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Discussion Starter #1
So since I bought my truck, the fuel gauge has never worked right. The needle moves a little, but it never really goes above 3/4s or below halfway, regardless of what's in the tank. I put a new fuel pump & sending unit assembly in it last summer and it didn't really change anything. I've just been filling it up regularly and ignoring the gauge, but it'd be nice to actually make it work.

How would I go about troubleshooting it? Is it more likely to be a bad gauge, bad grounds, or something different?

The truck is a 1990 S10 Blazer, 4.3L Auto.

Thanks.
 

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Since you have already replaced the fuel sending unit then that pretty much leaves the dash gauge or wiring. Assuming the wiring is okay the fuel gauge should move all the way one direction if the wiring is removed from the sending unit. Then move all the way the other direction when fully grounded. That will check the gauge. S10 trucks are notorious for the fuel gauge not being accurate.
 

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Discussion Starter #3 (Edited)
Do you check that with the key turned to the first position, or will the gauge move even with no key inserted? I'm a little wary of messing with 'hot' wires (especially around the fuel tank), but if it's not powered then I don't know if it'd do anything.

I remember the wires from the pump clipped onto the frame next to the bumper, but I don't recall the condition of the ground there. I'll check that again while I'm at it.
 

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Gauge only moves when the key is in the on position.
 

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Discussion Starter #5
So with the key on the first position, disconnecting the frame ground should make it read as empty, and disconnecting the pump/sending unit should make it read as full?
 

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With the key in the run position, disconnecting the frame ground should make it read either empty or full. I forget which way the gauge goes but yes. That bolt that holds those ground wires to the frame inside the left rear rail has to be clean and tight as that is the fuel tank, pump and gauge grounds.
 

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Discussion Starter #7 (Edited)
I finally got a chance to take a look at it today.

First I unbolted the frame ground, made sure it wasn't touching any metal, then turned the key. The needle didn't move. The threads on the frame were a little crusty and so was the bolt, so I cleaned them up with some sandpaper and a wire brush before bolting it back down.

Next I disconnected the wiring for the pump/sending unit, and when I turned the key, the needle went off the top of the gauge and disappeared. I reconnected it and tried again, and it slowly came back down to rest around the halfway point.

What do you guys think? I have absolutely no idea how much gas is actually in it, so it could be reading accurately now I've cleaned up the ground, but I thought it was meant to show empty when the ground was disconnected.
 

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Since yours is a 1990 it uses the older style 1st gen fuel gauge. 0-90 ohms. If there is an open circuit (power or ground) the gauge will see high resistance. ∞ is extremely high, so it will read over full. If a wire is shorted, 0 ohms resistance, it will read below empty.
Staying at 3/4's makes me suspect a stuck float or defective resistor in the tank. Especially since the gauge responded to being disconnected.

The 98 up GM models use a 40-250 ohms, but still would test the same for open or short.
Only Fords work the opposite since they're 73-10 after 1986.
 

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Discussion Starter #9
I put a new pump and sending unit in it about a year ago, and I had the same problem before that, so I would hope it's not a stuck float on both the new and old units.

When I fill it up at the gas station, I usually fill it until the pump shuts off, and it'll read around 3/4s with the tank full. It will move down to about the halfway point as it uses the fuel, but never displays any lower than that.

I'll drive it for a little while and see if cleaning the ground up has helped any, either way. Thanks for the input.
 
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