Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is performing a diagnostic on a modern vehicle with a no-start condition. There are no diagnostic trouble codes (DTCs) stored. They verify battery voltage is good. They then check for spark at the plugs and find none. Next, they use a scan tool to observe engine RPM during cranking, and it reads '0 RPM'. What is the most probable cause of the no-start condition?
- AA faulty ignition coil.
- BA defective Powertrain Control Module (PCM).
- CA malfunctioning Crankshaft Position Sensor (CKP).
- DAn open circuit in the fuel pump relay.
Show answer & explanationAnswer & explanation
Correct answer: C. A malfunctioning Crankshaft Position Sensor (CKP).
If the scan tool reads 0 RPM during cranking, it indicates that the PCM is not receiving a signal from the crankshaft position sensor (CKP). The CKP sensor is critical for the PCM to determine engine speed and position, which are essential inputs for both ignition timing and fuel injection. Without this signal, the PCM will not command spark or fuel, resulting in a no-start condition.
Why the other options are wrong
- A. A faulty ignition coil would cause a no-spark condition, but the PCM would still register RPM during cranking if the CKP sensor is working.
- B. A defective PCM could cause a no-start, but the 0 RPM reading points more specifically to an input signal issue first.
- D. An open fuel pump relay would cause a no-start due to lack of fuel, but the PCM would still register RPM during cranking if the CKP sensor is working.
Crankshaft Position Sensor (CKP) Importance
The CKP sensor provides the critical engine speed and position signal to the PCM, which is essential for determining ignition timing and fuel injection; a missing signal results in no spark or fuel.
- Primary input for engine speed.
- Required for spark and fuel timing.
- Faulty CKP often leads to no-start.
Memory trick: No RPM, no spark, no fuel – CKP's the king, it's the cruel rule.