Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing an intermittent starting issue where the engine cranks normally but occasionally fails to start. The fuel pump operates, and spark is present. Using a scan tool, the technician observes that the engine RPM signal is intermittently dropping to zero during cranking, even though the engine is clearly rotating. Which of the following components is the most likely cause?
- ACamshaft Position Sensor (CMP)
- BThrottle Position Sensor (TPS)
- CEngine Control Module (ECM)
- DCrankshaft Position Sensor (CKP)
Show answer & explanationAnswer & explanation
Correct answer: D. Crankshaft Position Sensor (CKP)
The crankshaft position sensor (CKP) provides the primary engine RPM signal to the ECM. If this signal intermittently drops to zero while the engine is cranking, the ECM will lose track of engine speed and position, preventing proper fuel injection and ignition timing, leading to a no-start condition.
Why the other options are wrong
- A. The CMP sensor primarily helps with cylinder identification for sequential injection, but the CKP is the main RPM source. While a CMP issue can cause starting problems, an intermittent *RPM signal drop to zero* points more directly to CKP.
- B. The TPS primarily measures throttle opening and is not directly involved in generating the initial engine RPM signal for starting.
- C. While possible, the ECM usually processes the signal; an intermittent signal loss points to the sensor itself.
Crankshaft Position Sensor (CKP)
An engine sensor that monitors the rotational speed and position of the crankshaft, providing critical input to the engine control module (ECM) for fuel injection and ignition timing.
- Primary source of engine RPM signal.
- Essential for engine starting and running.
- Faulty sensor can cause no-start, stalling, or misfires.
Memory trick: Crankshaft Position is KEY to starting.