I have the same problem with my 93 320E. Gauge works sometimes but never the correct Temp, most often it does not work at all. Thought it was the gauge rather than the sensor and fitted a replacement gauge but no change to the reading. My friendly Mercedes wrecker says the voltage supply to the gauge could be the problem?? Not sure about that but should be easy to check now that I have the knack for removing the cluster - (great procedure in the DIY section-superb photos and description). The idea of checking the gauge by substituting a 3K resistor sounds a great idea - only have to find the connector by the fuse box, as yet cannot see it- is it under the fuse box and do I have to remove the fuse box - anyone know?

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1993 320E 140,000km