The needle is flipping around like a fish on a hot dock.
Doesn't seem to matter if front or rear.
Goes way beyond full and empty.

It has been bouncing so hard the little cover flew off the pivot and the light shines out at night.

Is this a gauge or wiring problem?
My truck has a mechanical pump so I think the actual switch handles the gauge-->sender connection.
If so, it would make sense that it is somewhere in the dash, right?

 

Does it only happen when you're moving/when the fuel is sloshing around in the tank?

What I'd do is disconnect the level sensor at the tank connection and see what the gauge does then. If it pegs out (I believe it will go past full) and stays there, then you know the problem is with the wiring into/inside the tank or the level sensor itself.

If the gauge still acts up with the sensor disconnected then I'd pull the cluster and put an ohmmeter (preferably analog for it's fast response time) across the signal circuit while leaving the level sensor in the tank disconnected, and see what's going on there. If the ohm reading is steady now then you have a problem with the cluster or gauge itself.

If the ohm reading is all over the place, then you have a wiring problem between the cluster and tank connector.

Do this test on the tank which causes the problem. If it happens on BOTH tanks, then you can almost rule out a problem with the level sensors and signal circuits downstream of the tank selector switch. Not saying they couldn't both fail in the same way at the same time, but it's *really* unlikely. So if it does happen on both tanks, I'd skip testing the tank wiring for now and just disconnect the selector switch. Now put an ohmmeter on the gauge signal circuit behind the cluster. Stable reading = bad gauge. Wildly fluctuating reading = wiring problem between gauge and switch.

 

Happens sitting at the gas pumps.
On both tanks.

I have a Simpson 260!
It's the best for finding shorts and opens when you have to move the harness around for the fault to happen.

 

I believe the fuel gauge switches at the selector valve not at the switch, when you flip to select which tank you want the plunger inside the valve has contacts on it so when it pulls in, it comes of one set of contacts and goes to the other contacts for that tank, so if the valve goes bad and sticks on one tank the sender does as well. So it's either the gauge or in the wiring from the gauge to the selector valve, or inside the valve itself.

 

I have a mechanical pump, not electric.
Switch is different, valve is different.

 

It only happens sitting at a gas pump???

Either way, it should be pretty simple to isolate the problem. Since you have a Simpson 260 then I'd guess you probably have some experience with troubleshooting electrical. Basically just start by isolating sections of the circuit at common failure points, and work your way towards the fault til you narrow it down to one component. Then find out what's wrong with that component (shorted wire, faulty gauge, etc) and go from there.

Not sure how your fuel system selection is done, but when I say disconnect it at the selector switch, I mean disconnect it at whatever point the signal to the gauge is switched.

Good luck, post back if you need more help. I don't have schematics but I can certainly help with troubleshooting.

 

So you just have a 3-port valve with 2 wires going to it?

 

Yes the gauge is selected for the tanks at the switch on the dash.
Sounds like you may have a bad voltage regulator but I was thinking the voltage regulator also feeds the oil and temp gauges.
I would look at the traces on the back of the cluster and maybe change the voltage regulator.

I think it wired like this:





/

 

The ICVR crossed my mind, but the temp & pressure are stable.
I just don't understand the gauge swing to the stop both ways.
Open or short should have it either pinned or below empty.
Why both???

I may just do a SWAD-J anyhow if I have to pull the cluster.

 

No
I have a six port valve.
There are feeds and returns.
Its a two year wonder...

 

it *even* happens while sitting at the filling station.
(Key on, of course)

Gawd I hate this mobile site!