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Fuel gauge still pegged at 3 o'clock after sending unit replacement

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dj31499

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Just replaced the tank, straps, sending unit and all fuel lines on my 89 TBI Jimmy. The tank isn't fully in yet, still have to hook up the evap line and filler hoses, but I wanted to see if the gauge would move if I plugged in the sending unit and ground wire.

Some background info; when I got the truck, the fuel gauge was pegged at 9 o'clock, way past empty. Took the gauge cluster out and found out the circuit board was fried on the return connection from the fuel gauge. Once I replaced it, it moved all the way over past full, around 3 o'clock.

From there, I figured it had to do with the sending unit or the wiring to the unit. Found the ground bolt and wire from the sending unit, sanded the frame down around the bolt hole and still no luck. When I dropped the tank and saw the sending unit, the ground wire was pretty corroded at sending unit end. Figured that was the problem.

So with a new ground bolt and new sending unit, still having the same issue. I know there is a pink wire that comes out of the connector where the sending unit plugs in and runs down the frame to the front, but I lose it right near the front door and rocker, maybe that's the issue? Also read online somewhere that the fuel system and oil pressure switch are somehow connected; my oil pressure gauge always reads high (almost always between 45-60 psi, no matter what), wondering if that has something to do with it. Can't find any info in the 87-91 service manuals on how to troubleshoot this.
 
If the fuel pump runs then the ground wire is fine since they both share it. If you disconnect the wire harness plug at the fuel tank the gauge should peg to the 3 O’clock position. If you short the gauge wire (whatever the purple wire on the sending unit side of the harness corresponds to on the gauge side of the harness, grey wire is power for the pump) to ground, then the gauge should peg to empty.

That should help us narrow down where the problem lies.
 
rampage said:
If the fuel pump runs then the ground wire is fine since they both share it. If you disconnect the wire harness plug at the fuel tank the gauge should peg to the 3 O’clock position. If you short the gauge wire (whatever the purple wire on the sending unit side of the harness corresponds to on the gauge side of the harness, grey wire is power for the pump) to ground, then the gauge should peg to empty.

That should help us narrow down where the problem lies.
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Alright I’ll be sure to check that out once the weather clears up here. Just want to make sure I got this right: probe the engine end of the harness that connects to the purple on the sending unit, and ground it to the frame or something? Also should this be done keys on engine off or completely off?
 
Correct.
Can be done keys on engine off or started, doesn’t matter since you’re just simulating the resistance reading of the rheostat in the sending unit.

Also, I was hoping you’d answer my implied question earlier. Do you hear the fuel pump run with the key on or does the truck run? Looking to make sure the ground wire works.
 
rampage said:
Correct.
Can be done keys on engine off or started, doesn’t matter since you’re just simulating the resistance reading of the rheostat in the sending unit.

Also, I was hoping you’d answer my implied question earlier. Do you hear the fuel pump run with the key on or does the truck run? Looking to make sure the ground wire works.
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Yeah the truck runs, heard the pump hum on for a couple of seconds when I hooked it up today
 
rampage said:
If the fuel pump runs then the ground wire is fine since they both share it.
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Going to have to disagree with you on this, at least on my 89 Jimmy, my fuel gauge has not worked since I have owned it, and about 3 years ago I decided to check the sending unit ground to see if that could have been the problem and after trying multiple ground locations on the frame and the gauge still not working, then dropping the bolt into an unreachable crevice of the frame, I just left the ground unhooked, and my fuel pump is still working just fine.

To answer the original question though, in my experience, when the gauge pegs out past full, it indicates a problem with the wire that runs from the gauge to the sending unit, the one that is usually purple or pink.
 
76k5blazerr said:
Going to have to disagree with you on this, at least on my 89 Jimmy, my fuel gauge has not worked since I have owned it, and about 3 years ago I decided to check the sending unit ground to see if that could have been the problem and after trying multiple ground locations on the frame and the gauge still not working, then dropping the bolt into an unreachable crevice of the frame, I just left the ground unhooked, and my fuel pump is still working just fine.

