The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel gauge reads F all the time

The fuel gauge on my 1951 3100 was working fine but now only reads F all the time. I put in a new sending unit and voltage reducer a few years ago and it was working fine until this week. I became curious when I filled the tank and drove it for a week and the needle stayed on full. Any suggestions? It powers up when i turn on the key. The needle points to full. It goes to E when I turn it off.

You may not need the voltage reducer. I'd remove it, re-connect the gauge and see what happens. Second, do you have a handheld electrical tester? If so, you can check the ohm reading of your sending unit (which depending on what you bought could have already gone bad...there is one good sender unit out there and about a zillion crummy ones). Here is how to do that: remove the seat back to reveal the top of the gas tank, disconnect the wire in the center (that's the one that feeds resistance to your fuel gauge). Removing the wire ought to cause your gauge to go full. Connect it to a known good ground and that should cause your gauge to go to empty. Check the resistance between the post where the wire you removed connects and ground. It should be between 0 and 30 ohms. If it isn't, then something has gone wrong with it. Let's start there.

If you don't have a "handheld electrical tester" you can troubleshoot this particular problem with a piece of wire.

I'm assuming the gas tank is 1/2 full.

Attach one end of the wire to a good ground.

Turn on the key.

Touch the other end of the wire to the post on the gauge where the wire to the tank connects.

If the gauge still reads full the gauge is bad.

If the gauge reads empty, remove the test wire from the gauge and touch it to the post on the sending unit.

If the gauge still reads full the wire from the sending unit is bad.

If the gauge reads empty, either the sending unit is bad, or it has lost its connection to ground. Remove the test wire from the sending unit post and touch it to one of the screws holding the sending unit to the tank. If the gauge now reads the actual amount of gas in the tank (1/2 full) the ground for the tank is bad. If the gauge still reads full the sending unit is bad.

Turn off the key and remove the test wire.

usually F all the time is a bad ground

There’s a tech tip on troubleshooting the gas gauge circuit here.

Sorry, I forgot to say it has been converted to 12 volts.

If it still uses a stock 0 ohms empty, 30 ohms full sending unit my troubleshooting procedure using a wire to ground will work, 6v or 12v.

As Bill says the sender is not voltage specific, but the gauge is.

CLICK

Just curious, where does the other end of the grounding wire fasten to?. Oops, just read in the tech tip that it is marked "ground wire to cab". So somewhere in the cab hopefully is the connection.

My 54 has a ground from one of the sender to tank screws ,then a short 14 gauge wire to the closest behind the tank vertical support screwed with a Philips screw.

all my ground wires for the tank(s) go to the frame. But my tanks are all frame mounted.

Same for my '52 GMC

The ground has a pitiful torturous journey back to the battery. The sender has a gasket. The tank may have pads. The body has rubber pads. The motor is mounted on rubber....and sometimes paint along the way. The path: Tiny sender screw threads to crimped terminal to frame. Frame to body. Body to motor. Motor to battery neg post. So there are several jumpers used that jump across the rubber and questionable bolt contact. That's what those braided straps do. Those need to be double checked.

Always test a new gauge and/or sender to each other OUTSIDE the tank before installing. To be sure you get a reasonable reading. Then if you have trouble after intstall, you know its wiring.

I had the same problem after I installed the new free replacement tank last year. Who knows what was different to cause the gauge to read full all the time?

I ran a 12 ga wire directly from the sender to the battery. Problem solved.

Otto, that is a good way to also troubleshoot the problem. I have done that before. It's an OK way to install also. Sender center tap wire is that long, so why not the ground jumper.

By the way, that torturous path I mention is true for almost everything else. The path is even more involved and failure prone for lights.

Otto,
Is that running a wire from the sender's center post to the + post of the battery? Or a wire from one of the sender grounding screws to the neg post of battery?

Negative battery terminal to ground side of sending unit (mounting bolt on sender).

wouldnt it make more sense to ground there at the frame? small self tapping SS screw.
I`m asking cause I'm trying to learn.

It makes "sense" if it works. But if it does not work, and you have read my explanation on the path that the ground has to take, you will understand that a direct wire will fix it or prove it's not a ground problem. There are other methods to troubleshoot (per my link) and member suggestions. The direct wire is a reasonable fix nevertheless.

We all agree that the best idea is to use a backup jumper from screw to frame. The 60-66 trucks have a special tab on the sender to wire directly to cab wall. Chevy started to recognize the problem.

My sending unit had a ground wire going from the sender to the wire support screw under the driver's side of the cab. I restored that connection when I rebuilt the sending unit harness. So Chevy recognized the problem back in 1951.

No, I`m not testing/troubleshooting anything. I just have 3 tanks and 3 sets of tailights that for the ground I used the rail of the frame.
All work fine, just wondering if its appropriate.

I had a very original '52 that came wired just like that. A real head scratcher. Why would the sheet metal on the underside of the cab exposed to the environment be a better ground than the sheet metal at the cab support brace above the sending unit? An additional aspect of the original sending unit ground wire is the additional cost for each truck produced for that long copper wire.

This is all good info for us having tank reading issues to consider. Now as soon as the rains go away giving me time to work on my rig which is also showing a full tank all the time..........

A trade off of a screw connection for 1/2 cent worth of wire. Not like the price of copper today.

Probably would be a better environmental place to have sender ground. Maybe was not accessable, at assy sequence, to install? Just a guess, I don't have any AD's to look at.

Chevy did not delineate that wire on the wiring diagram like they should have if there was a factory wire for ground. They just show a "chassis" ground symbol like all the other components that use intimate contact/touching ground. The only place they show a dedicated wire is at the headlights. To a ground stud. Kevin and Carl do indicate/show a factory jumper wire to ground from sender flange.
(At neg post of battery the don't show a cable but that is obvious).

Chevy and GMC started to show a wire in 60.

No big deal just picky stuff. I have found lots of poor diagram drafting/old convention. The big one, in a diagram, is a curve/half loop as a wire passes over another.

Like with my tail light ground wire(s). There are some small unused factory holes on the rearend frame of my TF trucks that
once I scuffed them and used an oversized ring terminal I ran both ground wires from the lights to the same hole.
I never thought of running separate wires to the front of the truck.

Bought one of these last year, which made adding effective ground locations a one step move with a cordless drill, use std green electrical ground screws…..

https://www.homedepot.com/p/Klein-Tools-1-4-in-High-Speed-Steel-10-32-Drill-Tap-32239/304788331