BMW Cars 1999-06

Testing

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Fig. This chart illustrates how rapidly the sensor's resistance decreases as the coolant temperature increases

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.



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Fig. The engine coolant sensor was removed from a known good running engine and the resistance was documented as it changed with temperature

Use a BMW service tester, the OBD I blink code or on OBD II vehicles, use a suitable Data Scan Tool (DST) to check for faults and/or if capable, to read the values of the components. If this diagnostic equipment is not available, proceed as follows.


WARNING
When checking component values (volts or ohms) a suitable digital volt/ohmmeter must be used otherwise the circuit and/or component may be damaged.

  1. Unplug the sensor. Measure the coolant temperature as close as possible to the location of the sensor.
  2.  
  3. Measure the resistance between the terminals of the sensor. Compare the results to the accompanying chart.
  4.  
  5. With the ignition switch ON the sensor should receive 5 volts at one of the connector terminals. The other terminal is the return signal to the control unit, do not use it for voltmeter ground.
  6.  



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Fig. Use this chart on OBD I equipped vehicles with separate coolant sensors for the gauge and Engine Control Module



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Fig. The ECT sensor for the OBD II system, for the gauge and the ECM have been combined into one, 4 wire sensor. Terminals 1 & 2 are for the gauge, Terminals 3 & 4 are for the ECM



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Fig. The sensor's resistance is measured while the temperature of the water is monitored. The chart was derived by actual testing of a known good sensor



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Fig. The sensor's terminal positions are molded into the sensor, but difficult to see. This photo has been labeled to show the terminal positions of the sensor-OBD II sensor shown



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Fig. The OBD II temperature sensor four wire electrical connector shown with the terminals labeled



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Fig. The sensor can be removed and tested. Terminal positions 1 & 2 are measured with the sensor's probe submerged in a cup of ice water. The temperature is monitored by a thermometer and the resistance measured with a digital ohmmeter

Sensor Installed In Vehicle



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 terminals for the coolant temperature gauge. Write the measurement down for future reference. On sensors with 4 wire connectors, make sure to probe the proper electrical terminals (sensor positions 1 and 2 ).
  2.  
  3. Reconnect the sensor electrical connector and start the engine. Allow the engine to run for approximately 5 minutes, then turn the engine OFF .
  4.  
  5. Detach the sensor electrical connector and recheck the resistance of the sensor.
  6.  
  7. The resistance should have decreased from the previous measurement.
  8.  
  9. Reconnect the sensor electrical connector and start the engine again. Allow the engine to run for approximately 5 minutes, then turn the engine OFF .
  10.  
  11. Detach the sensor electrical connector and recheck the resistance of the sensor.
  12.  
  13. 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 specific Data Scan Tool (DST) will be necessary, or the sensor must be removed and tested along side a known good sensor and the resistance values compared.
  14.  

For comparison purposes, from a cold start, when tested on an E36 M44 model, the following results were obtained with an ambient air temperature of 65°F (18°C) with the engine idling.

Five minutes of engine idling:



Temperature gauge reading: 1 / 4 of total sweep (see photo)
 
Sensor positions 1-2: 1,380 ohms
 
Sensor positions 3-4: 570 ohms
 

Ten minutes of engine idling:



Temperature gauge reading: 1 / 2 of total sweep (see photo)
 
Sensor positions 1-2: 818 ohms
 
Sensor positions 3-4: 338 ohms
 

Fifteen minutes of engine idling:



Temperature gauge reading: Between 1 / 2 and 2 / 3 of total sweep
 
Sensor positions 1-2: 402 ohms
 
Sensor positions 3-4: 170 ohms
 
Electric engine cooling fan switch: Switch closed (fan running)
 



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Fig. Installing the test leads on the engine coolant sensor requires the use of a needle nose pliers-M44 sensor shown



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Fig. The engine coolant sensor shown with the test leads installed-M44 sensor shown



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Fig. After five minutes of idling, the coolant gauge just crept out of the blue cool zone-E36 M44 model shown



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Fig. After ten minutes of idling, the coolant gauge reached the mid point-E36 M44 model shown

Sensor Removed From Vehicle



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 at the terminals for the coolant temperature gauge (on sensors with 4 wire connectors, terminals 1 and 2). 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 did not decrease, the sensor is defective and should be replaced. 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 erratic operation or no resistance at all, replace the sensor.
  4.  



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

 
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