Inaccurate fuel gauge

[jackjeckel757]

Hey all I have searched the forum and can't find an answer for my problem so here goes...
I have a '77 C10 with dual tanks, I removed all the gauges to clean them up and paint the needles which were pretty badly sun burnt. All the gauges returned to working normally again except the fuel gauge. It was accurate before but now it reads about a half tank below what is in both tanks, so the left one I just filled reads half full and the right one which I know is empty reads about a half tank below the empty mark. Is there any way to calibrate the gauge? I have tried removing the gauge, disconnecting the battery and putting the gauge back in on full where I know the tank is and it just goes back down to a half tank lower. Does anyone know how to fix this? thanks.

Yep just realized I posted this in the wrong section...

[DnStClr]

There have been some posts explaining how to test a fuel gauge. One member, Haulin It, had a procedure that was added to the tech section. Search the site for "testing fuel gauge". ( I think) 
http://forum.73-87chevytrucks.com/smforum/index.php/topic,16071.msg125830.html#msg125830

[jaredts]

Your right tank is reading half tank below empty?  Did you remove the needle to paint it?

[bake74]

     That is a very good question, one that I would like to know the answer to.  I will watch this thread to see is someone gives you a solution and weather it worked for you or not.
     Keep us informed on what you find so we can learn something.

[jackjeckel757]

Nope, I did not remove the needle, only the gauge from the cluster, it was a few screws and pull it from the cluster.  I painted the needles on all the gauges in the house and put them back in the cluster. The gauge was accurate before on both tanks, now it still works, its just wrong by about a half tank less then what it should say. The temp, oil, volt, and speedo are all working fine.

[DnStClr]

Unless the gauge is bad, I'd be interested to know if the gauge works again if you remove it and scrape the new paint off the needle. Yeah, I know- it sounds dumb. But I'm wondering if the weight of the new paint is causing the needle to act sluggish. (hey, at least I'm thinking about a solution...)
  The gauge has 2 coils in it, and a small magnet between them. One coil is constant and one is a variable coil. When an electrical signal reaches the gauge, the amount of fuel in the tank determines a resistance to that signal- whether it's a strong signal or a weak one. So the gauge reacts to the resistance.  The magnet and coils then cause the gauge needle to move. If the only thing that has changed is the weight of the needle, it seems to me that the paint might be the culprit. Also, make sure the nuts on the gauge terminals are tight. 

[bd]

The fuel gauge plugs into three shiney metal contacts that clip through the printed circuit and cluster housing from behind.  Gently burnish (rubber eraser or crocus cloth) the PC board and the clips, then slightly bend the clip tangs to increase contact pressure against the PC board and increase spring tension against the gauge pins.

Then with the fuel tank sender unglugged and a ~45-ohm resistor (Yellow-Green-Black) inserted between the wire from the gauge and a good ground, the fuel gauge shoud read ~1/2 tank with the ignition switched on.

[79camper]

I've learned a lot already. My fuel gauge goes to almost empty when I turn the lights on - sounds like I have a contact in the wire somewhere pulling resistance.

[bd]

I've learned a lot already. My fuel gauge goes to almost empty when I turn the lights on - sounds like I have a contact in the wire somewhere pulling resistance.

Check the instrument cluster and dash grounds.