# Fuel gauge issues



## jjenkins (Sep 11, 2011)

when i fill up my 67 GTO my fuel gauge only goes to a little more than half tank,and when it gets around an 1/8th of a tank you better find a station or your walkin. not sure if it is a gauge issue or a sending unit issue.assuming it is the sending can this be changed without removing the fuel tank.
thanks for any answer i get


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## jjenkins (Sep 11, 2011)

no one can answer this question?


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## jmt455 (Mar 26, 2012)

You have to drop the tank to remove the sending unit.

Before you do that, check the sending unit ground (should be a black wire screwed to the floor pan, in front of the passenger side gas tank strap, above and behind the differential carrier). Remove the screw, clean the contact area and re-install it; might help.

To verify if the gauge is functioning correctly, disconnect the sending unit's 12V feed wire (tan wire) at the connector just below the trunk lid opening (between the rear wall of the trunk and the bumper). With that wire disconnected, the gauge should go to (or past) "Full".

Then take a jumper wire and connect that tan feed directly to a good ground. The gauge should go to "Empty".

If the gauge does those two things correctly, the problem is with the sending unit itself or the connections/wiring/ground.

You can check proper sending unit function by measuring the resistance between the tan wire and ground with the tank full and empty. Full, you should see 90 ohms between the tan connector and ground. Empty, you should see 0 ohms. 

Some suppliers have been selling new sending units with the incorrect resistance (0 to 30 ohms). That will cause the Fuel gauge to read about 1/3 when the tank is full and it will drop very quickly once it starts to move. You might have one of those stinkers in there...

HTH.


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## jjenkins (Sep 11, 2011)

Thank you


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## GTOBoston (Oct 18, 2012)

Hey,

I have read your threads and I am also having gauge issues, if I unplug the gauge (at the dash) it still reads (F) full. What do think the issue is....
Any help is welcomed

Thanks


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## geeteeohguy (Feb 2, 2008)

Boston, that's normal. You're just disconnecting the same wire at the other end of the circuit. Jjenkens, is your sending unit original or a reproduction? If original, it could be faulty. Most likely not the gauge, since it is registering within its limits and operating. If it's a reproduction, I've read on the "other" forum that there are a lot of repro sending units that have the wrong resistance value, and will not read accurately no matter what.


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## GTOBoston (Oct 18, 2012)

Thanks for the reply even if I disconnect both leads at the gauge ... It reads full
Wouldn't the gauge read empty like on the shelve...
Thanks


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## jjenkins (Sep 11, 2011)

not sure if sending unit is original or not,my guess is it is a repro. as the car was a frame off restore. at this time health issues keep me from doing anything to the car.as soon as i am able i willreplace the sending unit for good measure. by the way what is the proper resistance value.

Thanks,
Jim


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## Eddie.w (Nov 30, 2008)

This thread's been quiet a while but I was having a similar issue and wanted to post my solution.

My gas gauge was always reading full...after reading this thread I troubleshot the wire bundle in the trunk and with the wire hooked to the licence plate light next to it, my gas gauge would drop to empty. Obviously the problem was in/near the sending unit. I dropped the gas tank and trouble shot the wire from the top of the sending unit - it was good.

When I checked the resistance of the plug on top of the sending unit, the resistance only changed when i made contact with one small section of the plug - further inspection showed oxidation on the plug. I removed the sending unit and cleaned both sides of the plug (in the gas tank and sticking out of the sending unit) everything now works properly and all it cost me was some time and elbow grease....

Will let you know if it stops working again....but I'm pretty confident and am going to re-install everything this weekend. Thanks for the earlier threads on this, otherwise I was going to look at spending money that could go to other parts of my goat.


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## Eddie.w (Nov 30, 2008)

Cleaned the gas tank, replaced the strainer sock on the sending unit and re-installed the tank with new hardware and everything works...thanks again for the help!


