The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Gauge wiring

My fuel gage kinda works, but doesn't. When I turn on the ignition switch the gas needle moves up about 1/8" and thats where it stays. I know the tank is more than half full. I have checked the gauge wiring and the only thing that looks odd, but maybe it's not, is the fuel gauge power wire is wired to the ammeter. Shouldn't it be wired to the ignition switch, or does that even make any difference?? I know there have been lots of threads on gas gauges etc. but I don't see anything that answers this particular question. Electrical wiring is clearly not my strong suit, but I can learn. I do have a checklist for testing, ground etc, but nothing seems to fall in line yet. This is my 1948 Chev one ton 6 volt 216. Dale

Dale the power wire should be wired to the ignition switch as you mentioned. If that is done and your sender is working correctly (a gamble) all should work correctly.

Float could be stuck, especially if sitting for a while. If hooked like it is now, it is always on, while hooking it up properly to the ignition switch, be worthwhile to check grounds.

Ed

Thanks for the correct way to wire Jon, and I will make that change and see if that makes a difference. You’re right too Ed, the float could very well be stuck since this truck sat for many years in a collection after it was restored.That’s a different problem so I will re-wire first, the lesser of two evils,and then move on from there. Many thanks - I’ll let you know what happens. Dale

Always a good idea to make one change at a time.

After correcting the wiring...

Another possibility with the low reading is that fuel has leaked into the float. You will have to take the sender out and shake the float to see if you can hear gas sloshing around in there. There are plastic replacement floats available if that's the issue.

You can also see work I'm doing on the fuel gauge sender...a 3 stage project: first review of the common style senders, second an improvement on those and third a new design sender which is still being worked on as I write. I have a strong feeling your sender is part of the problem here.

I'm also kind of mentally preparing for that next step if it becomes necessary, and I'm wondering about this: the manual describes the sender unit replacement to include draining and removing the tank from the frame. Is that a standard and necessary safety precaution rather than just remove the sender unit? I've read a lot of threads on this topic and don't recall any mention of that part of the process in previous threads. Maybe I just missed it. While it's not an electrical problem what with the battery disconnected during the process, the manual sure describes draining and removal as part of the shop sender replacement procedure. Thanks for the float replacement idea too - I wasn't aware they were available. Dale

Ah, yours is the 1948, yes? That one won't have the tank mounted behind the seat but on the frame. It has been a while since I've seen one of those and I can't remember whether the sender can be removed without removing the tank.

Yes it is 1948, 3800. I’m not sure either Jon, never having changed one, but the open space between the top of the tank and the bed above is 10”. Is it possible to tip the sender as it is removed or does it have to come straight out. Seems like a lot of room. So is the reason for fuel tank draining and removal in the manual a safety concern, or is it a question of accessibility? Actually - I forgot, it’s a dump bed, so space available to remove a sender is virtually unlimited.

I'd say if it is a dump bed you have it made. But yes, you can tilt it coming out.

Before you remove the sending unit, short the wire that goes to the gauge at the sending unit to ground. No need to disconnect. If the gauge goes to empty, that tells you that the sending wire and the gauge are OK and the sending unit or the ground at the sending unit is the issue.

Got it Phil, anxious to try, but - what is the “ground at the sending unit”? It’s not the sending unit wire to the gauge, or the connection on the sending unit is it? There are no other wires to ground the tank that I can see.

Make sure to secure the bed while raised when you are crawling around in there?

Ed

Your gas tank but more specifically the sending unit needs the ground
When the gas tank moved into the cab the ground wire goes from one of the sending unit screws back out under the cab with the sending wire, the ground wire screws up onto the underside of the cab about under the drivers butt.
There should be an equivalent ground for your under box tank.
If not add one
But first do that experiment phak1 suggested above
-s

Maybe this diagram will clear it up. The jumper is only a test. If the gauge goes to empty while performing this test, the gauge and sending wire are good and it’s the sending unit or the ground thats the problem. The jumper goes from the terminal on the sending unit to a “known” good ground, essentially bypassing the sending unit. Don’t assume a ground is good. Go back to the battery if you have to as it’s essential to have a good ground. Next remove the jumper from the terminal that goes to the gauge, keeping the jumper on the ground and touch it to the body of the sending unit. This test will bypass the ground wire checking whether you have a good ground to the sending unit. If the sending unit is good, it should show the actual level of the tank. If it acts like it did originally, it’s time to remove the sending unit. Let us know your results and we’ll tell you where to go from there.

