Dual tanks help!

[nheller76]

Ok so this is what I've got, I have a 78 c20 that has both tanks but the driver side does not work. I replaced the selector value and I'm positive the switch works because when I push it it will change the reading on the gas gauge. So? What do u suggest I try? It looks like the power wire runs down the driver side and crosses over, am I wrong on this?
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[Hevy]

My 87 had a deteriorated hose in the right side tank fuel pump. I replaced the pump and hose.

[pontgta]

selector valve??? should have 12 volts...... do you have 6 port or 3 port?

[nheller76]

I have a 3 port. My fuel pump works because it runs it just won't suck any fuel from the drivers tank. And I know it's full ( found that out the hard way!)
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[Hevy]

My bad, I was thinking 87 not 78.

[mrpaintstick]

I think by default it will pull from the pass side. If there is a short or not proper voltage to the switch it will from the pass side has to have constant voltage to keep swithc open to that side. Had a similar problem with my 76 switch does not close all the way and pulls from both sides have been using only right side for now.

[hatzie]

Turn the ignition switch on and set the tank switch in the AUX position... Slide under the rig with a multimeter. Reading VDC from the green wire on the valve stud to a good ground the meter should read right at battery voltage or pretty close.

The single wire you see running from the RH to LH is the LH sender. The 73-80 dual tank system selects the sender that you're reading with the gauge using the switch in the cab.

Senders are Light Blue & Tan the valve should be Light Green...

[wilkin250r]

Bingo. Hatzie nailed it. With the 3-port valve, the fuel gauge is controlled at the switch in the cab, not at the valve. If you've got a bad valve, the fuel gauge might still work just fine.

Basically, it means that just because the fuel gauge switches, doesn't necessarily mean the valve is also switching. You may very well indeed have a bad valve.

Of course, there is also the distinct possibility you have a good valve but bad wiring, or a bad fuel hose, or a host of other possibilities. But just because your fuel gauge switches doesn't mean the valve is also switching, that logic would only apply to the newer 6-port valves with multiple wires running to them.

So, your next step should be to verify that you are indeed getting power to your valve, and that should be the green wire that attaches to the valve (the ground is made to the frame). It should read zero volts when the passenger side tank is selected, and +12 volts when the driver side tank is selected.

If that test passes (you get the correct readings), it's time to take the valve off and test it on a bench.