Audi Cars 1999-05

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

 
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