Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairHard
A technician is diagnosing a vehicle that exhibits a no-start condition, but cranks normally. There are no DTCs stored in the Powertrain Control Module (PCM). Using a scan tool, the technician observes that the engine RPM signal is present during cranking, but injector pulse width is 0 ms and ignition timing shows 0 degrees. Which of the following components should the technician investigate FIRST?
- AThe crankshaft position (CKP) sensor.
- BThe fuel pump relay.
- CThe ignition coils.
- DThe camshaft position (CMP) sensor.
Show answer & explanationAnswer & explanation
Correct answer: D. The camshaft position (CMP) sensor.
If the CKP sensor is working (RPM signal present), but there's no injector pulse or ignition timing, it often indicates the PCM isn't seeing a valid CMP signal. The CMP sensor provides cylinder identification, which is crucial for sequential fuel injection and ignition timing. Without it, the PCM won't know when to fire injectors or spark plugs.
Why the other options are wrong
- A. If the CKP sensor were faulty, there would be no RPM signal during cranking, contradicting the observation.
- B. A faulty fuel pump relay would prevent fuel delivery, but the primary indicator here is the lack of injector pulse and ignition timing, which the PCM controls based on sensor inputs.
- C. While faulty ignition coils would prevent spark, the primary issue here is the lack of ignition timing *signal* from the PCM, which points to a missing input rather than faulty outputs.
CKP & CMP Synchronization
The Powertrain Control Module (PCM) requires both Crankshaft Position (CKP) and Camshaft Position (CMP) sensor signals to synchronize engine operation, determine cylinder firing order, and enable sequential fuel injection and ignition.
- CKP provides engine speed and crankshaft position.
- CMP provides cylinder identification and camshaft position.
- Both are critical for 'no-start, crank-no-start' diagnostics.
Memory trick: Crank, but no spark or fuel? Check CMP for cylinder cues.