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## mwatson1647 (Feb 27, 2012)

We have a 68 Convertible, that's sat in our garage essentially unlicensed since 74. The fuel gauge is not working. I did the test that was suggested by removing the tan lead in the trunk going to the sending unit and it appears the fuel gauge is OK. When I jumped the tan lead on the connector going to the fuel gauge to ground, the gauge shows exactly empty. It moved all the way from pegged to empty. When the tan lead from the sending unit is removed from the connector, the gauge goes from empty to full. Actually, it pegs past the full line where you can't see it. That's the same gauge reading when the tan lead is connector. When I tested resistance from the tan lead to ground, I get nothing, (i.e. infinite reading). Is this rather conclusive that the sending unit is bad? Thanks in advance.


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## BearGFR (Aug 25, 2008)

Either that, or it's the wrong sending unit I'd say.

Bear


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## Parkdude65 (May 13, 2020)

jmt455 said:


> You have to drop the tank to remove the sending unit.
> 
> Before you do that, check the sending unit ground (should be a black wire screwed to the floor pan, in front of the passenger side gas tank strap, above and behind the differential carrier). Remove the screw, clean the contact area and re-install it; might help.
> 
> ...


Hello so I have done all the steps that are above and everything checks out near as I can tell. the sending unit measures 15 to 20 OHMS with the tank almost empty. The Tan 12V lead shows it has 8.25 volts on it not sure about that. Any input on this would be great. 
Thanks,
Ron


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## BearGFR (Aug 25, 2008)

GTOBoston said:


> Thanks for the reply even if I disconnect both leads at the gauge ... It reads full
> Wouldn't the gauge read empty like on the shelve...
> Thanks


 No. The difference is, sitting on the shelf there is no power being applied to that gauge. Internally, the gauge is essentially a volt meter. It's reading the voltage difference between the battery (12v nominal) and ground. The sending unit in the tank is actually a variable resistor (like the volume control on a radio). The lower the fuel level, the less resistance it has. The higher the fuel level, the more resistance it has. When the tank is full, it's effectively blocking electricity from flowing from the gauge to ground - and the gauge reads full. When the tank is empty, it's offering very little resistance - so current flows, there's very little voltage difference, and the gauge reads empty. When you disconnect the wire, no current can flow --- so the gauge reads full whenever power is applied to it.

Bear


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## BearGFR (Aug 25, 2008)

Parkdude65 said:


> Hello so I have done all the steps that are above and everything checks out near as I can tell. the sending unit measures 15 to 20 OHMS with the tank almost empty. The Tan 12V lead shows it has 8.25 volts on it not sure about that. Any input on this would be great.
> Thanks,
> Ron


The sending unit in my '69 measures 20 ohms when the gauge shows 1/8 of a tank and I know everything is good on mine, so it sounds like your sending unit is probably ok. There are only three other places the problem can be. Either you've got a failing wire between the gauge and the sending unit that's adding its own resistance to the circuit and causing the gauge to read high (although if it never goes above indicating 1/2 full I kind of doubt that), the gauge itself is bad, or the gauge isn't getting enough voltage to make it read full. If it still only shows 1/2 tank with the tan wire to the sending unit completely disconnected, then it's got to be either the gauge itself or insufficient voltage going to it. The original post on this thread said that the gauge is off "both ways" - it never reads full, and it also never reads empty. If the problem was low voltage going to the gauge, then I'd expect it to always indicate low - meaning that it always reads less fuel than you actually have, so it would show empty before you actually are. That makes my prime suspect the gauge itself. Another thing you can check is to disconnect that tan wire at the tank and connect it directly to a good ground. If the gauge doesn't show empty when you do that (ignition on, engine running) , I'm betting it's the gauge.

Summary... with the tan wire to the sending unit disconnected the gauge should read full. With the end of that tan wire that comes from the instrument panel/gauge grounded, it should read empty. If one or both of those things isn't true, then there's a problem "upstream" - the gauge itself or the wiring.

Bear


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