Points to bear in mind when diagnosing faults
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Points to bear in mind when diagnosing faults
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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.
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Important
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Diagnostic trouble codes should only be used for diagnostic purposes. They do not necessarily indicate a faulty component.
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However, the following points may serve as guidance when diagnosing faults in the ICE control module.
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1.
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Always begin fault diagnosis by using a scan tool to read any diagnostic trouble codes that may have been generated. Use the "ALL SYSTEMS" command to read all diagnostic trouble codes. Make a note of the trouble codes or save them in the scan tool's internal memory by using the "SAVE FAULT CODES" command. Diagnostic trouble codes stored internally can be displayed with the "RECALL FAULT CODES" command.
Trouble codes present:
Continue with "Fault diagnosis with diagnostic trouble codes".
No trouble codes present:
Continue with "Fault diagnosis with fault symptoms".
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2.
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Some fault diagnosis procedures involve unplugging connectors while the ignition switch is in the ON position. This can give rise to diagnostic trouble codes. Therefore, when you have finished fault diagnosis, always clear any diagnostic trouble codes that may have been generated. Clear all diagnostic trouble codes by using the "CLEAR" command.
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3.
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Check fuses 2, 21, 17 and 35 as well as MAXI fuse 3.
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4.
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Check all connectors, especially for corroded contact pins, excessive play, loose or retracted pins and anything else that may have an adverse effect on good electrical contact. If connector trouble is suspected, always spray KONTAKT 61 (part No. 45-30 04 520) contact cleaner into the female connectors.
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5.
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Be sure to check whether the "CHECK ENGINE" lamp (MIL) or any other warning lamp is on.
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6.
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Faults may be persistent or intermittent. A persistent fault continuously meets the fault criteria, i.e. the fault exists only under certain conditions. Examples of such conditions are:
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Faulty connectors (see point 4).
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Electromagnetic interference. Interference may consist of electromagnetic radiation emanating from the car's own equipment or from powerful broadcasting transmitters when the car is driven past radio and TV masts, high-tension transmission lines and places like airports, etc.
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Faulty components. Switching functions in relays, solenoids, etc. may cause interference through arcing as the contacts make and break.
Intermittent faults require careful analysis to ensure that components in perfect working order are not changed unnecessarily.
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7.
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In general:
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When connecting a BOB, make sure that the ignition switch is in the OFF position.
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When unplugging connectors, make sure that the ignition switch is in the OFF position.
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When taking resistance readings, make sure that the ignition switch is in the OFF position.
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8.
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Measuring the resistance of valves/relays. The nominal resistance is specified at 20
°
C (68
°
F). The resistance of the winding is strongly temperature-dependent and increases with rising temperature. Exercise caution when judging the results of such resistance measurements.
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9.
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Fault diagnosis strategy, with or without diagnostic trouble codes, is based on the occurrence of one fault only - the primary fault. However, such a fault may give rise to other faults - consequential faults. If the primary fault is rectified, then the symptoms of the consequential faults will disappear at the same time. The probability of two faults occurring independently is considered to be slight and is not dealt with in this manual.
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10.
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Always end fault diagnosis, with or without diagnostic trouble codes, by checking that the fault symptoms do not persist and that the trouble codes do not recur after remedial action has been taken.
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