Ram B1500, B2500, B3500, 1999-2003

Testing

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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.
  2.  
  3. Remove the radiator cap and place a mechanics thermometer in the coolant.
  4.  
  5. Using an ohmmeter, check the resistance between the sending unit terminals.
  6.  
  7. Resistance should be high with the engine coolant cold and low with the engine coolant hot.
  8.  
  9. It is best to check resistance with the engine cool, then start the engine and watch the resistance change as the engine warms.
  10.  
  11. If resistance does not drop as engine temperature rises, the sending unit is faulty.

    Click image to see an enlarged view

    Fig.

  12.  

  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.
  2.  
  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.
  4.  
  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.
  6.  

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.
  2.  
  3. Remove the radiator cap and place a mechanics thermometer in the coolant.
  4.  
  5. Using an ohmmeter, check the resistance between the sending unit terminals.
  6.  
  7. Resistance should be high with the engine coolant cold and low with the engine coolant hot.
  8.  
  9. It is best to check resistance with the engine cool, then start the engine and watch the resistance change as the engine warms.
  10.  
  11. If resistance does not drop as engine temperature rises, the sending unit is faulty.

    Click image to see an enlarged view

    Fig.

  12.  

  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.
  2.  
  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.
  4.  
  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.
  6.  

  1. Check for resistance across the sensor terminals. The use of an input-impedance (digital) VOM is recommended.
  2.  
  3. Resistance will vary with coolant temperature. Generally, resistance will be about 18kilo-ohms at 50ºF (10ºC), decreasing to about 600 ohms at operating temperature. If the measured resistance is at or near zero (shorted) or at or near infinity (open), replace the sensor.
  4.  
  5. Bring the engine up to or near operating temperature.
  6.  
  7. Disconnect the negative battery cable.
  8.  
  9. Unplug the connector on the sensor.
  10.  


NOTE
Be sure you are unplugging and checking the ECT, and not the temperature gauge sending unit; they are two different parts.

  1. Connect an ohmmeter across the sensor connectors and observe the reading over time as the engine cools.
  2.  
  3. As the engine cools the resistance in the sensor should increase slowly.
  4.  
  5. If it doesn't, replace the sensor.
  6.  
  7. If a scan tool is available, it can be used to check the sensor. Call up sensor voltage in the "live" mode. Start and run the engine observing the sensor voltage. It should decrease and increase smoothly as engine temperature increase and then decreases.
  8.  



Click image to see an enlarged view

Fig. The coolant temperature sensor can be tested using the Auto Xray® or equivalent tool's data display feature



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Fig. The ECT sensor's electrical connector terminal identification: (1) BK/LB; (2) TN/BK



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Fig. ECT sensor circuit diagram

 
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