EFI Conversion Dual Tank Wiring

[TommyGuns]

I converted my 83 K10 to EFI with a Pro-Flo 4 some months ago, and I’ve been driving around with Edelbrock’s Sump. That was always intended to be temporary, and the sump hasn’t been without issues. The main stumbling block I’ve run into with converting the rest of the fuel system is that my truck has dual tanks, and I want to keep them. I’ve referred to the 87 TBI fuel tank wiring as a reference, but according to the diagram, power for the fuel pumps was routed straight through the fuel tank selector switch and spliced off the wiring near the tank selector valve. If I update my wiring to match the 87 TBI dual tank wiring diagram, I have no idea how much current the tank selector switch can safely handle. The fuel pumps I’ve selected draw 8 amps of power.

If anyone has seen any hard numbers on how much current the fuel tank selector switch is rated to handle I’d love to see it.

Since the switch was an unknown factor, I thought about Gmachinz’ kits advertised in posts from years ago, which were apparently well made… if you ever received it and he didn’t just keep your money.

I decided to try to replicate his design from some of the photos he posted, also referencing the Pro Flo 4 instructions and also some DC wiring diagrams I found while googling. He used three relays from what I can tell, and I eventually came up with the attached wiring diagram. It seems to be the most logical way to handle this problem.

I would appreciate it if anyone could vet the design. I understand angry pixies to some extent, but electrical circuit design is not my day job.


Theory of Operations:

Design Notes:
Circuit numbers, wire colors and gauges are all based on the 1983 truck wiring manual. Circuits 53R and 54R (relay) are a larger gauge of wire which must replace the wiring originally routed from the firewall to the fuel tank selector valve. I went with GXL wire for these upgraded circuits as Wire Barn does not offer HDT wire.. at all it seems. Pin identifiers on the tank selector valve connector are shown, but may not be accurate. Double check the actual pinout of your tank selector valve to ensure accuracy.

Power Supply and Control:
The Pro-Flo 4 ECU module is capable of powering a fuel pump of up to 10 amps directly via it's Brown/White (-12v) and Orange (+12v) wire pair. Due to Edelbrock's circuit design however, the pump circuit is switched on and off using it's negative/ground lead. Since most in tank pumps are grounded at the tank, a relay is required to supply +12V power from the battery directly to the pump, with the relay being controlled by the ECU module. The bottom relay in the diagram is this On/Off control. The coil terminals on this relay are wired to the ECU’s fuel pump power supply, to power or disconnect the fuel system wiring from the car battery.

Switching Tanks and Fuel Pumps:
Above the Power Control relay are two more relays in the diagram, one for each tank. To avoid unnecessary wire crossings and make the diagram a bit easier to read, the top relay drives the Left Tank in the diagram, and the middle relay drives the Right Tank in the diagram.

The +12V power from the Power Control relay is connected to the Normally Open terminal (87) on both relays. The Normally Closed terminal (87a) on both relays are wired to ground. Coil Terminal (85) on both relays are also wired to ground. Coil Terminal (86) on both relays are wired to opposite sides of the tank selector switch in the cab. The tank selector switch receives +12V power from the driver footwell fuse panel and switches the polarity of circuit 53 (Light Green) and circuit 54 (Dark Green).

When a tank is selected in the cab, +12V power is sent from the tank selector switch to terminal 86 on that tank's relay (Middle or Top in the diagram). That relay's contactor closes, sending +12V from relay terminal 87, via terminal 30, out to the appropriate pin on the tank selector valve. Power to the fuel pump is supplied via a wire splice near the tank selector valve's weatherpack connector.

For the tank that is not selected, it's contactor remains open, and it's terminal 30 is connected to ground via terminal 87a, providing a -12V / ground connection for the tank selector valve via the non-selected tank's wire. The fuel pump connected to this -12V / ground wire remains off due to both of its leads being connected to ground.

Possible Failure Modes:
If the tank selector switch shorts out, the tank selector valve motor should remain in it's last position and both fuel pumps will run, drawing over 10A and blowing the fuse.

If the tank selector switch fails open, neither fuel tank relay will be energized. Neither fuel pump will receive power as a result. The tank selector valve will remain in it's last position.

[TommyGuns]

The diagram is difficult to read because of the attachment size limits, so here is another link to a clearer version:
https://pasteboard.co/e4oqXLeBxb71.png

[kwmech]

I think you are over thinking this. All the switch does is complete an intermittent circuit

[TommyGuns]

I disagree. I used this thread as a reference:
https://67-72chevytrucks.com/vboard/...81#post8151581

The tank selector switch from the factory is a momentary ON-OFF-ON switch on the carbureted trucks, but on the TBI trucks it is a two position ON-ON switch. You can replace the ON-OFF-ON switch on carbureted trucks with the ON-ON switch from TBI trucks and make no other changes, because the valve itself has an internal limit switch that shuts off its motor once the valve fully switches to a selected tank.

On the TBI trucks, the reason they switched from the momentary switch to the ON-ON switch was so they could add in-tank electric fuel pumps with as few changes as possible. They literally just changed the switch and spliced the fuel pump power circuit off of the tank selector wiring. This means that on a TBI truck with dual tanks, power for your fuel pump is passing through the switch on your dash.

I would do exactly this, but 10 PSI TBI fuel pumps do not draw as much current as more modern 40+ PSI EFI pumps. Upgrading the wire gauge to the tanks is not complicated, but the switch is another story.

I've seen that others in this situation have just left the tank wiring alone and wired up a new switch and circuit just for the fuel pumps. You would then switch tanks with the normal tank selector switch, and then quickly throw another switch to change fuel pumps. I really don't like that idea.

The most complicated part of this whole thing is the relay bank. It should work, but more eyes are better. You can look at something 20 times and not see a problem, but once someone else looks at it they can immediately see a boneheaded mistake.