Temperature gauge does not work on my 1970 340 Duster ?

DaveM said
No prob Mike, thanks for the update and confirmation the voltage readings appear to be good. I did ground the temp sensor wire and the temp gauge goes up to high/hot. I will have to ground the gas sensor wire ...but if that also works then ??

Is it possible that your engine hasn't made hot water enough to move that gauge needle? If the engine is up to normal operating temperature there should be somewhere between 40 and 25 ohms resistance between the senders contact post and chassis ground.
You don't absolutely have to go under the car to ground the fuel sender wire. It can be found in rearward harness connector behind left kick panel. Then again... To verify the ground jumper over the fuel senders hose connection and check all of the blue sender wire run, you must go under the car.
So why the service techs needed a 3 position tester... The sender wire direct to ground zero ohms test doesn't always prove good gauge. I've seen some that were soooo toasted inside that zero ohms could cause movement while within normal range ohms could not. Looking at those, the needle doesn't go all the way home at off. It'll stop very near the hash mark scale.
This 4th/off/home needle position isn't on the tester.