GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

Diagnosis and Testing


Service Precautions

To avoid damage to the computer control module (ECM/PCM) or other ignition system components, do not use electrical test equipment such as battery or AC powered voltmeter, ohmmeter, etc. or any type of tester other than specified.

When performing electrical tests on the system, use a high impedance multimeter, digital voltmeter (DVM) J-34029-A or equivalent. Use of a 12 volt test light is not recommended.
To prevent electrostatic discharge damage, when working with the ECM or PCM, do not touch the connector pins or soldered components on the circuit board.
When handling a PROM, CAL-PAK or MEM-CAL, do not touch the component leads. Also, do not remove the integrated circuit from the carrier.
Never pierce a high tension lead or boot for any testing purpose; otherwise, future problems are almost guaranteed.
Leave new components and modules in the shipping package until ready to install them.
Never disconnect any electrical connection with the ignition switch ON unless instructed to do so in a test.

Symptom DiagnosisSee Figures 1, 2, 3 and 4

The computer control module (ECM/PCM) uses information from the MAP and coolant sensors, in addition to rpm to calculate spark advance as follows:

Low MAP output voltage-more spark advance.
Cold engine-more spark advance.
High MAP output voltage-less spark advance.
Hot engine-less spark advance.

Therefore, detonation could be caused by low MAP output or high resistance in the coolant sensor circuit, and poor performance could be caused by high MAP output or low resistance in the coolant sensor circuit.

The best way to diagnose what may be an ignition-related problem, first check for codes. If codes, exist, refer to the corresponding diagnostic charts in Driveability & Emissions Controls . Otherwise, the accompanying charts may be helpful.

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Fig. Fig. 1: Ignition system schematic-2.3L engine

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