Functional Testing

Spark advance problems can result from an incorrect initial timing setting or a problem with spark advance during operation. Before attempting to check spark advance during operating conditions, the initial or "base" ignition timing setting should checked and adjusted.

This procedure varies between systems, but on TCCS equipped vehicles, it generally requires jumping terminals at an underhood check connector (DLC1) to default the TCCS system to initial timing. After checking or adjusting initial timing, remove the test wire to inform the ECM to reestablish corrective control over timing. Refer to the Repair Manual for details on performing this procedure.

Even with initial timing correct, it is still possible that the system is miscalculating ignition timing as a result of incorrect sensor inputs. For example, if an airflow meter indicates light engine load, when in fact, the engine is experiencing high engine load, the ECM may incorrectly respond by over advancing ignition timing to the point of causing detonation. Refer to course 850 and 873 handbook for additional information on spark advance control strategy.

If inaccurate sensor inputs are suspected on earlier EFI and TCCS vehicles, it is recommended that you perform standard voltage checks of all major sensor inputs to the ECM. Compare these readings to those listed on the standard voltage chart on the Repair Manual or readings obtained from other known good vehicles. On OBD-II vehicles, you may observe ignition timing and identify incorrect signal data using the Diagnostic Tester. Some of the more important spark control parameters include engine speed, engine load, throttle angle, and coolant temperature.

On early EFI vehicles, all spark advance is handled by mechanical means. This system uses a centrifugal advance mechanism to represent engine speed and vacuum advance mechanism to represent engine load. Resolving advance problems with this type system requires inspecting governor weights, springs, pivots, signal rotor, vacuum diaphragm, vacuum signal source, breaker plate, etc.

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