I've inherited a 1980 C10 that is still in pure stock condition (still running factory original water pump!). But there is one problem that I've traced to the fuel tank selector switch. If I leave the switch set to the "LH" tank, the battery goes dead overnight. If the switch is set to "RH", there is no problem. I've traced the power to the actual selector valve and this is a simple on-off mode valve. With the switch set to "RH", there is no power to the valve and fuel does indeed come from the RH tank. If the switch is set to LH, then the valve is energized and fuel comes from the LH tank. So the question is: is this normal in that the LH setting of the switch is a "constant on" setting (even with the ignition switch turned off)? It would seem to me that the switch should be connected through an ignition switched power line instead of directly to the battery. I'e examined the switch and it has 5 wires running to it, all different colors. The wires are all part of an original harness and they disappear up behind the cluster. Unfortunately, I can find no wiring diagram that includes the Fuel Selector Switch, an item that seems to have been forgotten by the electrical engineers and draftsmen.
More about the Fuel Tank Selector Switch I haven't fixed the problem yet. If I leave the switch set to LH, the battery will be dead overnight. But I have come up with the factory wiring diagram and here is where things get interesting. The wiring diagram shows a "Timer" which keeps the valve energized for 30 minutes after shutting off the ignition. What purpose this performs, I have no idea. It can't be pollution related (this was 1980 where they didn't really care that much). I suspect the real purpose might have had something to do with trying to keep the fuel lines pressurized to avoid vapor lock which was somewhat of a problem still plauging the engineers back in 1980. But then why wasn't the RH tank keeping the line pressurized?? I suspect that my "Timer" is stuck. When set to the LH tank, the timer never times out, therefore it drains the battery. Having a timer in the loop explains why this LH tank switch is connected directly to the battery position of the ignition (so that the timer can keep energinzed even after the ignition is shut off). This is a new one on me. Where is this "timer"? How big is it? I imagine that replacements are impossible to find therefore, I like to locate it and just wire around it eliminating the 30 minute countdown.
Just a question where have they or you got the power lead running off( change to keyed on power), is the left side running a electric fuel pump?.
I'm not sure what you mean by where is the power lead. I assume you mean at the actual dash mounted tank switch. This is a pure stock 1980 with no aftermarket parts so it still has the mechanical fuel pump. The OEM tank switch has 5 wires running to it and they all disappear into the rat's nest of wires behind the instrument cluster (remember this is pre-electronics age so things were quite messy). The more I study the factory wiring diagram, the more I get to learn about this. Two of the five wires run to the two different tank sending units (marked as circuit 30B and 30C). One wire runs to the actual fuel tank solenoid valve (circuit 53B). One wire (circuit 30A) goes to a yet un-identified place. The remaining 5th wire (circuit 953) goes to the infamous "timer" (marked as item 12004099). This "timer" has 4 leads which according to typical wiring diagram conventions make these look like the blade contacts of a "plug-in relay". The 4 leads of the timer are ground (circuit 150), the previously mentioned circuit 953 from the switch, and two circuits (139 and 140) which both run back to an ignition fused contact on the fuse block. Circuit 139 is the ignition fused contact while circuit 140 is the battery fused contact, both on the fuse block. When the timer is supposed to be functioning for its 30 minute countdown, the contacts that close are circuits 140 from the battery connection on the fuse block and circuit 953 on the switch. Could I simply cut wire #953 which is "dark green" and thereby render the 30 minute countdown ineffective without eliminating the actual switch-over between the tanks? Or would that mean I'd get no power for the fuel senders or the guage display. Perhaps I could cut wire #140 Orange which runs from the battery to the timer? Would that still allow normal function when the ignition is on (through circuit 139)? Here is an scanned image of that portion of the wiring diagram...... Anyone else concur that the root of my problem is a defective time/relay? . . . .
Ok I have to agree with you it most likely the the timmer, but disconect power form the Batt try to see if the cover will come off you might try to clean the contacts there sort of like points some time you can clean them and see it wore out really dirty see timer part gone.(basicly your right it a relay with timer). You should beable to bye pass it. But you should be able to still get that part throw GM dealer, Ac Delco dealer,after market supplier. I would try to clean it, if it work then you know that it get a new one relays save your truck a bad short. sorry for the other post was not sure if it had been updated the second time I read it quick just put down some idea's & question.
