1975 fuel gauge reading 3 oclock position

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fiatplus

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Aug 2, 2023
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Location
cali
First Name
chris
Truck Year
1975
Truck Model
c10
Engine Size
454
I own a 1975, the insurance company says it's a C10. Dual side tanks. As it sits a lot, I have had reoccurring rust in the fuel system issues for 35 years...
It recently started leaking fuel at the gas station when filling the tanks, so I concluded the rust had come thru the top of the tanks. Ordered new tanks, painted and sealed them, made sure the right tank was as empty as possible, then pulled the right tank.
The only rust thru was from the straps trapping moisture. And it was surprisingly clean inside. But I did discover the plastic "vent" on the center hard line of the tank sender was just an open hose, so I found my leak. Fuel lines 48 years old and showing it, so as I was tracing where the right tank hose attached to the tank solenoid, the plastic nipple fell off.
I got a new solenoid, and it's a five wire six terminal one. The old one was a one wire. I traced all of the wiring, and using the original three wires I installed the new solenoid. I then drained the left tank, replaced it and the hoses (discovered the same "open" vent on the middle sending unit nipple), connected everything, and the solenoid refused to swap between tanks.
I discovered voltage drops under load of five and eight volts from the new switch in the front and the solenoid harness, so I replaced all three wires. Now the solenoid swaps between tanks!

But, the fuel gauge now reads above full at the 3 oclock position. Swap tanks from an almost full one to an empty one and it moves, but not much. Ran grounds, jumper wires, tested continuity. Everything says the gauge should work, but it refuses to move back to where it has always been before I swapped all this tank stuff.

As I consider the fuel gauge as the most important gauge in the truck, what do I do next?
I am fresh out of ideas...
 

fiatplus

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Location
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chris
Truck Year
1975
Truck Model
c10
Engine Size
454
Sounds like you got the wrong ohm fuel level sender
Thank you for the suggestion, David.
Same senders as before, same gauge as before.
I got help finding wiring diagrams for early 1975, and now I suspect that I might have blown the fuse that powers the gauge. As all fuses test good under load, I am now wondering about the gauge. I need to spend more time with diagrams to find out where pink 39 gets power from. It says IGN on the aux fuel tank diagram... https://xwebforums.com/forum/index.php?threads/1975-chevy-c10-truck-fuel-gauge-issues.44262/
And tan 30, which changes to 18tan 30a should be providing power (as a ground) to the tank sender. It's dead at the control switch (terminal 4). I may need to pull the dash apart.
Or, I "rent" a sender from the parts store and play with the system manually.
 

Jgonick

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Location
West Texas
First Name
Joel
Truck Year
1985
Truck Model
K10
Engine Size
350
Usually if the gauge moves past three the gauge is getting current but not getting resistance info from the sender. First thing I would do is make sure the sender was grounded well. Not sure about 75- It might be grounded by the fuel tank being grounded so check the fuel tanks are grounded well to the chassis.
 
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