72 Cheyenne New Transmission Gauge Showing Hot Help!

[4dads72chevy]

I am new to this board. Please see if you can help my Dad with his 72 Cheyenne.

Took '72 Chevrolet Cheyenne to a mechanic and had a new transmission put in. The day he picked it up he drove 3-4 miles and looked down and the temperature gauge was showing "hot". He pulled over and checked the engine and all was okay (not hot). He knew it was something electrical. He carried it right back to the mechanic and he checked all the connections and wires (nothing pinched or incorrect). He traced everything and it was fine and there was no damage. He had the temperature switch/sending unit replaced and that didn't cure the problem. Then he put in a new tempature gauge and that didn't cure it either. It still shows it HOT.

He then took it to another station that could run computerized diagnostic tests. Being a '72 (35 yo truck w/ no computer or chips it couldn't tell him a lot of anything). He traced it with other things and found this:
Going and coming back from the tempature switch he's got 12 volts and
he is only supposed to have about 5 or 6 volts, at least that's what the guy told him.

Anyone have any suggestions or seen this before? Thanks!

[dave2953]

I think it is supposed to have 12 volts. My temp. sending unit is electric and I run it on 12V. Maybe a short in the wiring somewhere? Is is a factory temp. gauge or aftermarket?

[RustyParts]

I'm not sure if you're talking about the temperature of the engine water or the transmission oil, but the process is the same.

Sometimes the wire going from the sending unit to the gauge will get a bare spot and touch the frame creating a ground that will cause the gauge to max out.

The sending unit works by increasing or decreasing the resistance according to the temperature of the fluid and the gauge senses this and moves accordingly.

If the gauge moves to hot as soon as the key is turned on, you probably have a short circuit in the wire. Hope I have'nt confused you.

[4dads72chevy]

The first temperature gauge was original factory and after transmission was replaced it was showing hot and then it was replaced with an aftermarket and it showed HOT as well.

This is the engine sending unit I am talking about. The gauge goes to HOT when he starts the engine and it stays there and won't come back down at all, like it is locked.

Another question is could the new transmission being put in with the electrical hookup (passing gear hookup) be confusing the system in some way in going to his temp gauge? Everything worked fine with the original transmission but now he has had to replace a original temp gauge and original sending unit. All other gauges have been fine. Just the temp. gauge was showing HOT and staying HOT but the engine water is not hot at all.

Thanks for all of your replies!

[A.P.]

Disconnect the green wire that is connected to the sending unit. If it still goes hot as soon as the engine is turned on, you have short on the line some where. But if it stays in the cold position, then your sending unit is the problem.

[cooters]

because the gauge works on resistance, if you run the wrong size wire to the sending unit, the temp will do funny things.

[4dads72chevy]

Thanks for all the suggestions! I will tell my Dad and have him check the above and I'll let you know what he finds out, or I'll re-post if we have more troubles! Thanks, again!