This was on a gasser and not a mercedes.
I had a temp gauge that was not working as it should and I pulled the Sending Unit and found the end that is exposed to the Coolant had a thick build up of some white stuff on it similar to corroded Aluminum even thought the end was copper.

I have also had the loose connection issue and recently one that only had a few strands of wire that were still good that was causing a false reading.

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84 300D, 82 Volvo 244Gl Diesel