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!