Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A client complains of a 'check engine' light and rough running, particularly when the engine is cold. A scan tool reveals a P0301 (Cylinder 1 Misfire) code. After swapping the spark plug and coil-on-plug (COP) from cylinder 1 to cylinder 2 with no change in the misfire location, the technician decides to test the primary circuit of the ignition coil. What is the next logical step to diagnose the primary circuit issue, assuming the power supply to the coil is confirmed good?
- ATest the control circuit signal from the Engine Control Module (ECM) to the coil.
- BCheck the secondary resistance of the ignition coil.
- CInspect the spark plug for signs of fouling or damage.
- DPerform a continuity test on the coil's ground circuit to the engine block.
Show answer & explanationAnswer & explanation
Correct answer: A. Test the control circuit signal from the Engine Control Module (ECM) to the coil.
If the power supply to the coil is good and the coil itself (spark plug and COP swap) isn't the issue, the next logical step for a primary circuit problem is to verify that the ECM is sending the correct control signal to trigger the coil. This signal completes the primary circuit, allowing the coil to fire.
Why the other options are wrong
- B. Secondary resistance is a test of the secondary circuit, not the primary circuit after power has been confirmed.
- C. Spark plug inspection was effectively done by swapping it, and the question focuses on the primary circuit of the coil.
- D. While important, a continuity test to ground might not reveal an intermittent or weak signal from the ECM, which is the primary control aspect.
Ignition Primary Circuit Control
The primary circuit of an ignition coil is controlled by the Engine Control Module (ECM), which switches the circuit on and off to induce high voltage in the secondary circuit.
- ECM provides the ground/trigger signal.
- Signal timing is critical for ignition timing.
- Trouble can lead to misfires or no-start.
Memory trick: The 'brain' (ECM) sends a 'signal' to 'fire' the coil.