Temp Guage Check
Temperature Gauge-Warning Sensor
With several sensors on the A-housing (thermostat housing) we need to be sure to pick the right one for this test. The temperature gauge sensor is the taller of the two on the housing and it also has two male spade connectors on the top. There will be a temperature rating stamped on the housing (usually this is 117 or 118 degrees C) which is the temperature at which the switch will close and the warning light will be turned on.
Of the two spade connectors the larger one will have two wires with the same color (brown/white). One goes to the Instrument cluster and the other goes to pin number 4 on the diagnostic connector.
On the other smaller spade connector there will be on wire that appears to be brown in color. As per BMW wiring drawing it is brown/violet (brown/gray). This wire sends + 12 volts down the wire for the warning light with key on.
To test the temperature gauge take the wires on the larger spade connector and (while someone is sitting in the drivers seat to turn on the ignition and watch the gauge for you) touch it to the engine block only for an instant. This should make the gauge jump to all the way right into the red zone of the gauge. There should also be resistance on the sensor,(when checked with and Ohm meter) which should decrease as the temperature increases. The smaller spade connector should read infinite resistance until the rated temperature is reached closing the switch and turning on the warning light.
Before discarding the temperature sensor because you think it is bad try cleaning the threads as sometimes the sealant used on them will block the sensor from grounding. Also the batteries on the SI board have been known to create problems with many of the gauges on the instrument cluster.