Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is diagnosing an engine with a misfire on cylinder #3. They use a test light connected to battery voltage to probe the coil primary trigger wire while cranking the engine. The test light flashes brightly. What does this indicate about the ignition coil primary circuit for cylinder #3?
- AThe ignition coil driver circuit in the PCM is functioning correctly.
- BThe coil is receiving constant battery voltage and ground.
- CThe primary winding of the coil is shorted to ground.
- DThere is an open circuit in the primary winding of the coil.
Show answer & explanationAnswer & explanation
Correct answer: A. The ignition coil driver circuit in the PCM is functioning correctly.
Connecting a test light to battery voltage and probing the primary trigger wire (the ground side) of the coil, a flashing test light indicates that the PCM (or ICM) is successfully switching the ground on and off. This confirms that the coil driver circuit is functioning correctly and attempting to trigger the coil.
Why the other options are wrong
- B. While the coil needs constant battery voltage and a switched ground, the flashing test light specifically confirms the 'switched ground' action of the driver, not just constant power/ground.
- C. If the primary winding was shorted to ground, the test light would likely stay on or not flash correctly, as the ground would be permanent.
- D. An open primary winding would prevent current flow, and the test light would not flash or behave erratically, depending on where the open is.
Ignition Coil Trigger Test
Testing the primary trigger wire (ground side) with a test light connected to battery voltage helps verify if the PCM/ICM is correctly switching the ground on and off to command the coil to fire.
- Test light to battery voltage, probe trigger wire.
- Flashing indicates good driver signal.
- Confirms PCM/ICM output for ignition.
Memory trick: Flash, flash, goes the light, PCM's command, shining bright!