Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing a no-start condition on a vehicle equipped with a Coil-On-Plug (COP) ignition system. They have confirmed that the engine cranks, but there is no spark from any of the ignition coils. Using a multimeter, they check for power and ground at the COP connectors and find both are present. What is the NEXT logical step in the diagnostic process?
- APerform a compression test on all cylinders.
- BInspect the fuel pump relay.
- CReplace all ignition coils and spark plugs.
- DCheck for a signal from the Crankshaft Position (CKP) sensor.
Show answer & explanationAnswer & explanation
Correct answer: D. Check for a signal from the Crankshaft Position (CKP) sensor.
Since power and ground are present at the coils, the next step is to determine if the coils are receiving a trigger signal from the ECM. The Crankshaft Position (CKP) sensor is the primary input the ECM uses to determine engine speed and piston position to trigger the ignition coils. Without a CKP signal, the ECM will not command spark.
Why the other options are wrong
- A. A compression test is for mechanical engine health, not typically the first step for a no-spark condition across all cylinders with power/ground at coils.
- B. The fuel pump relay affects fuel delivery, not spark. While a no-start requires both, the current diagnosis focuses on the lack of spark.
- C. Replacing parts without proper diagnosis is inefficient and costly, especially when a common input signal is missing for all coils.
No Spark Diagnostics (COP)
When diagnosing a no-spark condition on a COP system, after confirming power and ground to the coils, verify the presence of a trigger signal from the ECM, primarily driven by the Crankshaft Position (CKP) sensor.
- Check power and ground at coil connectors first.
- CKP sensor is vital for ECM to initiate spark.
- No CKP signal = no spark from any coil.
- Also check CMP sensor, but CKP is primary for spark timing.
Memory trick: Power, Ground, then Signal, Check!