3. Check fuses 14, 23 and 28.
4. Check connectors and switches, particularly for corroded connecting pins, looseness, connecting pin slide-out, or anything else which may have an adverse effect on conductivity. If you suspect poor contact to be the cause of the trouble, always spray KONTAKT 61 contact cleaner (part No. 45-30 04 520) on the female connectors.
NOTE: Contact cleaner and grease must not be used on the oxygen sensor connectors.
5. Take care to check whether the "CHECK ENGINE' (MIL) lamp or any other warning lamp is alight,
6. Faults may be permanent or intermittent.
A permanent fault has fault criteria which are always fulfilled, i.e, the fault exists.
An intermittent fault has fault criteria which are occasionally fulfilled, i.e. the fault exists only under certain conditions. Examples of such conditions are:
^ Defective connectors (see step 4).
^ Electromagnetic interference.
Such interference may be caused by the car's own equipment or by an outside source when the car is driven past places where powerful transmitters are used, such as airports, radio and TV masts, and high-tension power lines.
^ Defective components.
Poor contact in relays, solenoids, etc. may cause interference through sparking.
Intermittent faults require careful analysis to avoid the replacement of perfectly good components.
7. Some generally applicable rules:
^ When connecting a BOB the ignition switch must be in the OFF position.
^ When unplugging connectors the ignition switch must be in the OFF position.
^ When measuring resistance the ignition switch must be in the OFF position.
8. Measuring the resistance of valves and relays. Nominal resistance is specified at 20°C (68°F). The resistance of windings is strongly dependent on temperature and increases with rising temperature.
Be cautious when judging the results of such resistance measurements.