VW Passat & Audi A4 1990-2000

Engine Coolant Temperature Sensor

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The engine coolant temperature gauge uses a heat sensitive sending unit to transmit an electrical signal to the gauge. The sending unit is a heat sensitive variable resistor that is located on or near to the cylinder head and threads into an engine coolant passage. The sensors are a Negative Temperature Coefficient (NTC) type. As the temperature increases, the electrical resistance of the sensor decreases. As the coolant temperature changes, so does the resistance of the sensor. The gauge is calibrated within the operating range of the sensor and interprets the resistance value to display the coolant temperature.

On models produced prior to 1994, the engine coolant gauge temperature sensor is a single wire terminal sensor.

Beginning with model year 1994, the engine coolant gauge and the Engine Control Module (ECM) temperature sensors were combined into one sensor with 4 terminals. The basic operation remains the same in that their resistance decreases as the coolant temperature increases, however the actual resistance values of the 2 sensor circuits are different. The electrical connector of the 4-wire terminal sensor ( 1 and 2 ) is keyed to prevent improper connection of the sensor's electrical circuit.


WARNING
When trouble shooting electrical components, always consult a wiring diagram for the particular year, make and model to ensure proper test equipment connection and diagnosis. Failure to use suitable test equipment and testing methods could permanently damage the electrical component(s).



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Fig. Beginning with model year 1994, the Engine Coolant Temperature Sensors combined the sensors for the ECM and the gauge into one sensor-1997 2.8L V6 sensor shown



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Fig. The electrical connector for the combined temperature sensors is keyed to avoid improperly connecting the sensor's wiring-1997 2.8L V6 connector shown

TESTING

Diagnosing the engine coolant temperature sensor should be done in a step-by-step systematic order to determine the cause of the problem. Using a process of elimination technique allows the system operation to be fully verified.


WARNING
When trouble shooting electrical components, always consult a wiring diagram for the particular year, make and model to ensure proper test equipment connection and diagnosis. Failure to use suitable test equipment and testing methods could permanently damage the electrical component(s).

The engine coolant temperature gauge uses an Engine Coolant Temperature (ECT) Sensor that is a heat sensitive variable resistance grounding circuit to monitor the engine's coolant temperature.

The coolant temperature information is conveyed to the instrument panel from the signal received from the ECT sensor. To test the sensor, the gauge and its related components, first test the gauge operation to verify the problem is not in the gauge or gauge wiring. To test the gauge and ECT operation, perform the following procedures.



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Fig. The Engine Coolant Temperature (ECT) sensor resistance decreases as the coolant temperature increases as shown in this graph of the ECM coolant sensor

Engine Coolant Temperature Gauge

WARNING
When trouble shooting electrical components, always consult a wiring diagram for the particular year, make and model to ensure proper test equipment connection and diagnosis. Failure to use suitable test equipment and testing methods could permanently damage the electrical component(s).

Single Wire Sensors
  1. Check the fuses and replace as necessary.
  2.  
  3. With the ignition switch OFF , disconnect the electrical connector from the gauge sending unit.
  4.  
  5. On models with a single wire coolant sensor, typically a yellow wire with a red tracer, connect the wire for the temperature sensor electrical connector to one of the leads of an 80-ohm resistor. Ground the other lead of the 80-ohm resistor to a known good chassis ground.
  6.  
  7. Turn the ignition switch to the ON position for a brief moment. The gauge should read hot, or 230°F (110°C).
  8.  
  9. Repeat the above steps using a 50-ohm resistor. When the ignition is switched to the ON position for a brief moment, the gauge should reach the warning zone and the overheat indicator light should blink.
  10.  
  11. If the gauge operates correctly, replace the sensor. If the gauge does not register, check for a damaged or shorted wire, electrical connector or a faulty gauge.
  12.  

multiple Wire Sensors

NOTE
On some models, terminal 2 (ground) is brown, and terminal 4 (signal) is blue/white. On other models, terminal 2 (ground) is brown/green, and terminal 4 (signal) is red/yellow.


WARNING
Use caution when identifying the coolant temperature sensor leads, as the signal for the ECM (terminal 3 ) could be brown/green on 1995-96 2.0L vehicles and can easily be confused with terminal 2 on 1.8, VR6 and V6 models, which is also brown/green. If the test leads are incorrectly connected when checking the coolant temperature gauge sensor, the ECM may be damaged.

