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

Diagnosis and Testing


Service Precautions

To avoid damage to the ECM 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 or quality digital voltmeter (DVM). Use of a 12 volt test light is not recommended.
To prevent electrostatic discharge damage, when working with the ECM, 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.
When performing electrical tests on the system, use a high impedance multimeter, digital voltmeter (DVM) J-34029-A or equivalent.
Never pierce a high tension lead or boot for any testing purpose; otherwise, future problems are 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 Figure 1

The ECM uses information from the MAP and coolant sensors, in addition to rpm to calculate spark advance as follows:

  1. Low MAP output voltage-more spark advance
  3. Cold engine-more spark advance
  5. High MAP output voltage-less spark advance
  7. 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.

Click image to see an enlarged view

Fig. Fig. 1: Ignition system schematic


The best way to perform this procedure is to use a spark tester (available at most automotive parts stores). Two types of spark testers are commonly available. The Neon Bulb type is connected to the spark plug wire and flashes with each ignition pulse. The Air Gap type must be adjusted to the individual spark plug gap specified for the engine. This type of tester allows the user to not only detect the presence of spark, but also the intensity (orange/yellow is weak, blue is strong).

  1. Disconnect a spark plug wire at the spark plug end.
  3. Connect the plug wire to the spark tester and ground the tester to an appropriate location on the engine.
  5. Crank the engine and check for spark at the tester.
  7. If spark exists at the tester, the ignition system is functioning properly.
  9. If spark does not exist at the wire, test the ignition coil, and other ignition system related components or wiring. Repair or replace components as necessary.