77-80's chevy 3\4 ton fuel sending unit trouble

hi all. I have a truck that was originally a 77 3\4 ton that was equipped on the dash for dual tanks (with the switch)..when i got the truck it only had one tank on it. I've since changed the 77 cab to an 81 and this cab also has the switch for dual tanks. I've determined that the sending unit in the tank is defective by using a ohm meter and temporarily run a jumper to by pass the corroded center pin strap on the lower section of the sending unit and now get reading on my meter..i'll replace the sending unit but have it like this just to try and get my fuel gage to read something other then way past full (3 oclock position)..The wiring under the truck is a rats nest for the fuel sending unit and am trying to trace out what is what. I'm only ever going to have one tank on this truck. The wires in this newer cab seem to go to a relay of some sort located beside the main fuse block then to the switch on the dash. I go under the truck and can't seem to get voltage on any of the wiring for the sending unit (s)..My main questions i guess are 1.) whats this relay between the switch and fuse panel and could it be defective? 2.) what voltage should the sending unit be getting? 12v or a lower voltage that comes from that relay. In the meantime i'll pickup a new sending unit but also trying to have the right voltage\wiring ready.. the original wiring harness\fuse panel from the 77 used glass type fuses and the new cab has push in auto fuses.

advice would be very helpful..

confused as hell, Jeff

Reply to
jrd
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I can tell you this: I would be very suprized if the wire from the gauge to the sender went through a relay. You are going to need a wiring diagram for this one im sure. the wire will go from the sender then it will be switched (two tanks) either at the switch on the dash OR a switch on the frame(for the fuel) then to the gauge. If you have power to the gauge you can find the wire by grounding it and watching the gauge for movement. Use a test light and ground through it so you dont short anything out and burn any wires. Again, a wiring diagram will save you on this one.

Reply to
ShoeSalesman

From memory (and it is flawed from age and abuse): The hot wire goes from the appropriate fuse and through the switch. Mine is tapped into a hot source in the fuse box and has an inline fuse (blade type). This wire goes to the selector switch on the dash.

Two wires from the switch go to the solenoid operated valve under the passenger side seat area on the frame rail (under the floor). This solenoid selects the sending unit and is connected back to the gauge with a single wire. The solenoid also operates the valve to change the fuel lines to the source matching the sending unit (hopefully the same tank). Power goes through the sending unit and to ground.

My description undoubtedly leaves a whole lot to be desired. I took a cheap meter, some paper and a pencil and set out to make sense of it all. A previous life had left the truck without the driver's side tank and the wiring was damaged also and the selector switch was smashed but a few days of thinking and some feedback from the men here was what I needed to make it all like new.

The box is still off if you need some pics.

- Regards Gordie

Reply to
The Nolalu Barn Owl

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