  1. Check the fuses and replace as necessary.
  2.  
  3. With the ignition switch OFF , disconnect the electrical connector from the gauge sending unit.
  4.  
  5. On 1994 and later models with the 4-wire electrical connector, locate the electrical terminals 2 and 4 of the electrical connector (refer to the illustration provided). Typically the terminal 2 is a blue wire with a brown tracer and terminal 4 is a brown wire, however this may differ from year to year and model to model.
  6.  
  7. With the ignition switch OFF , disconnect the harness connector from the sensor.
  8.  
  9. On 1995-96 2.0L ABA engines:
  10.  
    1. Using a multimeter, verify that terminal 2 is grounded.
    2.  
    3. If continuity does not exist between terminal 2 and ground, use the wiring diagrams to verify, trace and repair any damaged wiring. For additional information, please refer to the following topic(s): Chassis Electrical, Wiring Diagrams.
    4.  
    5. Set the multimeter to the DC voltage, and connect the test leads between terminal 4 and ground.
    6.  
    7. Turn the ignition switch ON and verify that voltage (approximately 5 volts) exists between terminals 4 and ground.
    8.  
    9. If voltage does not exist between terminal 4 and ground, use the wiring diagrams to verify, trace and repair any damaged wiring. For additional information, please refer to the following topic(s): Chassis Electrical, Wiring Diagrams.
    10.  

  11. On 1.8L, VR6 and V6 engines, the coolant temperature gauge sensor terminals are 2 (signal from gauge) which is typically a blue/white or black/brown wire, and terminal 4 (ground) which is typically brown.
  12.  


NOTE
Note that the polarity of sensor terminals 2 and 4 are the opposite of 4-cylinder engines.

  1. With the ignition switch OFF , disconnect the harness connector from the sensor.
  2.  
  3. Using a multimeter, verify that terminal 4 is grounded.
  4.  
  5. If continuity does not exist between terminal 4 and ground, use the wiring diagrams to verify, trace and repair any damaged wiring. For additional information, please refer to the following topic(s): Chassis Electrical, Wiring Diagrams.
  6.  
  7. Set the multimeter to the DC voltage, and connect the test leads between terminal 2 and ground.
  8.  
  9. Turn the ignition switch ON and verify that voltage (approximately 5 volts) exists between terminals 2 and ground.
  10.  
  11. If voltage does not exist between terminal 2 and ground, use the wiring diagrams to verify, trace and repair any damaged wiring. For additional information, please refer to the following topic(s): Chassis Electrical, Wiring Diagrams.
  12.  

  1. If both terminal circuits are in proper working order, and the coolant temperature gauge does not work properly, check the function of the sensor as follows:
  2.  

Obtain the listed resistors to achieve the following gauge results:



110 Ohms: Temperature gauge should read 194°F (90°C)
 
50 Ohms: Temperature gauge should read 248°F (120°C))
 


CAUTION
Failure to correctly identify and probe the correct terminals of the 4-wire electrical connector may cause severe damage to the control unit or other electrical components.

  1. Turn the ignition switch to the OFF position.
  2.  
  3. Connect one of the 2 resistors in series with 2 suitable electrical probes attached to terminals 2 and 4 .
  4.  


CAUTION
Do NOT force a test probe into the electrical connector. When using an electrical probe use care to not damage the connector. If an electrical connector is damaged it should be replaced.

  1. Turn the ignition switch to the ON position for a brief moment and monitor the operation of the engine coolant gauge. The coolant temperature gauge should register as outlined above.
  2.  
  3. If the gauge reads correctly, replace the sensor. If the gauge does not register, check for a blown fuse, an open circuit, a damaged or shorted wire, electrical connector or a faulty gauge.
  4.  
  5. For verification purposes, if necessary, test the sensor.
  6.  

Engine Coolant Temperature Sensor

The sensor can be tested using an ohmmeter either while installed or removed. Removal will require draining the engine coolant. Testing on the vehicle will require the use of suitable jumper leads due to the location of the sensor. The sensor's resistance decreases as the temperature increases. The measured resistance change for the coolant gauge sensor from cold to hot is rather significant.

When testing the sensor monitor both the measured resistance and the behavior of the sensor as it is tested. Actual readings may differ by as much as plus or minus 10% due to variances of the test equipment (ohmmeter and temperature gauge), though equally important is the sensor's behavior during testing.

As the temperature changes the sensor's resistance value should also change. At no time should the resistor develop a completely open or closed circuit. If the sensor's resistance does not change as the test temperature changes, or should the sensor develop an intermittent open or closed circuit during testing, replace the sensor.

Sensor installed

Testing the sensor in the vehicle requires the following:



An ohmmeter
 
Electrical test leads
 

  1. With a cold engine, measure the resistance at the sensor:
  2.  
    1. On single wire sensors, between the sensor terminal and ground.
    2.  
    3. On multiple wire sensors, between the terminals for the coolant temperature gauge.
    4.  

  3. Write the measurement down for future reference.
  4.  


NOTE
On sensors with 4-wire connectors, make sure to probe the proper electrical terminals.

