'75 Fuel Troubles

Beaster · June 28, 2016

Hey guys I'm having some trouble with the fuel system on my 3/4 ton. It quit on me the other day so we towed it home. If it sat for a while it would start and run for a minute then would die. Figured it was the fuel pump cause after it would die you'd turn it over and nothing would come out of the line when I disconnected it right at the carb. Put a new pump on it and it fired up and stayed running. However since it was in the shop I figured I'd drop the passenger tank and take a look inside of it cause I didn't think I could get fuel out of it. So I dropped it and looked inside and its bone dry. But when I was driving it and it quit on me, I had the switch set on the drivers tank. So it got me thinking that maybe the fuel pump is fine and maybe its my switch or solenoid that's acting up and sucking fuel out of the passenger tank. It doesn't make sense because before all this happened I could have sworn there was fuel in that passenger tank, yet when you switched to that tank it would run for 30 seconds then quit.

So my question is how does this solenoid work?

Does the selector switch in the cab change the polarity or alter the voltage to the solenoid to change between the 2 tanks? When it was set to the drivers tank there was 12 volts at the single wire for the solenoid, but when I switched to the passenger tank there was hardly any voltage there, 0.04V or something.

I took the solenoid out and you can here it click when I crossed it across the battery both ways, + to the stud & - to the body and reversed, - to the stud & + to the body. Anyways I ordered a new solenoid just in case, one of these:

KARNUT · June 28, 2016

Hey guys I'm having some trouble with the fuel system on my 3/4 ton. It quit on me the other day so we towed it home. If it sat for a while it would start and run for a minute then would die. Figured it was the fuel pump cause after it would die you'd turn it over and nothing would come out of the line when I disconnected it right at the carb. Put a new pump on it and it fired up and stayed running. However since it was in the shop I figured I'd drop the passenger tank and take a look inside of it cause I didn't think I could get fuel out of it. So I dropped it and looked inside and its bone dry. But when I was driving it and it quit on me, I had the switch set on the drivers tank. So it got me thinking that maybe the fuel pump is fine and maybe its my switch or solenoid that's acting up and sucking fuel out of the passenger tank. It doesn't make sense because before all this happened I could have sworn there was fuel in that passenger tank, yet when you switched to that tank it would run for 30 seconds then quit.

So my question is how does this solenoid work?

Original Solenoid.jpg

Does the selector switch in the cab change the polarity or alter the voltage to the solenoid to change between the 2 tanks? When it was set to the drivers tank there was 12 volts at the single wire for the solenoid, but when I switched to the passenger tank there was hardly any voltage there, 0.04V or something.

I took the solenoid out and you can here it click when I crossed it across the battery both ways, + to the stud & - to the body and reversed, - to the stud & + to the body. Anyways I ordered a new solenoid just in case, one of these:

New Solenoid.jpg

l had the same problem with an old truck I had after syphoning gas out of the tank. It would run 20 minutes than quit. I changed everything electrical and fuel wise. The problem was when I got it home everything worked for about 20 minutes. Turns out after long hours and almost selling it, the tank was dirty the sock in there was clogging after 20 minutes of driving. The was the excuse I need to move the tank from the cab to the back of the truck. My last mod on my old 65 GMC. Hope that helps.

txab · June 28, 2016

The valve is a simple one. 12 volts applied, valve moves to draw from driver's side tank, iirc. When 12 volts is not present it draws from the passenger tank

Beaster · June 29, 2016

l had the same problem with an old truck I had after syphoning gas out of the tank. It would run 20 minutes than quit. I changed everything electrical and fuel wise. The problem was when I got it home everything worked for about 20 minutes. Turns out after long hours and almost selling it, the tank was dirty the sock in there was clogging after 20 minutes of driving. The was the excuse I need to move the tank from the cab to the back of the truck. My last mod on my old 65 GMC. Hope that helps.

My tanks are absolutely spotless, I was completely shocked when I looked inside both tanks, no rust, debris or nothing.

The valve is a simple one. 12 volts applied, valve moves to draw from driver's side tank, iirc. When 12 volts is not present it draws from the passenger tank

Ok great, so the switch in the cab is working correctly. Thanks for the good news.

disenfranchised01 · July 1, 2016

I just bypassed the solenoid, ran a line from drivers side tank to fuel pump. Make sure there are no leaks (of course, duh...) otherwise should draw fuel. Could be an air lock in the line after problems with system... Keep drawing fuel through line... Should prime properly and work otherwise. Did you know there are no "Real" fuel filters in these trucks and maybe carb has debris in it... Anyways a few things to consider. Good luck with your project.

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disenfranchised01 · July 1, 2016

Check our my YouTube link for Emergency Fuel Bypass Surgery...

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Beaster · July 1, 2016

I just bypassed the solenoid, ran a line from drivers side tank to fuel pump. Make sure there are no leaks (of course, duh...) otherwise should draw fuel. Could be an air lock in the line after problems with system... Keep drawing fuel through line... Should prime properly and work otherwise. Did you know there are no "Real" fuel filters in these trucks and maybe carb has debris in it... Anyways a few things to consider. Good luck with your project.

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Good thinking, thanks for the idea.