OK I'm stumped..

[clinebarger]

Originally Posted by kris's66lwb

thanks tons cline but one question is the fuel tank slector and meeter valve under the bed with the lines connected to it?

Yes

[leegrady]

Thanks big time for the schematic on the twin tank set up Clinebarger! My brand new switch at the dash does nothing. It was missing when I bought the truck. Now I can trace the circuit. Most likely the selector valve is stuck.
Thanks again.

__________________

Bone stock 81 reg cab GMC K2500 longbox: 350/sm465/Dana 60/Corp.14 bolt

[kris's66lwb]

ok thanks on that i have this stupi relay under the dash it is on the left top side of the fuse block it is a 4 pin connector a straigh inline 4 pin relay and i knw it is bad and have benche tested it and i have tryed to find a replace ment i have been unsuccessful ive been told that its been discontenued and i have even pulled out the relay and got the only #s off it i could called every chevy dealer in town and every parts counter laughed at me and told me that 1 they dont have it or two theres too many # numbers to trace i just need to know what this relay is exaxtly for and the diagram to go with it im sure it controls the fuel servo ive been told that by other members with the same problem it seems to only be on 1980 chevy half tons and more commonly 1980 big tens when this relay is diconnected the guages go all outta wack but with it pluged in the right tank feeds and reads semi accurately but the left tanke reads wat past full and seems not to feed i have not tryed anything else tho so maybe im chasing my tail on this any help is very much appreciated

[78-5.3]

Originally Posted by clinebarger

Yes. This helps narrow down where problem is.

ok i disconnected the connector looked at the fuel gauge in the truck and it pops up too 1/4 fuel and stops i ground the spare battery to the frame for a good ground hook up the wire and the spare cluster goes straight to full pegs over ??
i have no clue why???

[clinebarger]

Originally Posted by 78-5.3

ok i disconnected the connector looked at the fuel gauge in the truck and it pops up too 1/4 fuel and stops i ground the spare battery to the frame for a good ground hook up the wire and the spare cluster goes straight to full pegs over ??
i have no clue why???

Has the fuel gauge ever worked? The first test is pointing toward a bad gauge.

On the second, did you have the sender unhooked? if you did what happened is normal.

Im wondering if the gauge was already bad but now you have a bad sender in the 87 tank.

Ok, lets try somthing else. The ohm range of the sender is 0 to 90 ohms of resistance to ground 0 ohms is empty, 90 is full, 45 ohms is 1/2.

Take a volt/ohm meter set to ohm scale, hook one lead to the negitive side of the battery the other to the sender wire and tell me what the ohms are.