GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Coolant Temperature Sensor

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OPERATION



The engine coolant temperature sensor is a thermistor ( a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms {100K ohms} at -40°F/-40°C) While high temperature causes low resistance (70 ohms at 266°F/130°C).

The PCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the PCM 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 PCM calculates the engine coolant temperature. The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 194°F (90°C) then stabilize when the thermostat open. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other.

Engine coolant temperature affects most systems the PCM controls.

TESTING



Sensor in Vehicle
  1. The Engine Coolant Temperature (ECT) sensor is located in the intake manifold, in a tapped and threaded hole in a water jacket.
  2.  

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Typical 3.8L Engine ECT Location

  1. Disconnect the electrical connector.
  2.  
  3. Attach an ohmmeter to the sensor terminals and then measure the resistance and compare with the accompanying chart on a cold engine. Measure the coolant temperature in the radiator or overflow bottle. Compare the resistance and temperature on the chart.
  4.  
  5. Tape an automotive thermometer tightly to the upper radiator hose. Start and run the engine to normal operating temperature, comparing the temperature on the thermometer to the resistance reading on the ohm meter. The readings should coincide with the chart.
  6.  
  7. If the sensor is not in specification, replace it.
  8.  

See Figure 2



Click image to see an enlarged view

Fig. Fig. 2: Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) Sensor Resistance Values vs. Temperature Readings

Sensor Removed From Vehicle
  1. Remove the ECT from the vehicle.
  2.  
  3. Immerse the tip of the sensor in a container of icy water (32°F/0°C), as measured with a calibrated thermometer.
  4.  
  5. Attach an Ohm meter to the sensor connector. The Ohms value should read approximately 9400 to 9500 Ohms.
  6.  
  7. Using the thermometer readings, compare the resistance of the sensor to the temperature of the water.
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  9. Repeat the test at two other temperature readings, heating the water as necessary.
  10.  
  11. Refer to the resistance value chart; if the sensor is not within specification, replace it.
  12.  

REMOVAL AND INSTALLATION



Use care when handling the ECT sensor. Damage to the coolant sensor will affect the operation of the fuel control system.

  1. Disconnect the negative battery cable.
  2.  
  3. Drain the coolant level below the height of the ECT and recycle the engine coolant.
  4.  
  5. Disconnect the electrical connector at the sensor.
  6.  
  7. Remove any obstruction to access the ECT.
  8.  

See Figures 3 and 4

Click image to see an enlarged view

Fig. Fig. 3: Location of ECT Sensor on a 3.4L Engine shown



Click image to see an enlarged view

Fig. Fig. 4: The Engine Coolant Temperature Sensor is tapped into the intake manifold and is able to transmit engine coolant temperature to the PCM.

  1. Using a deep well socket, remove the sensor.
  2.  

To install:
  1. Coat the threads of the new sensor with a suitable thread sealant and install it in the engine. Tighten to 17 ft. lbs. (23Nm).
  2.  
  3. Install the electrical connector.
  4.  
  5. Refill the system with coolant.
  6.  
  7. Connect the negative battery cable.
  8.  

 
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