Stovebolt.com Forums The Shop Area The Electrical Bay Fuel Gauge wiring

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!