Fuel Tank Switch

Looking for help with my 1978 Chevy P/U 3/4ton. The switch on the center of the dash which switches from R to L fuel tanks no longer works (3 months). I've checked the valve mounted on the frame under passenger side, I have no power getting to that point. I checked the switch on the dash which has 5 posts, only 1 of which has any power and it's only enough power to barely light the test light. I can't find a wiring diagram for the switch. When I put direct power to the valve under the truck I can hear the valve working.
Any help would be appreciated, where to start, where to go to next.

Looking for help with my 1978 Chevy P/U 3/4ton. The switch on the center of the dash which switches from R to L fuel tanks no longer works (3 months). I've checked the valve mounted on the frame under passenger side, I have no power getting to that point. I checked the switch on the dash which has 5 posts, only 1 of which has any power and it's only enough power to barely light the test light. I can't find a wiring diagram for the switch. When I put direct power to the valve under the truck I can hear the valve working.
Any help would be appreciated, where to start, where to go to next.

Hi,
I had the same failure on a 77 Chevy truck. It was caused by a wire knocked loose on a fuse panel above the drivers left foot.(big foot).
Maybe there is a bad fuse on yours. Mine looked like an added wire, but the dual tanks were original factory installed.

Bob B

The pink wires go to the guage, the green go to the tank (on an 86 anyway). Make sure the switch is plugged in tight. I thought mine was bad, but the connector was just lose (the plastic on the switch side). I'd already bought a new switch, but I think I could have superglued the back of the switch and got it working like new. I've got a wiring diagram for my 86 laying around (somewhere), I'll see if I can find it for ya. If I do find it, It'll be much later till I can get it posted. Maybe tomorrow morning early

Turned my filing cabinet upside down looking for that diagram... but it is nowhere to be found. Sorry to get your hopes up like that. I'll keep my eyes open for it, but don't count on me finding it.

Thanks for the replys, will check the fuse box wires again there are some under there (red) that look like possible candidates for tlhe unplugged wire.
Thanks for the search on the wiring diagram. If the fuse box turns up nothing I figured I'd try a continuity test from the valve to the switch to figure out which one goes from the switch to the valve. Need to try to figure out which one should be hot on the switch. I would think one of the five wires would be hot all the time.
But anyway thanks for the help.
Joe

Guess what I found?
http://files.automotiveforums.com/gallery/watermark.php?file=503/269357Dual_tank_wiring_diagram-med.JPG

I started circling what I knew was good on my truck, and crossing off what I knew was bad... still readable tho

Thanks for all the help, right now I feel about 2 feet tall. Sherman hit it on the nose the wire was unplugged from the fuse box. I didn't even give it a second look because it just looked like a wire someone ran for extra lights or radio or something that was no longer there. Plugged it in and whallah tank switch working like new. Now I can take back that 45 Dollar switch over valve.
Thanks for all the help!!!!!!!
P.S. I've saved the wiring diagram for future reference.
P.S.S. J-Ri, about welding the spiders in the rearend. A long time ago when I was working at a 4-Wheel Drive Center in South Carolina we had a guy bring in a truck that he couldn't keep a rear axle in. He had just bought it off a used car lot. The previous owner had welded the spiders in both the front and rear.
When making sharp turns on asphalt and concrete the outside tire turns faster than the inside tire (I hope you get what I'm to say) and eventually one of the axles would just twist and break. For off road (dirt road, gravel road) it would probably be great though.

J-Ri, about welding the spiders in the rearend. A long time ago when I was working at a 4-Wheel Drive Center in South Carolina we had a guy bring in a truck that he couldn't keep a rear axle in. He had just bought it off a used car lot. The previous owner had welded the spiders in both the front and rear.
When making sharp turns on asphalt and concrete the outside tire turns faster than the inside tire (I hope you get what I'm to say) and eventually one of the axles would just twist and break. For off road (dirt road, gravel road) it would probably be great though.

Thanks for the warning. I had some concerns about the gears shattering (someone told me that could happen, if it's not welded right). As for the axle twisting, I have the same rear axle that was on the tonner trucks. What axle was on the truck you're talking about? Did they constanly have weight in the bed?