GM Century/Lumina/Grand Prix/Intrigue 1997-2000

Engine Coolant Temperature (ECT) Sensor

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OPERATION



The Engine Coolant Temperature (ECT) 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 at -40°F.) while high temperature causes low resistance (70 ohms at 266°F.). The PCM supplies a voltage (about 5.0V) through a pull up resistor to the PCM's ECT signal circuit and measures the voltage. When the engine coolant is cold, the sensor (thermistor) resistance is high, therefore the PCM will measure a high signal voltage. As the engine coolant warms, the sensor resistance becomes less, and the ECT signal voltage measured at the PCM drops. With a fully warmed up engine, the ECT signal voltage should measure about 1.5-2.0 volts. By measuring the voltage, the PCM calculates the engine coolant temperature and engine coolant temperature affects most systems the PCM controls.

A technician's scan tool displays engine coolant temperature in degrees. After engine start up, the temperature should rise steadily to about 195°F (90°C), then stabilize and the thermostat opens. 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. A hard or even an intermittent fault in the engine coolant temperature sensor circuit will set a hard Diagnostic Trouble Code (DTC).

Most ECT sensor also contain another circuit which is used to operate the engine coolant temperature gauge (if equipped) located in the instrument panel.

TESTING



There are a few items you should check before beginning more extensive testing.



Check for poor connection at the PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections.
 
Look for a damaged harness or damaged wiring going to the ECT sensor, especially if heavy work has recently been done where the connector could have been damaged.
 

Sensor Installed in Vehicle


Click image to see an enlarged view

Fig. ECT sensor resistance value chart

  1. Disconnect the engine wiring harness from the ECT sensor.
  2.  
  3. Connect an ohmmeter between the ECT sensor terminals.
  4.  
  5. With the engine cold and the ignition switch in the OFF position, measure and note the ECT sensor resistance.
  6.  
  7. Connect the engine wiring harness to the sensor.
  8.  
  9. Start the engine and allow the engine to reach normal operating temperature.
  10.  
  11. Once the engine has reached normal operating temperature, turn the engine OFF .
  12.  
  13. Once again, disconnect the engine wiring harness from the ECT sensor.
  14.  
  15. Measure and note the ECT sensor resistance with the engine hot.
  16.  
  17. Compare the cold and hot ECT sensor resistance measurements with the accompanying chart.
  18.  
  19. If readings do not approximate those in the chart, the sensor may be faulty.
  20.  

Sensor Removed from Vehicle
  1. Remove the ECT sensor from the vehicle.
  2.  
  3. Immerse the tip of the sensor in a container of water.
  4.  



Click image to see an enlarged view

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

  1. Connect a digital ohmmeter to the two terminal of the sensor.
  2.  
  3. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the accompanying resistance value chart.
  4.  
  5. Repeat the test at two other temperature points, heating or cooling the water as necessary.
  6.  
  7. If the sensor does not meet specifications, it must be replaced.
  8.  

REMOVAL & INSTALLATION




WARNING
Care must be taken when handling the ECT sensor. Damage to the ECT sensor will affect proper operation of the fuel injection system.

3.1L and 3.4L (VIN E) Engines
  1. Disconnect the negative battery cable.
  2.  
  3. With the engine cold, relieve the coolant pressure by opening the radiator cap or opening the bleed screw near the thermostat.
  4.  
  5. Remove the air intake duct.
  6.  
  7. Detach the electrical connector from the ECT sensor.
  8.  
  9. Using a deep well socket and extension, remove the sensor.
  10.  



Click image to see an enlarged view

Fig. Engine Coolant Temperature (ECT) sensor and harness arrangement-3.1L and 3.4L (VIN E) engines

To install:

  1. Coat the ECT sensor threads with a suitable sealant.
  2.  
  3. Install the ECT sensor, and using a deep well socket and extension tighten the sensor to 17 ft. lbs. (23 Nm).
  4.  
  5. Install the remaining components in the order of removal.
  6.  
  7. Check for leaks and replenish the coolant, as required.
  8.  

3.4L (VIN X) Engine
  1. Disconnect the negative battery cable.
  2.  
  3. Remove the rear intake duct and MAF sensor assembly.
  4.  
  5. Drain the engine coolant, into a suitable container, to a level below the sensor.
  6.  
  7. Remove the exhaust crossover pipe.
  8.  
  9. Detach the ECT sensor electrical harness.
  10.  
  11. Using a 19mm deep well socket and a 6-inch extension, remove the ECT sensor.
  12.  



Click image to see an enlarged view

Fig. On this engine, the exhaust crossover must be removed to access the ECT sensor-3.4L VIN X Engine

To install:

  1. Coat the ECT sensor threads with a suitable sealant. Attach the electrical connector.
  2.  
  3. Install the sensor into the engine and tighten to 22 ft. lbs. (30 Nm).
  4.  
  5. Install the exhaust crossover pipe.
  6.  
  7. Install the rear intake duct and the MAF sensor assembly.
  8.  
  9. Connect the negative battery cable.
  10.  
  11. Check the coolant level and adjust as necessary. Start the engine and check for leaks.
  12.  

3.5L (VIN H) Engine
  1. Partially drain the engine coolant, into a suitable container, to a level below the sensor.
  2.  
  3. Locate and detach the ECT sensor harness.
  4.  
  5. Remove the ECT sensor.
  6.  



Click image to see an enlarged view

Fig. The ECT sensor is located on the flywheel end of the engine, on the right side (rear) cylinder head-3.5L (VIN H) engine

To install:

  1. Apply a suitable non-hardening sealer to the ECT sensor threads to prevent coolant leaks.
  2.  
  3. Install the ECT sensor and tighten to 15 ft. lbs. (20 Nm). Do not over-tighten. The cylinder head is made of aluminum. Attach the electrical connector.
  4.  
  5. Refill the cooling system.
  6.  

3.8L Engines
  1. Disconnect the negative battery cable.
  2.  
  3. Drain the engine coolant, into a suitable container, to a level below the sensor.
  4.  
  5. Detach the electrical connector.
  6.  
  7. Using a 19mm deep well socket and an extension, remove the ECT sensor.
  8.  



Click image to see an enlarged view

Fig. Typical Engine Coolant Temperature (ECT) sensor



Click image to see an enlarged view

Fig. Location of the ECT sensor-3.8L (VIN 1) engine



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Fig. ECT sensor location-3.8L (VIN K) engine

To install:

  1. Hand-start the ECT sensor into the engine, then tighten to 22 ft. lbs. (30 Nm).
  2.  
  3. Attach the electrical connector.
  4.  
  5. Refill the system with coolant.
  6.  
  7. Connect the negative battery cable.
  8.  
  9. Start the engine and check for leaks.
  10.  

 
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