Coolant Temperature Sensor/Switch (For Computer): Testing and Inspection

Figure 3-11: Coolant Temperature Sensor Circuit:

Engine Coolant Temperature Sensor Test

The coolant temperature sensor is a thermistor that provides a temperature signal to the PCM (Figure 3-11). The signal is in the form of a voltage that changes according to sensor resistance. At low temperatures, resistance and voltage are both high. As temperature increases, resistance and voltage both decrease.

A sensor fault will cause the check engine light to illuminate and the torque converter clutch to apply early. If the sensor fails in a cold mode, it can produce overrich operation and rough idle.

High Temperature-Low Resistance Test

1. Allow engine to cool down. Coolant should be at ambient temperature for test.
2. Connect scan tool, start engine, and note sensor temperature.
- If indicated temperature is 266 °F (130 °C) or above, continue test.
- If indicated temperature agrees with ambient, sensor is OK.

3. Disconnect sensor and note temperature indicated on scan tool:
- If indicated temperature drops to -22 °F (-30 °C), sensor has failed and should be replaced.
- If indicated temperature does not drop, problem is with sensor wiring or connections.

Low Temperature-High Resistance Test

1. Connect scan tool and start engine.
2. Note indicated temperature:
- If temperature is normal, problem is with sensor wires and connections.
- If temperature is at or above 266 °F (130 °C), continue with test. Disconnect sensor wires and note scan tool reading:
- If temperature now reads -22 °F (-30 °C), sensor has failed.
- If temperature is OK, problem is faulty sensor ground, reference wire shorted to ground, or PCM has fault.