My temperature gauge recently quit. For the first few weeks I owned the car, the gauge worked fine but jumped a bit if mashed the accelerator and let off. Now it doesn't work almost at all. When I turn the key to run, I can see the needle jump from it's resting place to right below the first mark, but it doesn't normally move from there. I changed the temperature sensor right by the upper radiator hose and that didn't fix the problem, but now the electric fan always kicks on when the car is barely warm. Also, once in a while, the needle will dance around while I'm driving, sometimes reporting the correct temperature for a few moments. I know there's a temperature sensor in the WUR, does that feed the gauge? Or is there some other temperature sensor on this car?

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1987 300SDL

The unit at the top radiator hose is the "212 switch", which lets the fan kick in when the coolant goes over 212 Fahrenheit and has nothing to do with the gauge. The sender unit for the gauge is at the back of the RHS bank of cylinders close to the firewall. You will see 2 senders. One is for the Cold Start Injector and the other is for the gauge. My guess is the sender is toast or the connection is poor after all these years.

Thank you for the help! I was able to find the temp senders and I found that the wire for the gauge had separated from the connector. I re-soldered the connector and it works perfectly now!

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1987 300SDL

Nice! I love simple fixes!