Jeep Wagoneer/Commando/Cherokee 1984-1998

Engine Coolant Temperature (ECT) Sensor Repair Guide

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OPERATION



See Figures 1 and 2

The engine coolant temperature sensor, or ECT sensor is mounted by the thermostat and provides the PCM with engine temperature information. The PCM supplies 5 volts to the ECT sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the PCM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance low), the PCM monitors a low signal voltage which it interprets as warm engine.



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Fig. Fig. 1: View of the Engine Coolant Temperature (ECT) Sensor-1996 4.0L engine shown



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Fig. Fig. 2: View of the Engine Coolant Temperature (ECT) Sensor-1996 5.2L engine shown

TESTING



See Figures 3 and 4

  1. With the engine cold, remove the ECT sensor.
  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 sensor 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 meet specification, it must be replaced.
  12.  



Click image to see an enlarged view

Fig. Fig. 3: Submerge the end of the temperature sensor in cold or hot water and check resistance



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Fig. Fig. 4: Temperature-to-resistance relationship of the ECT and MAT sensors

Removal & Installation



  1. With the engine cold, partially drain the cooling system. Record the amount of coolant drained.
  2.  
  3. Carefully disengage the ECT sensor wiring connector.
  4.  
  5. Unscrew the sensor.
  6.  

To install:
  1. Coat the threads of the ECT sensor with sealant and install it into the block. Tighten the sensor to 8 ft. lbs. (11 Nm).
  2.  
  3. Engage the sensor wiring connector.
  4.  
  5. Fill the system with the same amount of coolant as was drained and check for leaks.
  6.  

 
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