troubleshoot fuel gauge

[70chevlow]

Ok so im having trouble with my fuel gauge . It partially worked before i had to cut the wires to install some seatbelts due to the previous owners choice to run both connectors through the original seat belt mounting hole. Any way the tank is not located in the cab it is under the bed. The previous owner had ran the ground from the sending unit all the way into the cab would it be better to cut it back closer to tank and ground it inside the frame rail? Next question i tried to do this and the gauge now does nothing when turning the key or when truck is started its not until you put it in gear and drive a little bit that the gauge begins to work but it moves all over the place what would cause this?

[FRANKENSTEINC10]

The wire that comes from the fuse box to the tanks sending unit is a ground wire. If that wire is not connected to the tank and the fuse box then the gauge will not be grounded.
I'm sure someone with more knowledge than me will weigh in with more options

[MARKDTN]

If it is not too difficult, I would attack this in several steps.
1-I would do a good ground to the frame. Grind paint off a place around a frame hole, solder on an eyelet and bolt on the ground using a stainless star washer, nylock nut, and bolt. Paint over with battery corrosion inhibitor or something similar so that you don't build up corrosion later.
2-Test the wiring from tank to cluster. You should have no resistance and no short to ground.
3-Test the sending unit. Should be 0 ohms empty and 90 ohms full with no dead spots along the way.
4-Test the gauge. If you ground the input wire it should show empty. If you take it out of the tank, but leave wiring connected you can test various positions of the float and how the gauge reads.

Here is a good article also.

http://www.chevyclassicsclub.com/the...ry-1965-newer/

[70chevlow]

Ok i suppose im going to have to get a multimeter to test this thing. I grounded it to the frame now instead of the needle going past full it goes to empty with truck off when i turned it on no response and did this a few times to see if it responded and there was nothibg i than started the truck and nothing no response from gauge then i put truck in reverse and the gauge started fluttering from 1/8th tank to 1/4 after it bounced back and fourth i turned the ignition off and removed key the gauge continued to bounce back and fourth with key removed and ignition off what would cause this?

[PowerdbyChevy79]

definately check the wire.....you said it worked partially before.. well to me that says the wire is bad..but of course make sure everything is grounded...tank to cab, cab to frame and so on and so on....