dual tanks

[bakerboys]

any help on trouble shooting the fuel tank selector switch it is not switching between tanks

[panhandler62]

Can you provide some information on the year/model etc. of the truck and accesories?

[bakerboys]

1985 k20 was deisel. Now is 350 gas.
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[fixit-p]

Typically a dual tank circuit has a DPDT switch where 1 side turns on a solenoid which switches the suction and return ports on the tanks. The other side of the switch would switch between sending units. Pictures may help to further troubleshoot.

[hatzie]

Originally Posted by bakerboys

1985 k20 was deisel. Now is 350 gas.
Posted via Mobile Device

Your 85 should have a motorized Pollack valve not a single wire solenoid valve.
There are probably minor differences between the 85 CK and the 87-91 RV. The 87-91 RV have a fuel pump in each tank. The wires from the level senders to the selector valve may be a different color on the 85.

The '80s selector system is really quite a clever minimalist design. The way the selector switch is setup is commonly called a "phase reversing switch".
The switch reverses the output polarity to drive the DC motor in the selector valve left or right.
Electrically the selector valve switches the fuel gauge feed to the appropriate tank while an internal stop switch shuts off it's motor and reverses it's expected drive polarity.
Mechanically the valve switches the suction and return pair from one tank to the other.

This is a key for the attached wiring diagram.
Circuit # Function
120 +12v Power
150 Ground
920 +12v when RH tank selected or ground when LH is selected.
921 +12v when LH tank selected or ground when RH is selected.
30 Fuel level sender feed selector to bulkhead connection.
930 RH fuel level sender.
931 LH fuel level sender.

The TBI harness shown also feeds fuel pumps, that you don't have, from the selector switch wiring by splicing the pump +12v feeds to circuits 920 & 921.

The attached PDF is higher resolution than the picture.

[hatzie]

The first thing to verify is that you are getting voltage to the selector valve from the switch. If it's not there narrow it down to find your trouble.

Turn the ignition on, set the tank selector switch to RH tank, climb under the truck with your digital volt meter (DVM), and unplug the selector valve.

Select DC 20v or just DC on the DVM.
With the RH tank selected circuit 921 should be ground and 920 should be +12v. The circuit positions are in the schematic in my last post.

If you have volts it's probably a dead selector valve. Go to NAPA and get a new one.

No voltage at the selector plug? Unplug the selector switch under the dash and check for voltage there. The bottom 2 positions of the plug are the power feeds and your ground is the black wire.

Volts at the switch? You found it. Chase the wiring from the center pins of the switch plug through the firewall to the selector valve and fix the break.

No volts? Check from circuit 120 to a known good chassis ground.

Still no volts. Fix the + feed to the switch. I think it goes through the firewall to the big bulkhead connector behind the fuse panel on the firewall.

Circuit 120 to chassis ground gives you volts? You have a bad ground to the switch. Follow the black ground wire from the switch back to the buss bar behind the dash and fix it.

It's a PITA to chase wiring gremlins but this one's not too bad