Ram 1500, 2500, 3500, 2WD & 4WD, 2001 - 2005

Engine Coolant Temperature (ECT) Sensor


Description & Operation

The temperature gauge sending unit controls the gauge or warning light. Some models use two temperature sensor (one for the gauge or warning light and one as a PCM input) which may be located adjacent to each other. Refer to the illustrations.


A quick way to determine if the gauge, the MIL, or the sending unit is faulty is to disconnect the sending unit electrical harness and ground it (if two-terminal, jumper between the terminals) with the ignition ON. If the gauge responds or the light illuminates, the sending unit may be faulty. Proceed with the following sending unit test.

  1. Disconnect the sending unit electrical harness.
  3. Remove the radiator cap and place a mechanics thermometer in the coolant.
  5. Using an ohmmeter, check the resistance between the sending unit terminals.
  7. Resistance should be high with the engine coolant cold and low with the engine coolant hot.
  9. It is best to check resistance with the engine cool, then start the engine and watch the resistance change as the engine warms.
  11. If resistance does not drop as engine temperature rises, the sending unit is faulty.

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  1. The sending unit varies the resistance of the circuit in response to changes in coolant temperature. With a cold engine, resistance is high and gauge reading is low or the warning light is OFF. When the engine is hot, resistance is low and the gauge reading is high or the warning light is ON.
  3. Disconnect the wiring to the sending unit and connect a jumper across the two leads. When the ignition is turned on, the gauge needle should move to the maximum or the warning light will go ON. If this happens, the gauge or light is functioning properly and the problem is likely to be the sensor.
  5. If an ohmmeter is available, connect it across the sending unit terminals and start the engine. Resistance should decrease as the coolant temperature increases. If it does not, replace the sending unit.