[jims68ragtop]

Hi, I remember having the problem of the gas gauge needle being totally left or right and not working in the past, but I can't remember how I solved the problem. this is a 68 Lemans. It seems like maybe something was not grounded back then. With the gas tank still on the car, I can find one very small ground wire that runs from the top of the tank and is attached to a bracket that holds a vertical vent hose. I removed the wire and cleaned it and the contact surface, but this did not help. Please let me know why the needle won't respond when the car is turned on. Thanks.

[heups1629]

I had this exact same problem for about 3 years (my gauge was stuck on full). I tried cutting the power by disconnecting the line coming out of the trunk - that would make the needle go to empty. Eventually, I figured the sending unit was bad, and a few weeks ago I dropped the gas tank and replaced it. That ended up fixing it. It wasn't nearly as difficult a job as I expected (and sending units aren't that expensive) - took only a couple hours.

[jims68ragtop]

Thanks for the response Heups!

[Greg Reid]

Which way is the needle reading now? Reading full all the time means one thing, empty another. Several other possibilities as well...

[jims68ragtop]

Greg, I have no idea. I can't see the needle even using a flashlight and trying to look at an angle. It is totally pegged in one direction.

[Greg Reid]

It would be helpful if you could find out. Generally, if everything else is working and you short the 12v supply going to the sender to ground, the needle will peg past empty. If you open the 12v OR the ground the needle will float past full.
If you do both of those and nothing happens the next thing to look at is whether you have voltage to the circuit or if there is a short somewhere upstream and finally the gauge itself if all that is good.

[jims68ragtop]

Greg, I really appreciate your help, but I can't see the needle no matter what I try. I even tried to angle my smart phone into the bezel and take photos. The needle is buried very deep to one side or the other. It does not move at all when the key is turned on or the car is started.

I may just need to drop the tank and check out the sending unit. As I stated in the first post, this happened before when I painted the gas tank, but I can't remember how I corrected the situation. It seems like something was not grounded at the sending unit. I cleaned the wire(s) off in that location and the gauge started working. Hopefully that is the problem, but at least I can figure things out once the tank is dropped.

[Greg Reid]

What I'm telling you to do should still help you isolate it to either a wiring problem, bad sender or bad gauge. Have you had someone watch while you either disconnect the ground or short the + side of the sender to ground with the key on? (Just briefly 'brush' the + side wire to ground).
It's basically a single wire circuit with a gauge and a sender in series between the 12v source and ground. Like this-
12v-------(+Gauge-)------(+Sender-)---->>Ground.
Any open or short in that series will cause a peg to empty or a float to full. If the gauge doesn't move no matter what you do, it's likely the gauge.
Your gauge may be shot if it doesn't move after those checks. Gauge is always a last resort for me. I hate pulling instrument panels.

[tekheavy]

Originally Posted by Greg Reid

It would be helpful if you could find out. Generally, if everything else is working and you short the 12v supply going to the sender to ground, the needle will peg past empty. If you open the 12v OR the ground the needle will float past full.
If you do both of those and nothing happens the next thing to look at is whether you have voltage to the circuit or if there is a short somewhere upstream and finally the gauge itself if all that is good.

I didn't think the sending unit had voltage on it at all. If I remember correctly, the sending unit was just a variable resistor reading somewhere between 0 and 90 ohms to ground depending on how much fuel was in the tank.

[Greg Reid]

Resistance doesn't mean anything without voltage/current. That 0 to 90 variable resistance is what determines how much of the current is being passed through the circuit to ground.
It's not going to be the entire 12v as the gauge will have a certain amount of voltage drop but it will have some voltage.

[ponchjoe]

Find the ground wire underneath remove it, sand the contact area and reinstall. When you take the wire off the chassis, have someone watch to see if the needle moves.

[dataway]

At some point in the next year or so I'll be to the point where I'm installing the tank and sender ... would it behoove me to hook it all up before I install the tank? Seems like I could ground the sender, power up the dash and see it works before installation.

[Greg Reid]

You can do that.

[north]

Maybe your gauge needle just fell off?