I have been chasing down some funny temp readings on my 84 300d. At idle it generally reads around 82C. When I drive it with the AC off, it will hover between 80-85C. When I turn the AC on it will go up to just under 100C and sometimes cool down or rise very quickly to the red and once it hits the red, the guage goes crazy and I also lose my fuel guage. When it has risen over 100C and into the red, I have pulled the car over and felt the hoses and the lower hose is hot as expected, so the thermostat and water pump appear to be working fine. I also put a thermometer in the collant tank and it was reading 87C (sounds reasonable to me on a warm day running the AC on full blast).

I don't think the car is overheating but when the temp sending unit drops to a certain ohm (I am assuming it drops with more heat), it is causing something to go crazy in the guage (maybe trips a ground issue???). I replaced the sending unit, checked the thermostat and flushed the radiator. I am somewhat confident that it is not overheating and it is a faulty guage but would appreciate some advice. I had an ohm meter on the sending unit yesterday and as the car warmed up, the ohm's dropped so I am assuming this is not the problem.

Is there any more reliable way to check the engine temp than in the tank? This is driving me crazy, because I don't know if I can believe the guage or not, but sometimes it looks like it is really working.

 

Is the auxiliary fan coming on when you turn on the a/c?

 

The aux fan is coming on at around 100C when the guage is working, runs for a few minutes and turns off when at idle and turns off pretty quick if I drive the car at 25+ MPH.

 

It sure does sound like the gauge is wonky then. Is there one you can borrow to substitute in just to check it out?

 

I just bought a used cluster and am going to swap out the guage when it gets here. Will see if that works...

 

If I unplug the wire from the sending unit for the temp guage, I would assume that the needle should either go all the way up or all the way down. It stays wherever it is at that time and sometimes rises. This really makes me think it is the guage.