Fuel gauge sending wire disconnected= normal engine performance?

[2jduenas]

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 manua

Thank you for that info, I’m going to dig back into the loom this weekend. I’ll let y’all know how it goes.

 

[1998_K1500_Sub]

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

If you leave the gas cap loose, so the tank can breathe, does the problem persist?

^^^ I had this thought after reading @Schurkey's comment about checking the vapor recovery.

 

[Road Trip]

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.


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

Greetings 2jduenas,

The difference between the DTCs reported with the purple wire connected vs disconnected
is interesting, and may give additional insight as to the root cause of the problem. I've
been digging through the FSM, and I have a possible theory, but first I need to know
the answer to the following:

Q: Was your '98 K3500 equipped with dual gas tanks? If so, then I may have a possible
troubleshooting avenue to explore.

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.

I think that you have come to the right place. There's plenty of analytical talent combined with
hard-won experience in here, and if you are motivated to keep one of these GMT400s on the road,
we're motivated to help you get this sorted out.

Please reply back with whether your truck was built with single or dual gas tanks. Meanwhile,
I'm going to get some practice on those DTCs you posted...

 

[2jduenas]

Greetings 2jduenas,

The difference between the DTCs reported with the purple wire connected vs disconnected
is interesting, and may give additional insight as to the root cause of the problem. I've
been digging through the FSM, and I have a possible theory, but first I need to know
the answer to the following:

Q: Was your '98 K3500 equipped with dual gas tanks? If so, then I may have a possible
troubleshooting avenue to explore.



I think that you have come to the right place. There's plenty of analytical talent combined with
hard-won experience in here, and if you are motivated to keep one of these GMT400s on the road,
we're motivated to help you get this sorted out.

Please reply back with whether your truck was built with single or dual gas tanks. Meanwhile,
I'm going to get some practice on those DTCs you posted...

Thank you, @Road Trip I appreciate you taking the time to dig on this issue. This truck does not have dual tanks nor has it ever from what I’m told.

 

[2jduenas]

Thank you, @Road Trip I appreciate you taking the time to dig on this issue. This truck does not have dual tanks nor has it ever from what I’m told.

If you leave the gas cap loose, so the tank can breathe, does the problem persist?

^^^ I had this thought after reading @Schurkey's comment about checking the vapor recovery.

Thank you @1998_K1500-Su I just tried this today and yes, the problem still persist.

 

[2jduenas]

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.

Thank you @Schurkey, If I wire everything according to the ABCD version, using the original square plug as a reference, and set the wires in the new style connector starting from the left when looking at the back of the harness connector (ABCD), the fuel gauge works, but the fuel pump does not.

 

[1998_K1500_Sub]

I’ve probed the wiring extensively to ensure there are no shorts

This thing's an enigma.

That "purple wire" is not what it seems. Something is not right with that (and other, evidently) connections.

You need to dig deeper.

Visually inspect the relevant wire(s) for ANY signs of being changed since leaving the factory, along their entire length.

ANY splices, aftermarket connectors, electrical tape, etc. are an indication that someone's ****ed with the electrical system previously.

You may have two wires connected together somehow, e.g.,

- previous owner's sloppy splicing
- worn insulation allowing two wires to come in contact

Or, you may be the victim of some other, prior changes / "fixes" to the vehicle wiring, e.g., the installation of a car alarm or remote starter.

I would begin by confirming the continuity of the wire(s) (purple, etc.). This can be done any number of ways.

In the past I've used a small, 9V battery to test individual wires. I'll connect the 9V battery "-" to vehicle ground and "+" to the wire under test (e.g., your "purple" wire you've mentioned) and then go to the OTHER end of the wire and verify that IT measures +9V, and NO OTHER wires measure voltage.

 

[1998_K1500_Sub]

By the way, did you say that you have (or have not?) checked the condition / function of all the ground connections...

- ECU ground connections on the engine intake manifold

- body / frame / engine ground straps, found behind the engine on the passenger's side, sort-of between the exhaust manifold and the body

- I think there's a strap near the fuel tank that grounds to the frame, too, IIRC

 

[2jduenas]

This thing's an enigma.

That "purple wire" is not what it seems. Something is not right with that (and other, evidently) connections.

You need to dig deeper.

Visually inspect the relevant wire(s) for ANY signs of being changed since leaving the factory, along their entire length.

ANY splices, aftermarket connectors, electrical tape, etc. are an indication that someone's ****ed with the electrical system previously.

You may have two wires connected together somehow, e.g.,

- previous owner's sloppy splicing
- worn insulation allowing two wires to come in contact

Or, you may be the victim of some other, prior changes / "fixes" to the vehicle wiring, e.g., the installation of a car alarm or remote starter.

I would begin by confirming the continuity of the wire(s) (purple, etc.). This can be done any number of ways.

In the past I've used a small, 9V battery to test individual wires. I'll connect the 9V battery "-" to vehicle ground and "+" to the wire under test (e.g., your "purple" wire you've mentioned) and then go to the OTHER end of the wire and verify that IT measures +9V, and NO OTHER wires measure voltage.

Thank you for your willingness to help.

Yes, I did open all the looms up to the ECM and found a set of wires spliced together near the fuel tank. This included the sending ground signal wire for the sending unit and the tank pressure sensor ground. Initially, I thought this was the cause of all my problems. To verify, I checked three trucks at the junkyard, dropping their fuel tanks to compare their wire harnesses and splices. Surprisingly, I found that all three had identical wiring and splices, with black tape used from the factory.

Despite hoping to find an issue there, everything matched my truck’s wiring. I now suspect you might be right that a connector was replaced and pinned incorrectly. I will follow your suggestion to connect a 9V battery and check if any other wires carry 9V. Thanks again.

 

[1998_K1500_Sub]

I will follow your suggestion to connect a 9V battery and check if any other wires carry 9V. Thanks again.

It’s a start.

This is not going to be an easy road.

Use any similar method… It doesn’t have to be a 9 V battery, it can be an ohmmeter measurement,… any means of confirming continuity on the wire in question and isolation from other wires nearby.

Use whatever method is convenient for you, but I think this level of inspection is the next step.