I think I have covered all of the bases now, and I am still at the same place: the gauge comes up just about a 1/16" when I turn on the ignition = empty. Specifically: 1) I have grounded the gauge as Phaq1 has suggested, no change when I turned on the ignition, 2) I Attached a Ground cable to the sending unit and grounded it to the base of the cab as Stan indicated, no change when I turned on the ignition. (this was new as there was no ground wire at all before) but still no change. Checked and tightened all wiring to the instrument panel including adding a bolt on the instrument cluster housing that was missing. Pretty sure everything is now as it should be, but no change in the fuel gauge response.

So, it looks like, based on all of your help in diagnosing, the sending unit seems to be the culprit? I shall order it post haste and reply again after I have received and installed. Thanks for all you help so far! Dale

Testing so far hasn’t ruled out anything. The tests I recommended were simple, easy to do and required no disassembly. The next diagnostic steps will require some disassembly. When you grounded the sending unit wire you basically sent the signal that the sending unit would send with an empty tank. If the gauge didn’t go to empty, you have other issues. Could be a bad gauge, a bad ground inside the gauge cluster or faulty wiring, gauge or the wiring going to the gauge including the wire from the sending unit.

The first thing to do is recheck the wiring. Make sure you don’t have the wires switched on the gauge because it would destroy the gauge requiring replacement. I’m posting a picture of your wiring diagram in case you don’t have one.

I assume you corrected the power supply wiring to the gauge you initially said was coming from the ammeter. Check over the wiring and compare it to the diagram, then we’ll go from there.

Is the wiring diagram the one you posted in your earlier post Phil? I think I have the wiring pretty much covered based on that test diagram you provided. The power is wired from the ignition to the gauge and to the right post in the gauge. The gauge does have the other leg labeled for only the sending wire, and that appears to be correctly attached. About the only thing I haven't done is a continuity test on the sender wire, but I''m not sure how to do that to make sure that wire is good. It looks fairly new, but it was strung across the bottom of the truck, hangs over the driveline and then disappears into a wrap of cables running along the frame. I'm just hoping it's the right one coming into the cab. I have a voltage meter so I'll read that book and see how to do the continuity test. Grounding of the instrument cluster - is that accomplished when the cluster housing is bolted to the dashboard? I don't see any other form of grounding on the wire side of the cluster. Still looking --

Sorry, I failed to post it. I fixed it in the original post.

The next steps are as follows:

* Test for 12 volt supply on the gauge (the one from the ignition switch) if the gauge works, fix the supply wire.

* Do a continuity test on the sending unit wire to the gauge. If the gauge works fix or run a new wire.

* Run a jumper wire from a known good ground to the back of the gauge cluster to insure the cluster has a good ground to the dash. If you gauge works you’ll need to clean up the area where the cluster touches the dash.

* Unfortunately the next step is to remove the gauge. This requires you to remove (or drop) the gauge cluster which houses the fuel gauge. I was able to lower mine sufficiently in order to remove the gauge. The bezel is crimped in a few spots which holds the assembly together. Using a small screwdriver, carefully pry open the crimps. Once the bezel, glass and inner trim are removed the gauge is visible. To remove the gauge, the adjacent gauges need need be loosened. There are two tabs (visible in the picture) that are behind the adjacent gauges and will prevent you from removing the gauge. Once loosened, you can remove the gas gauge. There are a few things to check. At this point, use the battery (or an external battery such as a motorcycle,tractor, auto, etc) so we eliminate any bad ground issues.
First let’s check if the gauge functions properly. Connect a jumper from the negative side of the battery to the sending unit terminal (the one with the red warning label). Connect a second jumper from the negative side of the battery to the gauge ground (shown in the picture). Connect a jumper from the positive side of the battery to the power supply terminal. Your gauge should go to empty. If it does not, the gauge is bad.

