S10 Pickup, Sonoma 2000-2004

Diagnosis & Testing


Before beginning any diagnosis and testing procedures, visually inspect the components of the ignition system and engine control systems. Check for the following:

Discharged battery
Damaged or loose connections
Damaged electrical insulation
Poor coil and spark plug connections
Ignition module connections
Blown fuses
Damaged vacuum hoses
Damaged spark plugs

Check the spark plug wires and boots for signs of poor insulation that could cause crossfiring. Make sure the battery is fully charged and that all accessories are off during diagnosis and testing. Make sure the idle speed is within specifications.

If an open or ground in the Ignition Control (IC) circuit occurs during engine operation, the engine will continue to run, but using a back-up timing mode (controlled by the ICM) based on preset timing values. The Malfunction Indicator Lamp (MIL) or SERVICE ENGINE SOON light will not illuminate at the first appearance of a break in the circuit. However, if the IC fault is still present once the engine is restarted, a Code will set on the OBD II system in the PCM and the MIL will illuminate. Poor performance and fuel economy may be noticed while the engine is running under back-up timing.

When attempting to search for ignition troubles, keep in mind the various sensor inputs which the PCM uses to calculate timing which affect engine performance. The PCM will alter timing based on sensor inputs 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

With this in mind, DETONATION could be caused by low MAP output or high resistance in the coolant sensor circuit. POOR PERFORMANCE could be caused by a high MAP output or low resistance in the coolant sensor circuit.