To answer the original question though, in my experience, when the gauge pegs out past full, it indicates a problem with the wire that runs from the gauge to the sending unit, the one that is usually purple or pink.
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Then you must not have the anti-squeak pads on your tank straps. The tank is electrically isolated by rubber hoses and the anti-squeak pads, that’s why there’s a need for the ground strap. And there are only two wires going to the tank, 12V for the pump and the sending unit wire. I have an ‘89 Jimmy also.
 
rampage said:
Then you must not have the anti-squeak pads on your tank straps. The tank is electrically isolated by rubber hoses and the anti-squeak pads, that’s why there’s a need for the ground strap. And there are only two wires going to the tank, 12V for the pump and the sending unit wire. I have an ‘89 Jimmy also.
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My new tank didn’t come with the rubber matting and the one on my old tank is pretty rotted. Is that going to be an issue?
 
dj31499 said:
Also read online somewhere that the fuel system and oil pressure switch are somehow connected; my oil pressure gauge always reads high (almost always between 45-60 psi, no matter what), wondering if that has something to do with it. Can't find any info in the 87-91 service manuals on how to troubleshoot this.
Click to expand...

I *think* on your vintage truck the oil pressure switch and oil pressure sending unit are separate pieces. I want to say '90 is when they went to the three wire combination setup. I've never dealt with one of those vintage trucks, so I'm not sure where the sender/switch setups were located, but I would assume as earlier either/and above the oil filter and to the drivers side of the distributor. Three wires going to one fairly long cylindrical unit means it's the combo switch/sender, one wire to a bell shaped piece is a sending unit, and two wires on a short cylindrical unit will be the switch assembly.

Either setup can have a fault with the sending unit/sending unit portion, easiest test you can do is to ground the sender wire, then leave it open and see if the gauge responds properly (maxes out one direction, then the other). If the gauge responds properly, everything is working right except the sending unit.

The oil pressure switch is ONLY a redundancy for the fuel pump relay. When working properly, the truck will start and run with either one disconnected, however with the relay disconnected, it will take extra cranking to get the switch to close and activate the fuel pump.
 
dj31499 said:
My new tank didn’t come with the rubber matting and the one on my old tank is pretty rotted. Is that going to be an issue?
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The anti-squeak pads? You can buy them from some place like LMC, but they’re basically the same thing as heavy roofing felt. Or you can use something like a spare inner tube. I used some shower pan liner I had leftover.
 
rampage said:
The anti-squeak pads? You can buy them from some place like LMC, but they’re basically the same thing as heavy roofing felt. Or you can use something like a spare inner tube. I used some shower pan liner I had leftover.
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I'll check to see if anyone's got it. Is it necessary though? Will it cause an issue with signal to the pump/sending unit?
 
No signal issue, but it’ll probably rattle and make noise, hence their name.

I don’t know if it’s such a good idea to have metal rubbing on metal though, especially when the tank has a corrosion protective coating on it.

Just find some house that’s having their roof torn off and ask if you can take some felt, double it up if it’s only 15# felt.
 
dj31499 said:
I'll check to see if anyone's got it. Is it necessary though? Will it cause an issue with signal to the pump/sending unit?
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I would assume under the proper conditions it could wear through the tank and/or help the tank/straps to rot. The tank should be galvanized, if the galvanizing gets worn through, it will rust. I painted mine on top of using the roofing felt I had laying around to make new "pads".

Rampage beat me to it. ;)
 
Old bicycle inner tubes work well for the strap insulators..
A few layers of duct tape works good also..

However,I've noted having rubber against steel often ends up letting water get trapped in between and rotting things out there--like where the rubber body mount bisquits sit on the frame brackets,and your rim beads ..some grease or silicone grease might help prevent that,or a bead sealer type stuff..

I doubt the tank gets "isolated" by having felt or rubber on the straps,because the straps have threaded rod ends to tighten them against the tank brackets,which in turn bolt to the frame..it might not be a great ground,but grounded none the less..the ground wire at the sending unit is just double insurance it'll be grounded good..
 
rampage said:
No signal issue, but it’ll probably rattle and make noise, hence their name.

