Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairEasy
A technician is diagnosing a vehicle with an intermittent no-start condition. Using a scan tool, they observe that the engine RPM shows 0 during cranking, even though the engine is physically turning over. All other sensor readings appear normal. What sensor is most likely at fault?
- ACrankshaft Position (CKP) sensor.
- BEngine Coolant Temperature (ECT) sensor.
- CMass Air Flow (MAF) sensor.
- DThrottle Position Sensor (TPS).
Show answer & explanationAnswer & explanation
Correct answer: A. Crankshaft Position (CKP) sensor.
The Crankshaft Position (CKP) sensor is crucial for the ECU to determine engine speed (RPM) and piston position, which are necessary for proper fuel injection and ignition timing. If it reads 0 RPM during cranking, the ECU won't know when to fire the injectors or coils, leading to a no-start.
Why the other options are wrong
- B. The ECT sensor provides coolant temperature. A faulty ECT can affect fuel enrichment but wouldn't cause a 0 RPM reading during cranking or a complete no-start if other systems are functional.
- C. The MAF sensor measures air intake, but a faulty MAF wouldn't prevent RPM from being displayed during cranking; it would typically cause poor running or stalling once started.
- D. The TPS monitors throttle plate position. A faulty TPS might cause idle issues or acceleration problems, but not a 0 RPM reading during cranking.
Crankshaft Position (CKP) Sensor
The CKP sensor is a critical input to the ECU, providing information about engine speed (RPM) and the exact position of the crankshaft. This data is essential for controlling ignition timing and fuel injection.
- Usually a magnetic reluctance or Hall effect sensor.
- Generates pulses as a toothed wheel on crankshaft rotates.
- Loss of signal prevents engine from starting or running.
- Often works in conjunction with Camshaft Position (CMP) sensor.
Memory trick: Crank, no start? Check the CKP, the engine's heartbeat.