GM Astro/Safari 1985-1996 Repair Guide

Coolant Temperature Sensor (CTS)

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OPERATION



See Figure 1



Click image to see an enlarged view

Fig. Fig. 1: Engine coolant temperature sensor

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 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.

TESTING



See Figure 2



Click image to see an enlarged view

Fig. Fig. 2: Coolant temperature sensor resistance chart

  1. Remove the sensor from the vehicle.
  2.  
  3. Immerse the tip of the sensor in container of water.
  4.  
  5. Connect a digital ohmmeter to the two terminals of the sensor.
  6.  
  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 chart.
  8.  
  9. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  10.  
  11. If the sensor does not meet specification, it must be replaced.
  12.  

REMOVAL & INSTALLATION



  1. Disconnect the negative battery cable.
  2.  
  3. Drain the cooling system below the level of the sensor and disengage the sensor electrical connection.
  4.  
  5. Remove the coolant sensor.
  6.  

To install:
  1. Install the sensor and engage the electrical connector.
  2.  
  3. Refill the cooling system and connect the negative battery cable.
  4.  

 
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