Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A vehicle's coolant temperature gauge consistently reads maximum (H) even when the engine is cold or has just started. The engine cooling fan operates normally, and there are no overheating symptoms. A technician tests the coolant temperature sensor's resistance and finds it to be within specifications for the current ambient temperature. Which of the following is the MOST likely cause?
- AA faulty engine control module (ECM) input for the sensor
- BA short to ground in the coolant temperature sensor signal wire
- CAn open circuit in the coolant temperature sensor signal wire
- DA malfunctioning thermostat stuck in the closed position
Show answer & explanationAnswer & explanation
Correct answer: B. A short to ground in the coolant temperature sensor signal wire
Coolant temperature gauges typically read cold (low) with high resistance and hot (high) with low resistance (or a short to ground). If the gauge reads maximum (H) when the engine is cold and the sensor itself tests good, it indicates that the gauge is receiving a signal equivalent to a very low resistance or a direct short to ground, which would be caused by a short in the signal wire.
Why the other options are wrong
- A. While possible, the ECM processing the sensor signal, the sensor itself is testing good, and the fan is working, suggesting the ECM is receiving *some* valid data, making a wiring short more direct.
- C. An open circuit in the signal wire would typically cause the gauge to read minimum (C) or fail to move, as it would represent infinite resistance.
- D. A stuck thermostat would cause actual overheating, which is not present, and wouldn't directly cause the gauge to read max when cold if the sensor is good.
Coolant Temperature Gauge Operation
The gauge displays engine coolant temperature based on the resistance of a thermistor-type sensor. Typically, resistance decreases as temperature increases, causing the gauge to move from cold to hot.
- High resistance usually means cold (C) or open circuit.
- Low resistance or short to ground usually means hot (H).
- Sensor output is critical for both the gauge and ECM.
Memory trick: Cold engine, hot gauge? The wire's shorted, not the stage!