Fuel gauge isn't receiving expected input

[SJSnelling]

1987 GMC TBI dual tanks

Fuel gauge was reading above full on both leaking tanks. I ordered replacement tanks and pumps. Installed the left.

Fuel gauge continued to read above full on both tanks. After viewing a schematic, I traced wires and found two (pink and tan) had been cut in the engine compartment. Cleaned and connected those. Fuel gauge now reads empty for both tanks. If I unplug the right (leaker) tank electrical connector, the gauge shows over full on that tank, and empty on the new tank.

I'm not certain how to proceed with troubleshooting. Any suggestions would be greatly appreciated.

 

[SJSnelling]

With the truck running on the new (left) fuel tank, I switched to the right (disconnected bad tank) and the engine immediately stopped. If nothing else, I believe that shows my tank switch is working...

Next, I repaired the ground wire/connection on the bad (right) tank. The fuel gauge appears to properly indicate empty. Comparing it to when I switch back to the left (new) tank, which results in the gauge dropping to less than empty.

Though I'm making progress, it now seems to be with the wrong tank.

 

[SJSnelling]

Now I have removed the right (rusted) tank/pump/sender, and will install the new one as soon as a replacement bracket arrives. I believe, since my initial posting, I have the right side reading correctly on the gauge.

Now, to plug-away until I figure out the issue with the left (primary) tank/sender. I'm wondering if the sender isn't responding...voltage on both the grey and purple wire when disconnected.

 

[Snoots]

less than empty

How!? What?

 

[1987 GMC Jimmy]

Now I have removed the right (rusted) tank/pump/sender, and will install the new one as soon as a replacement bracket arrives. I believe, since my initial posting, I have the right side reading correctly on the gauge.

Now, to plug-away until I figure out the issue with the left (primary) tank/sender. I'm wondering if the sender isn't responding...voltage on both the grey and purple wire when disconnected.

So you saw that your right sender was rusted? Perhaps the same fate for the left? Ethanol, moisture, and sitting for long periods of time will wreck a sending unit.

 

[SJSnelling]

How!? What?

The left tank, which it and the pump/sender are new, pegs the fuel gauge beyond the empty mark. When I disconnect the left tank plug, it pegs to way above full. The right tank, which I'm in the process of replacing, reads empty as would be expected (needle does not continue to the position shown by the new left tank.

 

[SJSnelling]

So you saw that your right sender was rusted? Perhaps the same fate for the left? Ethanol, moisture, and sitting for long periods of time will wreck a sending unit.

No. Both tanks were rusted and leaking. I replaced the left tank and pump/sender. I am in the process of replacing the right tank and pump/sender. So, the left is already new, yet not reading correctly on the gauge. The right tank was reading correctly on the gauge (based on empty...after I reconnected two wires which had been cut).

At this point, I believe one of two things is happening with the improper gauge reading on the new tank. One, the pump/sender I installed may be hung up inside of the tank, or more likely I have a wiring issue on the sender wire (purple-to-pink).

 

[1987 GMC Jimmy]

No. Both tanks were rusted and leaking. I replaced the left tank and pump/sender. I am in the process of replacing the right tank and pump/sender. So, the left is already new, yet not reading correctly on the gauge. The right tank was reading correctly on the gauge (based on empty...after I reconnected two wires which had been cut).

At this point, I believe one of two things is happening with the improper gauge reading on the new tank. One, the pump/sender I installed may be hung up inside of the tank, or more likely I have a wiring issue on the sender wire (purple-to-pink).

I gotcha. I would agree with your latter hypothesis that it’s more likely a wiring issue. I’ll tag someone who knows the wiring stuff better than anyone and can offer further suggestions. You probably already have this, but I’d continue to follow this diagram and see what’s cut or broken. Looks like there’s at least one other wire in the circuit that could be giving you problems. @chengny

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[SJSnelling]

I gotcha. I would agree with your latter hypothesis that it’s more likely a wiring issue. I’ll tag someone who knows the wiring stuff better than anyone and can offer further suggestions. You probably already have this, but I’d continue to follow this diagram and see what’s cut or broken. Looks like there’s at least one other wire in the circuit that could be giving you problems. @chengny

You must be registered for see images attach

Thank-you for the reply, and the schematic. I've been using a similar one, yet possibly my cranial wet-drive is working better in the morning, than later in the day...after viewing the schematic you sent, and thinking about what I've done up to this point, I'm now wondering if the solution is right in front of me. Since I believe the RIGHT (I incorrectly typed left) pump/sender was reading correctly (empty) before I removed yesterday, the issue with the left tank (new one) should be between the sender and the fuel tank selector valve. I should probably connect my new right tank's new pump/sender (out of the new tank) and operate the sender manually while watching the gauge. If it does in fact then operate correctly, I should be able to isolate where my problem is... (thinking out loud)