Dual tank solenoid valve

[legopnuematic]

Thank you, I do have this book. Is there a better one?

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Nope, that's what you want!

As a companion there is also the unit repair manual (PDF on the site, or find on eBay for a few $$). The unit repair manual goes into details on specific things, steering gearboxes for example. The service manual will tell you basic on truck stuff and how to remove it, whereas the unit manual will go through rebuilding a box and the specs/adjustments. If the service manual doesn't go into much detail, head for the unit manual.

 

[AuroraGirl]

@AuroraGirl is this part of the fuel system too? Fuel filter?

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Is that by the AC compressor? Im not sure what that is, but it looks like a muffler for an AC compressor

 

[AuroraGirl]

Thank you, I do have this book. Is there a better one?

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We have matching manuals! I wanted a GMC one but i couldnt find one back in the day. Mines a little bit more rough looking

 

[75gmck25]

It sounds like you have a single terminal solenoid. These will default to the main tank with no power, and to the opposite tank when 12 volts are applied.

The “main” tank varies over the years. For my ‘75 ( and up through ??) the passenger side was main, and/or the only tank for a single system. Then GM later switched to the drivers’s side tank for single/main.

The 6 port solenoid switches the fuel feed to the pump and the return from the pump. The vent lines are connected to a T near the solenoid and then (for your truck) they are connected to a steel line that runs up to the vent canister.

 

[Grit dog]

It sounds like you have a single terminal solenoid. These will default to the main tank with no power, and to the opposite tank when 12 volts are applied.

The “main” tank varies over the years. For my ‘75 ( and up through ??) the passenger side was main, and/or the only tank for a single system. Then GM later switched to the drivers’s side tank for single/main.

The 6 port solenoid switches the fuel feed to the pump and the return from the pump. The vent lines are connected to a T near the solenoid and then (for your truck) they are connected to a steel line that runs up to the vent canister.

Help refresh my memory. I know 1 wire solenoid is like you said. Actually gotta figure out which is which on the 77 (aftermarket 1 wire). Seems smart to use the powered side only when necessary and not leave switched to that perpetually. (But unsure if it’s an ignition on only which would be safer/preferred)
For the 6 port multi wire valve/solenoids like the 80s trucks are they also default tank power off opposite tank power on? Or is the solenoid energized in both positions?

 

[tophat36]

Is that by the AC compressor? Im not sure what that is, but it looks like a muffler for an AC compressor

Yeah I got a better look of where it was coming from and yes it’s goes to behind the compressor. Sorry about that, for a second I thought it was fuel related It’s leaking something there though. AC doesn’t have a belt so I’m assuming it doesn’t work, the things do spin on the front though which may be a good sign. Is there a fuel filter on this truck? Also is the fuel pump located in the engine bay?

 

[legopnuematic]

Fuel pump is a mechanical pump bolted on the passenger side front of the block (under the alternator).

If the truck is still rocking a Quadrajet carburetor the filter is at the fuel inlet of the carburetor itself.

 

[legopnuematic]

@Grit dog my 76 is a factory dual tank truck, one wire solenoid, it receives keyed power to the solenoid. No power defaults to one tank (could be either depending on which hose goes where), ask me how I found that out .

 

[AuroraGirl]

@Grit dog my 76 is a factory dual tank truck, one wire solenoid, it receives keyed power to the solenoid. No power defaults to one tank (could be either depending on which hose goes where), ask me how I found that out .

doesnt it get stuck, leading to filling up tank without someone knowing lol

 

[Old Guy Bill]

When I rebuilt my '78 I swapped the lines on mine so the default tank is the left side tank, that is when the solenoid is de-energized...
Also had to flip the in dash switch..

 

[Grit dog]

@Grit dog my 76 is a factory dual tank truck, one wire solenoid, it receives keyed power to the solenoid. No power defaults to one tank (could be either depending on which hose goes where), ask me how I found that out .

Because of your post I spent 5 minutes and tested the 77 tonight. Solenoid is keyed power and power on, on drivers side tank.
Now I know which side to leave the most gas in lol.

 

[75gmck25]

Ref post #35 -
I’m not sure how the multi-wire solenoid and dash switch works inside. It is motorized inside, and stays in whatever position you leave it (constant power not needed). I’ve also seen dash switches that tell you to push and hold the switch to change tanks. It sounds like it applies power when you hold the switch, and that moves the internal solenoid motor to the tank position you select; then it stays there.

 

[Broken85]

Ref post #35 -
I’m not sure how the multi-wire solenoid and dash switch works inside. It is motorized inside, and stays in whatever position you leave it (constant power not needed). I’ve also seen dash switches that tell you to push and hold the switch to change tanks. It sounds like it applies power when you hold the switch, and that moves the internal solenoid motor to the tank position you select; then it stays there.

81ish- 87 all used the same Pollack valve. Up until 1986 they were momentary switches and you held it down just long enough for the valve to move and the gauge wire to switch because the mechanical pumps were pulling fuel and not pushing like an electric pump. 87 with EFI turned to persistent switches.

When I rewired the 85 and put a sniper in it, the only thing I changed was the switch in the dash from an 85 to an 87’ switch (LMC has them for $10). There is one prong of the 87 switch that you will need to grind nub off of and make it flat for it to fit in the original dash wiring harness. I used the light green and dark green wires in the switch valve harness before the Pollack valve as trigger wires for the two fuel pump relays to activate the in-tank pumps.

I was worried about leaving power flowing to the pollack valve to keep the relay triggered, but no matter where I looked, everywhere stated there was no difference in a 1987 pollack valve from prior years. I have a thousand miles on it so far, no issues.

 

[gilby959798]

There is no need or benefit to remove/lift the bed to drop the tanks on these trucks.

These trucks are very simple, but one of your best friends should be a factory service manual (not a Haynes or chilton manual) for your year truck, along with, if applicable, a unit repair manual. If you have not found them already, there is a small library in the reference section of this forum with pdf copies of that literature. Especially in the late 70s-early 80s era as there were quite a few changes over that period and one or two year type things that are not the same as earlier or later trucks.

Revamping the fuel system in a square isn’t really that expensive of a thing to do, about $100/tank, reuse the old senders if in good shape and still function, figure about 8 feet each of 3/8, 5/16, and 1/4” fuel hose, if needed new straps about $20/pair, the switching valve has the potential to be a bit of a pain due to the various types these trucks used and the varying availability of some of them.

But having a fuel system that is fresh and clean is absolutely worth it in my opinion.

If you can bend lines, can make the hard lines real nice as well. I got a selector valve from Napa for my 77, its a single terminal unit with 3 ports, 2 to the tanks and 1 up to the pump.

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[Ricko1966]

Yeah I understand the confusion I just think a truck would actually start even if switched to an empty tank bc there may still be some fuel present. It would die but possibly start, sorry for the assumptions. I’ll see if it starts when I get home and see if the saga continues! Thank you for ur help.

Just for your info,(thats not smartass) lierally just so you know,a lot of these trucks won't start on residual fuel in the system. Pages and pages and thread after thread of my truck won't start if I park it for 30 minutes,overnight,1 week,1 month.