My XR7 started overheating intermittently. I have replaced thermostat, have correct thermo and restrictor plate, and had radiator checked. I just noticed that temparature goes up when lights turned on. When turn off lights, temp goes down. I have bought new original gauge and original regulator, but hesitant to take panel off to replace as big job. Has anyone else had this problem and what you think?
This almost always turns out to be the radiator. How did they check it?
Strong flow and no signs of rust and has been replaced, not original. But why turning on lights effects temp on gauge?
Get yourself an infrared temp measurer and check block temp at the location of the temp sensor - with lights on and then again with lights off. You will probably not see any difference, and will then know it’s an electrical problem. Engine temp should measure close to rated thermostat temp when idling on a winter day with thermostat open (radiator warm).
Two issues are unrelated. Is the temp gauge you are referring to the stock gauge or after market? Does it get hot going down the highway or idling around town?
the gauge i bought and regulator bought are both original from 1970 in original boxes. I have not installed yet. The gauge in car is likely original. Do not know about regulator, as have not removed panel. I noticed also the amp meter needle shakes when I turn on AC or lights or fan for AC even when ac not on. Amp gauge steady when no ac or fan or lights. Does not seem to make difference if driving or idling. Actually temp gauge goes down when lights, brake light, and fan all off and driving. When brake for a long time temperature gauge goes up. I probably will install a radiator hose temperature gauge to see temp when gauge says overheating, before taking off instrument panel. Its weird
1 Like
excellent idea. i was going to splice in a gauge in engine, but this is easier. will get today
1 Like
Sounds like an electrical gremlin, grounds? You didn’t mention the temp sender replacement, yea its a PITA 
One of the 1st things I did on my 73 was replace the oil and temp gauges, Cleve’s run hot to begin with
Make sure your ground from the body to the engine is good.
2 Likes
Start with cleaning the battery to chassis to engine block ground cable(s) and connections. The gauges work on resistance, the more resistance, the lower they will read. Switching on the lights should have no effect on the readings in a properly grounded system.
Lower resistance at the sensor actually causes more current flow through the gauge making it read higher. So higher sensor resistance actually causes lower gauge readings. But all sorts of strange things can happen without good grounds, and making sure they are clean is a good place to start.
1 Like
Ooofff, I need to finish my coffee before getting on the forums. Calicat is 100% correct. Higher resistance gives a lower reading, and vice versa. If you ground the sender wire, the gauge will peg at the high end of the scale.
Regardless, there is something going on with your electrical system and the grounds are always the first thing to check.
3 Likes
replaced sending unit as that was easy. Bought gauge and regulator, just hesitating to take off instrument cluster till sure that is problem. This is great site, love all the comments on my problem
I’d start with grounds after all the info you’ve posted. Grounds and Vacuum gremlins will have you pulling your hair out
1 Like
At least on the 67-70’s the aftermarket temp sender units will usually read way off. Once you establish the actual operating temp of your engine, and make sure the grounds are good, you can start work on calibration of gauge and sender unit if needed. Agree with putting off pulling the dash until you absolutely have to.
1 Like
You can’t fix electrical problems by looking at them. You need to get a multi meter and take a few measurements. Some multi meters also measure temperature but I prefer the non contact infrared thermometers. The only place you need to measure temperature is at the location of the temp sending unit. Then you need to measure the resistance across the sender. Then we can look at the numbers and see what is happening.
2 Likes