1980 gmc k2500 gauge issues

[2bigredneck]

i have a 1980 gmc truck and i cant figure out why my fuel gauge is not working right. it wants to peg hard to the 3 o'clock position. all the grounds at the harness plug to the cluster are good. i changed the fuel sending unit. every ground on the truck i unbolted and cleaned to bare metal. and reinstalled. with no difference. i also have 3 different instrument clusters and all 3 do the same thing. can anyone give me a direction here going crazy. thanks oh and when i got the truck it only has one gas tank. other is missing

[K20Detroit]

Can't imagine it'd be this simple, but did you try cleaning the contacts on the back of any of the fuel gauges?
On my '84, it was as simple as that. I scrubbed them clean with a brass brush, bent the pins in a bit for tighter contact, and mine has worked good ever since.

[blazer74]

Check your sender wire for a break. 

If your grounds are good to the gauge.

Sounds like no sender input IIRC.

The cluster circuits board could be dirty at the connections.

Also the wires can come loose and push back in the connector not making contact.

Hard to say what's been done to the wiring and selector switch with the tank removed.

There are wire diagrams in the tech section on this site.

Start tracing wires. A multimeter will be very helpful.

[2bigredneck]

the switch to the fuel tanks still there i hear it click when i use the switch so i know that's good..

[2bigredneck]

i put a brand new fuel sending unit in it yesterday

[blazer74]

Pretty sure 3:00 is no input from sender.
Broken or disconnected sender wire.
Sender has a ground wire to the frame.

[blazer74]

Last time I did engine work on my 74 blazer a I left the sender wire disconnected inadvertently and that's what I had 3:00

[blazer74]

Verified on my own vehicle that if I disconnect the tan sender wire the fuel gauge goes to 3:00.

[VileZambonie]

open circuit. Take the signal wire at the sending unit and ground it with the key on. If it goes to empty the problem is from there to the sending unit. If there is no change, check the continuity of that wire to the gauge and if it's good then you have a faulty gauge, if it's open, then you are looking for an open in the wiring

[2bigredneck]

will a 81 to 87 cluster work in my truck??

[2bigredneck]

thanks the two ended plug at the firewall was corroded..