I have a 1970 442 with factory tic toc tac (not reprouction). The rally pack was sent out once for rebuild and worked fine up until now. The fuel gauge needle is stuck above the "FULL" mark. I thought I was getting good gas mileage until I had driven it enough to realize I am getting impossible gas mileage for a 455 c.i. engine. I found the ground wire at the gas tank has broken in two. I spliced it and sanded the terminal where it screws to the trunk sheet metal. I test the ohms of wire (ground coming from tank) and I am getting resistance. The needle does not move whether I have the ground connected or not connected and I turn on the ignition key. The other gauges (oil and water) work fine. Any ideas or suggestions?

 

You are correct that an open circuit is the cause of your gauge going above Full. You've checked the sending unit ground, but you also have to check the wiring coming from the gauge to the rear of the car to the tank. That wire should connect to the wire coming from the sending unit in the rear of the car. Pull that connector apart, ground the side coming from the gauge, and turn the key on. The gauge should go straight to E and stay there. If it does, there's nothing wrong with the gauge or the wiring to it. If not, then there is a short in the wire from the gauge to the rear of the car.

If the gauge and wiring check out and you've addressed the ground side of the sending unit, all that's left is the sending unit itself, which means getting it out of the tank and most likely putting in a new one.

 

I grounded the wire coming from the fuel gauge to the tank and the needle DID NOT go to "E." I tested the wire (gauge to tank) and it is showing 3.5 ohms. If it was shorted, wouldn't it show infinity?

 

No! The exact opposite.

A short would be zero Ohms. An open circuit would register infinite Ohms. That's why a gauge goes not just to Full when there is an open, but well past it. The sending unit is just a variable resistor with resistance between 0 Ohms (E) and 90 Ohms (F). Because infinity is much bigger than 90, the gauge goes past F when there is an open circuit.

Your measurement of 3.5 ohms is close enough to zero that I suspect that you have a partial short in the wire from the gauge to the rear of the car. You'll have to trace that wire.

Just to verify, when you grounded that wire coming from the gauge and found that it did not go to E, you did this with the key in the ON position? The ignition has to be ON, or there is no power to the gauge.

 

I did turn the ignition on. Is there a most likely to short part of that wire length where the wire would more likely than not short,like up by the gauges. I don't look forward to going underneath the carpet to hunt it down.

 

If there is a short in the wire (that would be grounding it) The gauge should go to Empty with the key on. So there is either a broken wire or the gauge is no good. Now you need to get behind the dash and ground the gauge directly to verify it works. If it does then all that's left is to find where the open wire is, or run a new one.

 

You're right. My apologies. I had that backwards. If grounding the wire from the gauge with the key on does not send the gauge to E, then there is an open in the wire, not a short. If the gauge was stuck on E all the time, that would be an indication of a short.

 

I have little electronic experience so, how do I ground the gauge?

 

I don't know, but I think a wiring diagram would help. But if I were you, unless you have some reason to assume the gauge itself has been damaged or something, I wouldn't start there. I would assume the gauge is ok and look for a break in the wire between the gauge and the rear of the car, starting at the rear.

 

There are 2 wires on the back of the gauge, pink is your 12v+ and the tan coming from the tank. Ground the tan wire terminal, the gauge should go to Empty. There is a harness connector in the trunk that all the wires from the front feed through. The wire color is tan to the sending unit.

 

I have similar problem a few people here helped me diagnose...in my case when i grounded the feed wire it went to empty so i have a tank sensor to deal with

Sender not sensor

 

I know this is for a Chevelle, but a lot of the wiring is very similar.

http://documents.clubexpress.com/doc...qFRxOz5Cfko%3d

 

I'd pull the gauge pack out and check to make sure the resistors are on correctly and nothing is touching where it shouldn't. They should be in this configuration:
Resistorpositions442gauges_zpse0db39c6.jpg

 

I took the gauge harness off and sprayed the terminals and contacts and it finally did to to "E" when I shorted it. All seems to be working as it should now. However, I have the enviable task of replacing a heater core when I found the carpet damp. I have a new post on that project.

 

It's a never ending amount of little things that line up to go wrong with older cars. Glad you got the fuel gauge fixed.

 

Thanks everyone for your help. You really got me pointed in the right direction.

 

I love it when the solution to the problem is really simple!