Fuel gauge sending wire disconnected= normal engine performance?

[2jduenas]

Since I took ownership of my 1998 Chevy Silverado K3500 7.4L 454, the fuel gauge has never worked. I have replaced the fuel pump assembly, including the sending unit, three times. All the other parts I have put into this vehicle have been from the dealership, either AC Delco or GM original. I’ve also had the instrument cluster tested, and it checked out fine. I’ve probed the wiring extensively to ensure there are no shorts, and I even replaced the ECU, thinking it might be a computer issue since the fuel signal runs to the ECM before reaching the cluster. Additionally, I changed the fuel pump relay and checked all my fuses. Despite all this, the problem persists.



Additionally, the truck runs poorly at idle. Interestingly, if I cut the purple sending unit wire at the fuel pump, the truck runs perfectly, though it throws multiple codes, including:

- Ignition on: P0452 and P0463

- Engine running: P0107, P0405, P0463, and P1107



If I leave the purple wire from the fuel pump connected, it will only throw two codes:

- Ignition on: P0463

- Engine running: P0463 and P0102



When I fill the tank up completely, the truck will stall out, but once the fuel is around half empty, the truck runs fine.



I am at a loss and need help. If anyone has experienced this issue and found a solution, I would greatly appreciate your advice. I’ve been dealing with this for a few years now.

 

[Schurkey]

1998 Chevy Silverado K3500 7.4L 454,

the fuel gauge has never worked.

HOW has it "never worked"? Shows all the way full? All the way empty? Needle somewhere in the middle and never moves? Needle moves around at random? Something else?

I have replaced the fuel pump assembly, including the sending unit, three times.

Probably not the sending unit, then.

I’ve also had the instrument cluster tested, and it checked out fine.

Assuming they did a proper job, I guess it's not the instrument cluster, either.

I’ve probed the wiring extensively to ensure there are no shorts, and I even replaced the ECU, thinking it might be a computer issue since the fuel signal runs to the ECM before reaching the cluster. Additionally, I changed the fuel pump relay and checked all my fuses. Despite all this, the problem persists.

So there's no problem anywhere in the fuel gauge circuitry. Very, very odd.

if I cut the purple sending unit wire at the fuel pump, the truck runs perfectly, though it throws multiple codes, including:


- Ignition on: P0452 and P0463

- Engine running: P0107, P0405, P0463, and P1107

If I leave the purple wire from the fuel pump connected, it will only throw two codes:

- Ignition on: P0463

- Engine running: P0463 and P0102

Ok, that's genuinely weird. Trace the purple wire using the schematic in the Genuine GM service manual.

When I fill the tank up completely, the truck will stall out, but once the fuel is around half empty, the truck runs fine.

Verify the fuel tank vapor recovery plumbing including the charcoal canister and the purge valve/plumbing.

 

[docstoy]

As Schurkey stated use service manual 8nformation. Any vehicle that is capable of setting DTC's and being able to be read with a scanner it should be diagnosed with applicable trouble code chart that will lead you to problem. Fortunately we have a source for those hear on the forum. They are located above in sticky threads titled 88-98 service manuals that can be downloaded, you will need to look in the fuel and emissions manual for your DTC trouble codes and electrical diagnosis and wiring diagrams manual for help with locations/routing/diagnosis. There's more than one purple wire in all of our wiring harnesses . Very odd what's happening with yours, but if you look at manuals I'm sure you will be able to locate and repair problem. Best of luck and let us know what you find.

 

[docstoy]

Another thought if someone in the past has replaced a connector and indexed wire(s) in wrong slot in connector that could be problem but your DTCs are trying to tell you something especially by running when fuel tank full that changing resistance on that wire, just some thoughts

 

[2jduenas]

Thank you @Schurkey and @docstoy for the quick reply, first off to answer your question

HOW has it "never worked"? Shows all the way full? All the way empty? Needle somewhere in the middle and never moves? Needle moves around at random? Something else?
Thanks, Schurkey sorry, probably should’ve mentioned that. Yes, the needle I’m sure at one point in it’s lifetime has worked however, at this time the needle appears to be stuck on empty, when the engine is off, and the ignition is turned to the off position, the needle will raise to less than a quarter tank.

Probably not the sending unit, then.


Assuming they did a proper job, I guess it's not the instrument cluster, either.
Supposed to be a pretty reputable spot that I got it inspected I pulled the cluster out and took it to them and they charged me $85 to hook the cluster up to run tests. I sure hope they’re legit.

So there's no problem anywhere in the fuel gauge circuitry. Very, very odd.
Yes, very very odd. I’ve printed out diagrams to follow the sending unit signal and everything seems to check out as far as continuity.

Ok, that's genuinely weird. Trace the purple wire using the schematic in the Genuine GM service manual.


Verify the fuel tank vapor recovery plumbing including the charcoal canister and the purge valve/plumbing. Purge valve/plumbing and charcoal canister good even switched out the purge valve sensor, thinking that wouldn’t really cause the needle to be stuck on empty but only because of how on a full tank of gas, the truck bogs out.

 

[2jduenas]

As Schurkey stated use service manual 8nformation. Any vehicle that is capable of setting DTC's and being able to be read with a scanner it should be diagnosed with applicable trouble code chart that will lead you to problem. Fortunately we have a source for those hear on the forum. They are located above in sticky threads titled 88-98 service manuals that can be downloaded, you will need to look in the fuel and emissions manual for your DTC trouble codes and electrical diagnosis and wiring diagrams manual for help with locations/routing/diagnosis. There's more than one purple wire in all of our wiring harnesses . Very odd what's happening with yours, but if you look at manuals I'm sure you will be able to locate and repair problem. Best of luck and let us know what you find. Yes I’ve downloaded diagrams so as to trace the wires correctly and all the continuity tests check out I’m not the very best with a scanner, but I have sent the truck to two different mechanics who are and this darn truck through them for a loop.

 

[2jduenas]

Another thought if someone in the past has replaced a connector and indexed wire(s) in wrong slot in connector that could be problem but your DTCs are trying to tell you something especially by running when fuel tank full that changing resistance on that wire, just some thoughts.

Yes, I was thinking the same thing since how once the purple sending unit signal wire is disconnected I’m throwing codes for multiple sensors, also the fact that I’m getting high and low circuits is strange and when I have a full tank of gas, it’s adding more resistance, which is probably throwing off one of the sensors, causing the truck to bog out

 

[docstoy]

As probably several here that would like the opportunity to repair your truck, I always like a challenge just sorry that your not alot closer. It's not going to be a 10 minute fix but it can be fixed, it's just how much want to , that you have. Best of luck, be patient and don't try to shortcut the process as is seen now many others have. If any questions just ask. Again good luck

 

[2jduenas]

As probably several here that would like the opportunity to repair your truck, I always like a challenge just sorry that your not alot closer. It's not going to be a 10 minute fix but it can be fixed, it's just how much want to , that you have. Best of luck, be patient and don't try to shortcut the process as is seen now many others have. If any questions just ask. Again good luck

Thank you very much. I appreciate it. I’ll keep y’all posted.

 

[Schurkey]

With most GM vehicles, when the sending unit wire is open--high resistance, broken wire, corroded connection--the gauge goes past "Full".

When the sending unit wire is grounded, (bare spot in the insulation, or some other problem) the gauge goes below "Empty".

But verify with the service manual.