GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

Diagnosis And Testing



The computer control module 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 Engine Coolant Temperature (ECT) sensor circuit. Poor performance could be caused by high MAP output or low resistance in the coolant sensor circuit.

If the engine cranks but will not operate, or starts, then immediately stalls, diagnosis must be accomplished to determine if the failure is in the ignition system or the fuel system. Refer to the individual component tests.


The ECM will set timing at a specified value when the diagnostic TEST terminal in the ALDL connector is grounded. To check for EST operation, run the engine at 1500 rpm with the terminal ungrounded. Then ground the TEST terminal. If the EST is operating, there should be a noticeable engine rpm change. A fault in the EST system will set a trouble code 42.