300E Weird Temp Gauge...sometimes
 

Once in a while, my temp gauge, when car is warm, hops around around and drops from 100 to around 80 then stabilizes and goes back to the 95-100 degree range. Anyone else ever have this happen? I've only noticed it recently. Electrical? Sensor?

Sounds like either a faulty temperature sensor or a bad ground/wiring on the instrument cluster for the temperature gauge.

Ground
 

My guess is a bad ground. Are other gauges/clock affected?

Everything else is fine. I was thinking maybe a bad temp sensor. Wiggled all of the wires and still had same problem but just very temporary. I'll keep an eye on it. Is this the temp sensor near the firewall or the in the middle of the valve cover under the air housing? Thank you!

Bad ground up to the cluster panel, very common problem.

We had the same issue with the fuel level gauge on a 1992 300E a few years ago. It would - only below half a tank - flicker between empty and half full intermittently and the dash illumination on the left didn't work either. The common reason was a faulty rheostat, although the car moved on before we had it checked.

I wonder if a dying voltage regulator could cause those same symptoms?

Yes. It could be something like the voltage regulator although mine is new. I can reach in there an reseat it. Hasn't happened again since my last reply. Where is , what is, the rheostat?

What is the fix for the bad ground?

Happened again coming home. I noticed it happens when it is around 55 degrees outside. Warm outside, doesn't do it. Hmmm...