1985 300D.
I am having an issue with either coolant temp or coolant temp readings and hope someone can help. Our temp gauge started reading low around 40c to 60c. It had always read around 80c before then. I assumed the thermostat.

I replaced it, and am still having the same issue. I tested the old thermostat, and it opens in some hot water no problem, so I have to assume there is another issue. (unless both thermostats are weak?) As I'm fairly certain there is no air bubble, and the aux pump seems to work (heat and defrost work at idle). The coolant temp sensor is my next suspect, unless there are some other options you all may advise. After some research that seems the likeliest culprit though. What say you guys? Anything else that can cause this condition? Thanks!

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1984 Astral Grey 300TDT - The French Fry
1979 Yellow 300SD - The Pomme - SOLD
1985 Light Ivory 300DT - Fritz
1987 Anthracite Grey 300SDL - ?