Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A technician is diagnosing a complaint that the vehicle's tachometer reads erratically, jumping between correct readings and zero, while the engine runs smoothly and has no performance issues. The engine control module (ECM) reports a consistent and correct engine RPM signal via a scan tool. Which of the following is the MOST likely cause?
- AA faulty crankshaft position sensor
- BAn intermittent open circuit in the wiring from the crankshaft position sensor to the ECM
- CA faulty instrument cluster tachometer circuit
- DA faulty ignition coil
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty instrument cluster tachometer circuit
Since the ECM reports a consistent and correct RPM signal, it indicates that the crankshaft position sensor and its wiring to the ECM are functioning properly. The issue must therefore lie downstream of the ECM, specifically within the instrument cluster itself, where the tachometer receives and displays the RPM signal.
Why the other options are wrong
- A. A faulty crankshaft position sensor would cause engine performance issues and the ECM would report an incorrect or absent RPM signal.
- B. An intermittent open circuit in the CPS wiring to the ECM would cause the ECM to report an erratic or incorrect RPM signal, which contradicts the given information.
- D. A faulty ignition coil would cause engine misfires and performance issues, but the ECM would still likely report an RPM signal, and the core problem is the tachometer's display, not engine operation.
Tachometer Signal Path
The tachometer displays engine RPM, typically receiving its signal from the Engine Control Module (ECM), which in turn processes data from the crankshaft position sensor.
- Crankshaft Position Sensor (CPS) is the primary source of RPM data.
- ECM processes CPS data for engine control and sends RPM signal to cluster.
- Tachometer is the final display component in the instrument cluster.
Memory trick: Engine's fine, ECM's green? Tachometer's circuit, not the machine!