GM Full Size Vans 1987-1997 Repair Guide

Coolant Temperature Sensor (CTS)



See Figure 1

The coolant temperature sensor is a thermistor (a resistor which changes value based on temperature). Low coolant temperatures produce high resistance (100,000 ohms at -40°F/-40°C) while low temperatures causes low resistance (70 ohms at 266°F/130°C). The sensor is mounted in the coolant stream (usually on the intake manifold) and the ECM supplies a 5 volt signal to the sensor through a resistor in the ECM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the ECM knows the engine coolant temperature effects most systems the ECM controls.

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Fig. Fig. 1: Coolant temperature sensor location


See Figures 2, 3 and 4

  1. Remove the sensor from the vehicle.
  3. Immerse the tip of the sensor in container of water.
  5. Connect a digital ohmmeter to the two terminals of the sensor.
  7. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance illustration.
  9. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  11. If the sensor does not meet specification, it must be replaced.

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Fig. Fig. 2: Submerge the end of the coolant temperature sensor in cold or hot water and check the resistance

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Fig. Fig. 3: Coolant temperature (CTS) sensor wiring diagram

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Fig. Fig. 4: Coolant temperature sensor temperature vs. resistance values


  1. Disconnect the negative battery cable.
  3. Drain the cooling system below the level of the sensor and disengage the sensor electrical connection.
  5. Remove the coolant sensor.

To install:
  1. Install the sensor and engage the electrical connector.
  3. Refill the cooling system and connect the negative battery cable.