If your gauge checks out good, look at the back of the gauge where the gauge ground makes contact with the gauge cluster frame and clean/sand the contact area, insure there is no paint, rust or anything else inhibiting a good ground contact. Reinstall the gauge back into the cluster frame and tighten the nuts. Re-perform the gauge test but use the gauge cluster frame as the gauge ground. If the gauge now test good it’s time to reassemble. Tighten the adjacent gauges to the cluster frame.

There’s one more thing we need to check before we reassemble. Some of the reproduction gauges are made slightly thicker and the needle gets caught on the inner trim piece inhibiting the needles movement, making the gauge look like it’s not functioning. If you find this condition, a gentle push with your fingers will give you the additional clearance you need to keep it from binding. Mind you I do mean gentle as it moves easily and you don’t want to distort the inner trim piece. Once you have determined the needle moves freely, reassemble the rest of the cluster. The first time I installed the bezel, I used a screwdriver and a small hammer to crimp the bezel to the cluster and I felt it jared the cluster too much. The second time (yes, it took two times to get it right) I was able to use a needle nose pliers to get the crimps in. Protect the chrome from getting marred. Maybe so other “Bolters” have some other ideas on how to crimp the bezel on to the cluster. Now reinstall the cluster and hopefully it works!

* At this point, we have eliminated everything but the sending unit. If your gauge still does not work, remove the sending unit from the tank. Be prepared to buy or fabricate a new gasket, as it’s usually stuck to the tank. First, shake the float to see if it has been leaking. If there is any gas in it, it’s shot and will need to be replaced. We’ll test the sending unit first to see if we replace the whole unit or just the float. To test, use a multimeter and set it to the lowest ohms scale (mine was 0-200). Place the positive (red) lead on the terminal that goes to the gauge and the negative (black) lead on the sending unit body. Now move the float. At empty (float all the way down) it should read 0 Ohms (or close to it). At full (float all the way up, it should read 30 Ohms (or again close to it). If the unit shows not reading or it is erratic, you will need the replace the entire sending unit. If the unit works and the floatis the culprit, you can buy a new plastic float from eBay. Look for one that fits a Jeep, 1955-1983 CJ5, 1955-1975 CJ6, 1976-1986 CJ7, 1981-1986 CJ8.


Hopefully, at this point, your issue is solved!

One other simple test is to remove the wire that goes to the sending unit at the connection on the back of the gauge.

When you turn the key on the gauge should read full.

If it does not there are 4 areas to check.
- Is there a full 12 volts at the gauge? I assume there is some voltage because you do get the 1/8 reading.
- Ensure that the gauge is grounded properly through the instrument cluster to the chassis. You can check that with a jumper wire connecting the back of the gauge housing to the chassis.
- The needle is sticking against the bezel because it is a poorly made reproduction unit. If that is the problem follow Phil's instructions. I fought that same problem last year on a '49 3100.
- The gauge is defective. If you can find a NOS gauge it will avoid the aggravation of the needle sticking. Some people have removed material from the gauge bezel so the needle has room to move.

I confess - I jumped the gun and installed a new fuel tank sender today hoping that would solve the problem. It did not! My bad.

Went back to basics: wiring, first looking again at the ground wire, which I re-wired again, and looked for and found a different place on the tank to try for a ground. Cleaned the area around one of the screws to the gas filler pipe, a last ditch effort, made the connection, and viola, the gauge now registers 1/2 full and goes to empty when I turn the ignition key off --- just like it's supposed to. Lots of learning in this process, and I went back and read again the many times that many of you stressed: the ground is often the prime suspect.

My many thanks to all of you who have given so much good testing advice to help me get to here. I am very pleased to have a working gas gauge (without having to replace the gas gauge!).

Dale

I’m glad that you have solved the issue. Keep the sending unit as now you have a spare. Those overseas repop’s have a high failure rate!