  1. Reconnect the sensor electrical connector and start the engine. Allow the engine to run for approximately 5 minutes, then turn the engine OFF .
  2.  
  3. Detach the sensor electrical connector and recheck the resistance of the sensor.
  4.  
  5. The resistance should have decreased from the previous measurement.
  6.  
  7. Reconnect the sensor electrical connector and start the engine again. Allow the engine to run for approximately 5 minutes, then turn the engine OFF .
  8.  
  9. Detach the sensor electrical connector and recheck the resistance of the sensor.
  10.  
  11. The resistance should decrease from the previous measurement, until the coolant temperature reaches its normal operating temperature, at which time the resistance should stabilize. If the sensor's resistance does not change as the engine's coolant temperature changes, or should the sensor develop an intermittent open or closed circuit during testing, replace the sensor. To test the sensor for accuracy, the use of a suitable model Scan Tool or special factory-approved will be necessary. The sensor must be removed and tested alongside a known good sensor and the resistance values compared.
  12.  

sensor removed

Testing the sensor once it's removed from the vehicle requires the following:



An ohmmeter
 
A suitable hot plate
 
Electrical test leads
 
A safe working environment
 
Thermometer with a range of 30-212°F (-1-100°C)
 
A suitable pan capable of containing water and being heated safely to boiling; 212°F (100°C).
 

  1. With the probe of the sensor submerged in ice cold water, measure the resistance as follows:
  2.  
    1. On single wire sensors, between the sensor terminal and ground.
    2.  
    3. On multiple wire sensors, between the terminals for the coolant temperature gauge.
    4.  



NOTE
On sensors with 4-wire connectors, make sure to probe the proper electrical terminals.

  1. Write both the water temperature and sensor resistance down for future reference.
  2.  
  3. Begin heating the water. The resistance should decrease as the water temperature increases. If the resistance does not decrease, the sensor is defective and should be replaced.
  4.  
  5. To test the sensor for accuracy, monitor the water temperature and resistance change. As the temperature changes, so should the resistance. If at any time the sensor shows signs of an open circuit, erratic operation or no resistance at all, replace the sensor.
  6.  



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Fig. When the sensor was placed in a cup of ice water, the resistance increased instantly

REMOVAL & INSTALLATION

CAUTION
Always allow the engine to cool below 100°F (37°C) and then drain the coolant before removing the Engine Coolant Temperature (ECT) gauge sending unit; otherwise, the heated engine coolant can spray out during removal, causing severe burns.

  1. Allow the engine to cool below 100°F (37°C) before working on the cooling system.
  2.  
  3. Note the anti-theft code and radio presets for the radio.
  4.  
  5. Disconnect the negative battery cable.
  6.  
  7. Locate the Engine Coolant Temperature (ECT) gauge sending unit.
  8.  
  9. Drain the engine coolant until it is just below the level of the sensor into a sealable container.
  10.  
  11. Detach the gauge sending unit electrical connection.
  12.  
  13. If the sensor is held into the coolant passage with a retaining clip:
  14.  
    1. Remove the sensor retaining clip, and pull the sensor straight out of the housing.
    2.  

  15. If the sensor is threaded into the coolant passage:
  16.  
    1. Loosen the sensor using a crowfoot wrench, a suitable boxed end wrench or a thin-walled 3 / 8 inch drive 12 point deep well socket and breaker bar.
    2.  




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Fig. This Craftsman® 3 / 8 inch drive thin-walled, 12-point, deep-well socket can be used for removing a threaded coolant sensor



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Fig. The Craftsman® socket slides over the sensor without risking damage to the electrical connector housing



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Fig. If the coolant sensor is held in position with a retaining clip, remove the clip and carefully pull the sensor straight out of the housing

To install:

  1. If the sensor is held into the coolant passage with a retaining clip:
  2.  
    1. Install a new O-ring on the sensor.
    2.  
    3. Using a small amount of fresh coolant or an anti-seizing compound, lightly lubricate the O-ring, and push the sensor in the housing.
    4.  
    5. While pushing inward on the sensor, install the retaining clip.
    6.  

  3. If the sensor is threaded into the coolant passage:
  4.  
    1. Install the sensor with a new crush washer. Tighten the sensor to 12.5-14 ft. lbs. (17-19 Nm).
    2.  

  5. Plug in the harness connector to the sensor.
  6.  
  7. Refill the cooling system with the proper type and mixture of coolant and bleed as necessary. For additional information, please refer to the following topic(s): General Information and Maintenance, Fluids and Lubricants.
  8.  
  9. Reconnect the negative battery cable and re-code the radio as necessary.
  10.  
  11. Start the engine, allow it to reach operating temperature and check for leaks.
  12.  

 
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