Help for a newbie

[nwsteve]

First the good news. Found this 79 GMC 3/4 ton for $1250. Think I did alright. A real "grandpa's" truck, never even had a radio installed in it. All original from bumper to bumper.





This should be an easy one. I haven't done any crawling around uder this truck to chase this down (hoping to get an easy answer first) so can someone tell me how the dual tanks work on this truck. There is a switch on the lower dash which switches the guage between each tank, does it also switch the tanks or is there another switch for that somewhere else?
I got $60 of gas in the right hand tank I want to use!

Also have this module under the dash, never seen anything like before what is it.

[Slonaker]

The switch on mine controls the tanks and the gauge. The switch is cheap and easy to replace, but the actual mechanism under the truck appears to be more complicated. Brandon, with the Big10, has some experience repairing a setup like that. You might talk with him.

I have a similar thing mounted under the dash. Mine is a brake controller for electric trailer brakes installed by the previous owner, who used it to tow a travel trailer. It looks like yours is too.

Slonaker

[crossy]

Slonaker is right on both as usual :-) , that brake controller looks old and probably has the old 'board' mounted underhood somewhere that has springs on it. ? crossy

[79BIG10]

Thanks for the plug Slonaker

On the dual tanks on your truck like mine it should have the switch on the middle lower section on your dash. The top button pressed is for the left (driver side) tank and the other one is for the right (passenger side) tank. The way it works it the switch is wired into your tanks to show the level of the fuel on the tank it's drawing from. Also it has a wire going to the tank selector valve. The valve is located under the passenfer behind the cab inside the frame rail itself. Both lines come into it and and then it selects the correct line you have pressed then feeds it into the main line going into the fuel pump.

The valve has a green wire headed to it. This is the power on/off. When it has power to it it goes to the left side tank. When the power is off it runs on the right side tank.

Let me know more in detail what you've done and what it's doing and I can help you figure this out. It's a pretty easy setup on these models.

[magnethead]

good find. Any rust or cancer anywhere? What drivetrain?

[nwsteve]

Originally Posted by 79BIG10

Let me know more in detail what you've done and what it's doing and I can help you figure this out. It's a pretty easy setup on these models.

That's the info I needed BIG10, thanks. It's exactly as described. Some suspicious splices in the wires however.

I don't know if there even is a problem yet, I am running it with the switch on the RH tank and watching the guage to see which tank empties.

I have a feeling it is always pulling from the LH tank regardless of the dash switch position. From what you described I believe that is possible if the tank selector valve is stuck. If I find that is the case, I'll get back with more specifics.

(How come the $20 manual I bought for this truck doesn't say a thing about this!)

[nwsteve]

Originally Posted by magnethead

good find. Any rust or cancer anywhere? What drivetrain?

2WD, 454 w/400 tranny.

Worst part of the body are holes on doors where some very large side mirrors were attached at one time, and a crinkle in the bottom of the post behind DS door. No rust which is surprising for a truck lived in Oregon all its life.

Its all coming together now, this was orginally purchased and used for some serious towing, wonder how much life is left in the transmission.

[magnethead]

it had the trailering mirrors i bet, like mine does. I'm willing to bet they were tossed because where they mount rotted/rusted/cancered out.

which manual did you get? My haynes seems vague too.

[79BIG10]

Originally Posted by nwsteve

That's the info I needed BIG10, thanks. It's exactly as described. Some suspicious splices in the wires however.

I don't know if there even is a problem yet, I am running it with the switch on the RH tank and watching the guage to see which tank empties.

I have a feeling it is always pulling from the LH tank regardless of the dash switch position. From what you described I believe that is possible if the tank selector valve is stuck. If I find that is the case, I'll get back with more specifics.

(How come the $20 manual I bought for this truck doesn't say a thing about this!)

Best way to check to see if the valve is stuck is to turn the key on but don't turn the ignition over. Make sure to have the brake set and get under the truck. Pull the wire off valve and see with a test light if you're getting power. See what side the switch uses to to power the valve. Then with the power on the wire place it back on the stud. Listen for a clicking sound when you place the wire on and off.

If you don't have power on this then we can narrow it done a bit more for you. On the manual I've had a Chilton's and Haynes but nothing has been better than a LMC catalog for pics and a GM Shop manual for how to it works.

[boxrodz]

My Dad and Grandfather on their trucks had that same brake controller when they pulled travel trailers. Like crossy said, the 'circuit board' is probably under the hood, maybe mounted on the fender. Old technology, fer sure...

Check out this thread http://67-72chevytrucks.com/vboard/s...d.php?t=201901 to view what the inside of the fuel valve looks like. BTW, make sure hold down the fuel selector switch when attempting to change tanks to give the fuel valve motor time to cycle to the other tank.

Like 79BIG10 said, a Chevrolet Shop Manual has much more detail then the aftermarket manuals. You can get them at Helms as well as a few other places. They are a bit expensive, especially for the pre-81 models since they are in reprints.

[79BIG10]

Originally Posted by boxrodz

Check out this thread http://67-72chevytrucks.com/vboard/s...d.php?t=201901 to view what the inside of the fuel valve looks like. BTW, make sure hold down the fuel selector switch when attempting to change tanks to give the fuel valve motor time to cycle to the other tank.

That's actually one for a later model with the 6 wire port but you're right the inner workings should be very close. The one he needs is like this one below. You can see the stud you're looking for there. Also this shows the 6 port (GM#467513) or you can get the non-emissions type that doesn't have all the ports just 3 (GM#467511). The 6 port has a return line from the fuel pump and to the tanks (smaller lines). The other ports are the fuel supply lines. I went ahead and changed mine out when I had mine out.

[magnethead]

kinda OT, but is it me or does mine not look anything like that? both wires from my sending units go to the front of the cab (blueish), I only see 1 wire going to the valve (whitish), and I have a 6 port non-emmisions.

[79BIG10]

Originally Posted by magnethead

kinda OT, but is it me or does mine not look anything like that? both wires from my sending units go to the front of the cab (blueish), I only see 1 wire going to the valve (whitish), and I have a 6 port non-emmisions.

Not sure what that is. Can you post a pic in a new thread and show us to help you out?

[nwsteve]

Thanks for all the great input guys. Guess it's worth it to pick a real shop manual. Seems to me back in the day Chiltons were much better. They had some color pics and like a 20pg troubleshooting section.

Checked out the switch as suggested and it works just fine, no worries.