I don’t know if it’s such a good idea to have metal rubbing on metal though, especially when the tank has a corrosion protective coating on it.

Just find some house that’s having their roof torn off and ask if you can take some felt, double it up if it’s only 15# felt.
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Alright, picked up a roll of #30 roof felt today, would wrapping the straps in it be good? diesel4me brought up a good point of water getting stuck in between the felt and the tank. Wrapping the straps in it should solve both problems right? Dampening and water lock up? Also, how'd you go about adhering it to the tank or straps?

Tested the pink wire from the engine to sender connector, grounded it with key on and the gauge moved. Here's a link to a video I took:

So what's the next step rampage? :haha:

Thanks for all your input everyone, really appreciate it.
 
I went batty trying to get my gas gauge working--truck had factory dual tanks,and the previous owner butchered the wiring badly,they left the switch in the dash unplugged,tore out the electric fuel solenoid,and put a manual valve in between the tanks instead..
Since one tank leaked (drivers side) I just removed it (I only need one tank,I go nowhere long distance) and had to hunt down a wiring diagram to figure out which wires to jump at the dash switch plug to get the gauge to read..I just ran it off the passenger side tank..

All was fine till that tank started leaking a few years later.
I decided to put another good used tank back on the drivers side,it was easier to fill on that side..I had to extend the wires from the passenger side tank to reach,the factory wires on the drivers side were junk,so the gas gauge would work..

About a month later--the sending unit decided to :poo: the bed,now it just pegs the gauge at 3 o'clock like yours..:mad:

I happened to have several good used ones from gas powered trucks,and one brand new one,but they are a bit different than a diesel--a diesel has a water sensor built into it and and 2 more nipples,one for the return line and another for a water drain..

To test if it was the sending unit,I was able to unplug the one on the tank (just enough room between the stepside bed & cab to get at it,not easily),and I plugged in the known good unit,turned on the key,and moved the float up & down,and the gauge responded..so I know the sender is NG..I could probably swap just the rheostat onto the old unit,but I only want to take the tank down once..

I just started writing down the mileage to keep track of when to fill it up again--and now 3 years later--that tank is leaking under the strap..so I'll have to replace the tank and sending unit..

This helped me quite a bit when I was diagnosing my gas gauge troubles..

Troubleshooting Your Chevy Truck Gas Gauge Failure - Chevy Trucks Blog
 
dj31499 said:
Alright, picked up a roll of #30 roof felt today, would wrapping the straps in it be good? diesel4me brought up a good point of water getting stuck in between the felt and the tank. Wrapping the straps in it should solve both problems right? Dampening and water lock up? Also, how'd you go about adhering it to the tank or straps?

Tested the pink wire from the engine to sender connector, grounded it with key on and the gauge moved. Here's a link to a video I took:

So what's the next step rampage? :haha:

Thanks for all your input everyone, really appreciate it.
Click to expand...

No need to wrap the straps with the felt. It would most likely just fall off eventually. Just cut a strip slightly wider than the straps, about 1/2” should be good. You can keep it held in place while installing the tank by using some spray adhesive on the straps. It’s been 20 yrs since I did mine, but IIRC you place some on top of the tank also, someone else who’s done it more recently will know.

So it seems like everything from the plug to the gauge is in working order since it’s responding. I take it you grounded the wire several times and then one long one at the end since the gauge isn’t instantaneous and responds slowly on purpose.

I know you replaced the sending unit with a new one, but that doesn’t mean it’s working properly. If the delicate wiper arm on the rheostat got bent during handling (not just you but the shippers also) then the gauge will peg at 3 0’clock. Only way I know of to test that is with a multimeter. Depending on how much fuel you have in the tank then the meter will read anywhere between 0 and 90 ohms. You do have a multimeter and know how to use one, right?

And one other thing I forgot to mention after re-reading you first post - put some dielectric grease under the ground strap at the frame to help make a better connection and prevent corrosion.
 
I want to say that yes, there are felt strips at the front and rear top of the tank, where the curved metal locates the tank in the frame.

Can't view the video here, but is the tank bolted up? Or is it easy to pull the sender out to test the resistance with a meter as you move the float arm?
 
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