Temperature Senders

[courtenay]

I've got an '80 Fi. There are two temperature senders in the engine block - one red, one black. Based on a tip from Craig Nelsonk, I have disconnected the wire to the black sensor. Two things have happened. First, referring to my cooling system thread, the temperature gauge is behaving as I would expect it should under normal circumstances. The T-stat is opening just under 190 deg - and the fan comes on just over 190 deg. When I reconnected the wire to the black sensor, the needle jumped almost to the red line. Second, when the black sensor was connected, the needle would jump when I turned the headlights on - doesn't do that now with that connector off.
I just want to confirm that the only thing I am missing with the black sensor disconnected is an overheat light that would come on (somewhere) if the car got too hot. Correct???

[divace73]

Hi courtenay
The Rear sensor (closest to the firewall which from memory I think is the black one) is a switch only. It enables your gauge to go to max when a certain temp is reached (I don't know what that is). I have a problem with mine (I have a 1980 FI as well) that i leave disconnected as when plugged in the needle moves closer to the hot zone even when the car is functioning normally, it makes me panic for no reason.
However with the rear disconnected and front connected I never let the needle go much past half way if she's getting hot.
I should one day get a new black sensor but I have a fear that it is welded shut???
The headlight movement may be an earthing, or pwer supply issue??
I have also sent you a PM

[manoa matt]

Sounds like the overheat switch is stuck open/closed, or is not grounding. Was the sensor wrapped with teflon tape prior to install, or some kind of thread sealant? Try adding an additional ground wire from one of the studs on the cam shaft retaining plate at the back of the cam carriers to the firewall.