GM Full-Size Trucks 1988-1998 Repair Information

Engine Coolant Temperature (ECT) Sensor

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OPERATION



See Figures 1 and 2

The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the ECM monitors a low signal voltage which it interprets as warm engine.



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Fig. Fig. 1: Engine Coolant Temperature (ECT) sensor location-4.3L, 5.0L and 5.7L engines



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Fig. Fig. 2: Engine Coolant Temperature (ECT) sensor location-7.4L engines

TESTING



See Figures 3, 4, 5, 6 and 7

  1. Remove the ECT 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 illustration.
  8.  
  9. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  10.  
  11. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced.
  12.  
  13. The sensor may also be checked in the vehicle. Unplug the sensor and attach a digital ohmmeter to the two terminals of the sensor.
  14.  
  15. Using a calibrated thermometer, compare the resistance of the sensor to the ambient air temperature.
  16.  
  17. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  18.  
  19. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced.
  20.  



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Fig. Fig. 3: The ECT sensor is usually located near the thermostat housing



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Fig. Fig. 4: Using a thermometer, a DVOM and some jumper leads, check the resistance of the ECT sensor and compare your readings to those in the chart



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Fig. Fig. 5: Submerge the end of the coolant temperature sensor in cold or hot water and check the resistance



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Fig. Fig. 6: Engine Coolant Temperature (ECT) sensor wiring diagram



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Fig. Fig. 7: Engine Coolant Temperature (ECT) sensor temperature vs. resistance values

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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