68 fuel gauge maxes out at half

[connella08]

Hey all,

My gas gauge has never worked since I got the car. I finally decided to replace the old rusting tank with a stainless kit from CJ's. After doing the swap, I went to the gas station and filled up and the gauge looked like it barely moved. If I ohm out the sending unit, I get 9 ohms to ground and if I ground out the wire up to the dash, it only goes to half a tank. I did replace the constant voltage generator with a solid state one and it measures 4.9xx volts down to the sending unit.

 

[GOvert]

It is possible that the float arm may be bent such that it has the resistor maxed out thinking that it is full when it is not. This would require draining the fuel, removing the sending unit to bend the float arm to remedy the situation. The task could be performed on ramps but is a lot easier on a lift of some sort. Use extra caution when dealing with gasoline. Don't be backed in a building where you could get trapped during a fire and don't use a droplight with an incandescent bulb that could turn into an ignitor if dropped. Both of those horrific scenarios have occurred within two miles of me.

 

[connella08]

Brand new sending unit and I didn't have any difficulties installing it. I thought if I ground out the wire that goes to the dash, that its supposed to make the gas gauge go all the way to full.

 

[GOvert]

That's correct. You must have some resistance in a wire somewhere. Are you grounding to the tank or the frame?

 

[connella08]

if I use the tank as a ground, it measures 9 ohms. however, for some reason someone had clipped the wire under the dash and I just twisted it back together for now. If I ground that to the body of the car, it only brings the gauge to half.

 

[GOvert]

The twisted back together wires would be an indication that someone else has had a similar issue that you are having. One positive thing about it all is that the fuel, temp and oil gauge/s all function the same. It would be a little trouble to take your fuel sending unit out to test but if you can, attach the sending post to the water temp circuit and a clip lead from the "frame" of the gauge to battery negative. Have a helper turn on the key to run and watch the temp gauge while you move the float arm from empty to full, pause, then move to half. That would totally rule out a sending unit problem but if you have grounded the yellow wire under the dash already as you have said, then there is a problem past that point. I haven't encountered just the dash gauge part partially working. In my experience, they either work or don't.
When I go to someone that has parts for sale, individually or at a car show etc., I take a 3-volt battery pack with me to test the individual fuel, temp and oil gauges. 3 volts read over half on these old Mustang instruments. I glued two 1.5 c cell battery holders together to make my tester, but an old "C" or "D" cell flashlight could be modified to be a power source. I saw this test procedure in a 60-year-old repair manual.

 

[connella08]

There is a lot of wiring damage in this car that I am slowly trying to fix. It was in pretty rough shape when I got it. But yes, someone clipped the yellow wire under the dash about where the heater box is. If I take the section that goes up to the gauge and ground it out, it only goes to half full. I will try the grounding test today and see what results I get. Maybe thats why most of my gauges don't seem to do much.

 

[GOvert]

Does your instrument cluster have a good ground wire?

 

[connella08]

I think so. my instrument lights appear to be nice and bright.

 

[GOvert]

The most recent voltage regulator/limiter that I purchased for a '69 turned out to be bad. I found a used one attached to the back of some old "loose" instruments and installed it. It worked! These wiring systems from 55 to 60 years ago are not severely complicated however previous owners have sometimes made a mess of things trying to track down a wiring issue or hack into something while adding a stereo or the worst that I have seen is one hacked to install an alarm system. I just had a flash memory of someone installing a new ignition switch connector/pigtail and they wire nutted all of the wires instead of soldering and heat shrink tubing for insulation.
Let us/me know what the problem ends up being.

 

[connella08]

I think I'm just going to pull my gauges and isolate everything. I will put power to it and simulate the sensors to make sure everything works.