fuel selector valve question

[rgunlock]

I've got a 78 GMC K15 that has the dual saddle tanks. The PO had rigged a valve with a handle under the seat to switch between tanks, but that didn't handle switching the fuel gauge. The dash still has what I believe is the original switch, but its not doing anything. Does anyone have a picture of the actual fuel selector valve, its hose and electrical connections, and its original location? Its been removed from my truck so I don't know what I'm replacing. Thanks!

[ray_mcavoy]

Hi Rick,

If no one else chimes in with pics, I can get some of my dad's 77 GMC K15 tomorrow.

Just be aware that GM used a few different setups over the years. The early style ones use a solenoid valve to switch the tanks. It only has 1 wire connected to it and the dash switch takes care of switching the gauge sending unit connections. The later years use a motorized valve that switches the tanks and the gauge sending unit connections. These have a 5 wire plug. I'm not sure what style is correct for your 78 but I know 77 was still using the older 1-wire solenoid valve.

There are also 3 and 6 port selector valves. The 6 port ones are used on trucks that have a return line running from the fuel pump back to the tanks. A quick look at your fuel pump will let you know which one you need.

[rgunlock]

Thanks for responding Ray. I bought a fuel selector valve from rockauto.com that is supposed to be for my truck. It has 6 ports and takes the 5 wire plug, but my fuel pump doesn't have a return line so I believe I only need 3 ports. The valve has covers on 3 of the ports and I assume I can still use it. I don't have the plug so I will have to find one of those. I'm not sure which of the ports are connected to the tanks, and which goes to the fuel pump. The valve didn't come with any instructions.

[ray_mcavoy]

Hi Rick,

Yeah, I don't see any reason why you can't just leave the return line ports unused on that 6-port valve to use it like a 3-port one.

If you ignore the 3 unused return line ports, the valve should have two "inlet" ports on one side of the unit. And the single "outlet" port that goes to the fuel pump on the opposite side.

I mentioned earlier that I wasn't sure which style (1-wire or 5-wire) would be correct for your 78. But I just remembered a quick way you could check (assuming the dash switch is original). The 1-wire solenoid valves use a dash switch that simply flips between the two positions and stays where you leave it. The 5-wire motorized valves use a dash switch that is spring-loaded so it returns back to a center neutral position after you release it.

[hatzie]

If the '78 cab wiring is intact the gauge is selected by the dash switch.

The '78 NL2 wiring is bloody simple.
More often than not the cheap plastic dash switch was the problem.
Standard Motor Parts still makes the FV1 solenoid selector valve for the original 1978 system.

[rgunlock]

Thanks for the help! My switch stays in RH or LH position, so sounds like I got the wrong selector valve. Found the FV1 valve and it looks like its pretty straightforward and what I likely needed in the first place, and doesn't require the 5 wire plug (that I dont have). It also looks like the mounting holes are spaced differently than what I have, so maybe I can figure out where it originally mounted since the valve I got doesn't appear to line up with any existing holes.

[ray_mcavoy]

Hi Rick,

Yeah, what hatzie just posted confirms that GM was still using the 1-wire solenoid valve in 78. So that 5-wire one isn't correct for your truck.

Here are a couple pics I took of Dad's 77 this morning. This should be the same setup as your 78 with the exception of the return lines (3 vs. 6 port valve).

This first pic is looking up at the valve from underneath. It is located on the passenger side frame rail near the back of the cab.

The silver thing off to the right in this pic is a heat shield that was used with dual exhaust.

This second pic is with the camera held up close to the floor to get more of a sideways shot of the valve.

I labeled all of the hose connections but you can just ignore the return ones ... feed lines will still hook up the same way.

And finally, here is the wiring diagram for the dual tanks. I figured you might find this helpful if the former owner has removed more than just the selector valve and taken out some of the wiring as well.

[rgunlock]

These pictures are great. Thank you very much!

[rgunlock]

Resurrecting this thread to add more info. I'm finally re-wiring my truck. Ray's wiring diagram is very helpful but I also sorted out the fuel related wiring from my old harness to add some pictures. Even though it wasn't originally working on my truck, as far as I know this wiring and these connectors are factory.

First is the switch and the plug going into it. Pink w/black stripe goes off presumably to my old fuse box. Showing the connector on it for reference - not sure it was original.

[rgunlock]

From the plug on the back of the switch, 4 wires (lt blue, green, lt tan, tan) go off to another connector that plugs into the backend of a bulkhead connector passing through the firewall. Both sides of the bulkhead connector are male.

[rgunlock]

First two pictures are the engine bay side connector into the bulkhead connector. From here, the harness runs back to the driver's side tank where the light blue wire (3rd picture) goes to the fuel sender. The rest of it crosses over to the passenger side. Green goes to the fuel switch solenoid, tan to the passenger side sender.

The other tan wire and connector with the male pin I'm not so sure about. Must be going back to the fuel gauge? Couldn't find anything to connect to this in my old harness. I did read on a thread that the fuel guage wire goes out on a circuitous route?

[hatzie]

You are correct about your connection assumptions. The plug on the pink connector looks like a Wye for one of the IGN slots in the 73-78 AGC fuse panel. I believe the 79-on NL2 harnesses use a Pak Con connector on the ATC fuse panels as there are a lot more accessory connections on those panels.
You'll note that the tan wire to the gauge has the same kind of pin as the senders.
GM setup the wiring harnesses so the NL2 equipped and single tank trucks had the same harness.
Look for a tan wire on the RH frame rail with a sender connector. At least I think its on the RH rail. The pin connector from the NL2 harness plugs into it. You see the tan NL2 to cab wire connection in Rays' pictures.

I would check for voltage at the green plug with the ignition on, the switch in AUX, and the pink/black wire plugged into the fuse panel. If you don't get any replace the switch. It's cheap. You could just hook the pink wire to the battery with alligator leads to test the switch.