1973-1991 Dual fuel tank systems theory of operation

[v30crewcab]

91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!

[hatzie]

Originally Posted by v30crewcab

91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!

NO!! You haven't lost the ground... The valve motor wires from the dash switch change polarity depending on the switch position. The dash switch is a polarity reversing switch commonly used to control the direction of DC motors. Go back and read the 81-91 section in post #2 of this thread. Pay close attention to the power routing diagrams I drew up.

The only wiring modifications you need to make are at the two position Weatherpak plugs attached to the pigtails dangling from each sender. DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS.

Remove the fuel pump power wire from the Weatherpak plugs in each sender pigtail and replace it with a green Weatherpak cavity plug. Fold over the fuel pump wire and tuck it into the split poly loom around the sender pigtail.
You only need to mod the sender pigtail. AGAIN!!! DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS. If you ground these wires you'll have a fire because either could be hot depending on the dash switch position.

[v30crewcab]

ok, so it should still be switching without the fuel pumps in the loop? I've got power and ground at the switch, but the valve is not switching, even with a new switching valve. I'll have to investigate more. Thanks.