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Fuel Gauge Wont Go Past 1/3 tank

Richcz28

Richcz28

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My truck has had this problem since I picked it up but I am just now getting into dealing with it. When the tank is filled the gauge doesn't rise past 1/3 full. It goes down with some driving, but I do not know if it is accurate below 1/3. I am thinking this could be because the float has been saturated with diesel over time? This would cause me to think I need to replace the sending unit?

The truck is an m1009 blazer 1986 diesel
 
most likely the sending unit. But check / clean the ground near the fuel tank before you replace.
 
My '83 diesel pick-up did the same thing. Difference is with a pick-up you have two tanks, and the gauge acted the same on either tank. I took an educated guess and replaced the gauge itself. Problem solved.

Rene
 
I removed the ground wire connecting the sending unit to the frame from the frame and turned the key. This should create a full tank simulation on the gauge or possibly past full? My gauge read 7/8ths full. I think my gauge is not working correctly.
 
Removing the ground should put the gauge about an inch past the full mark...basically the needle should be pegged as far as it can go.

Rene
 
Yeah it does not go past full with the ground off but it goes above where it did before. So replace the gauge?
 
Yeah, easy job, they're cheap to find and it's most likely what is wrong. I've had a few of the factory gauges go bad on me, one oil pressure (needle moved, but max OP it ever showed was 10 psi, actual OP was 50 PSI), two voltage gauges (just dead, no needle movement at all) and one fuel gauge now.

It's a lot easier than swapping a sender too...might as well try it first.

Rene
 
Was this resolved?

My truck (also an 86' m1009) has started doing the same thing.
Various troubleshooting guides would lead me to think the gauge is working properly and I have a bad sending unit.
Unfortunately It's been nearly impossible to find a proper replacement, and I'm not convinced that the sender is actually the cause.

It's acting as though 1/3 on the gauge is the full mark.
Going down slowly as I burn fuel.

It can't be a mechanical float issue (such as a damaged float) because on more then one occassion I have observed it return to the proper level while driving for a minute then go back down to the low reading.
For a float to do that it would have to magically regain it's bouyancy, then loose it again.

Corrosion on the sender coil is a possability, but I'm leaning towards faulty wiring or gauge.

I was thinking the issue could be a voltage regulator, but I don't see one, and I can't find any mention of one in the technical manuals.

Ground to frame was filed, cleaned, and solid. Sender wire looks undamaged from the sender all the way to the main wiring harness at the firewall.

Removed the gauge and sanded (more like polished as 2000 grit was the lowest paper on hand) the gauge pins and the connector.

If it is in fact a bad sender It looks like I'm going to have to remove-clean/repair-reinstall, as the GM part number for the sender doesn't match anything but aviation controls. The senders available for the civvy K5's all have the wrong tank capacities and -1 outlet.

Anyone know what readings I should be getting at the three gauge connectors (voltage, resistance, etc.)

I don't recall exactly, but I believe I got something like;
TOP-LEFT
0-volts.........................0-ohms to ground
TOP-RIGHT
(less then batt)-volts......(steady reading)-ohms to ground
BOTTOM
Batt voltage............didn't bother with ohm reading

I havn't re-installed the dash pad yet, I'll be doing some other work before I put it all back together. Taking the gauge out is simple, so if I can check the connection at the gauge for the proper reading that would eliminate everything but the gauge itself, unfortunately I can't find these specs. I'll check it again tomorrow after work (tank should be about half or less) then fill the tank up and check the readings again.
 
I replaced it with a gauge from another truck and it worked fine. I pulled the ground on the new gauge and it went past full.
 
Ive lost two in my twin tank 83 c10. Both did the 3/8 tank as full. With another new-to-me used gauge its working fine again. It was bad when i got it, swapped in a used gauge from an 82, worked for like 9 months or a year then went again. Newused one is working fine so far. Although ive noticed the stock gauge doesnt really move until im in the last 50 miles of the tank.
 
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