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1966 Mustang Engine Overheats

I’ve just finished cleaning up my 66 mustang (6 cyl) and it starts and sounds great. The problem is that the engine heat gauge immediately flys up over “H” (within 1 min of running). We seem to have circulation of fluid in that time and the coil therm. seems to open & close just fine. Any ideas?

It cant possibly go from stone cold to overheated in only one minute.

What does the gauge read when you remove the wire from the temp sender? The wire will “probably” have a rubber boot & just pulls off.

It goes back to cold

Sounds like the sender is whacked (thats a technical term) & i’d replace it for 7 or 8 bucks.

Or if you like playing around with this sort of thing & have a candy thermometer & pan of water you could test it. The test info is at this link. As the water warms up resistance should slowly drop.

Edit OOPS wrong link. Try this one

OUCH, i’m getting my cars mixed up. That second link is also incorrect. It’s for the 2 wire Engine coolant temp sensor, not the single wire coolant temp sender that you are working on.

However the sensor works the same. High resistance slowly becomes low resistance as the sender warms up.

If you turn the heater control to hot and run the fan is it blowing super hot air at the 1 minute mark when the temp guage goes to full Hot? If the heater isn’t blowing very hot air then this confirms a problem with the guage and/or the sending unit. If the sending unit is replaced and the guage still has the problem then there is a ground wire not making good contact somewhere in the circut or the guage itself is bad.