Tech / Special Interest → Automotive > 1985 Ford E350 Dual Fuel Tank Problem

1985 Ford E350 Dual Fuel Tank Problem

HI. I got the RV out for a test run this weekend and discovered a problem. I bought it and only filled one tank---which is the rear one, and ran off of that until this weekend.
I stopped to fill up and asked to fill the front as well as the rear. It turned out the front tank is already full. If I select the front tank using the dash switch the gas guage drops to dead empty and the engine dies as soon as the fuel in the carb is used up.
From what I can find online there is a separate electric fuel pump in each tank. Then there is a 6 port relay thing somewhere on the frame.
I need some ideas on what to check first? I was leaning toward the fuel pump being the problem but can't understand why the gas guage doesn't read full instead of empty?
Anyone come across this problem or have any ideas?
Here's a link I could find but a diagram would be great. »www.ehow.com/list_7695936_1985-f···ecs.html

Always start with the easy stuff first. Check all the fuses, make sure all are good.
I don't know about that particular vehicle, but there may be a separate fuse for each pump / tank. Also, there may be more than one fuse block. Be sure to check them all. If you have the owner's manual, that would be a good place to look for fuse locations and assignments.

Just a quick better than nothing answer. I had a 1987 and you could hear the relay "thunk" when you flipped the switch. If it doesn't switch then you won't get a reading.

Fairly common failure of the fuel tank switching valve. However as noted, you need to test it, and examine the wiring at and to the valve. It should be mounted on the frame rail near the forward tank. Rusty connections and especially the ground connections to the frame are a good culprit.

I'm into a real mess with this now. I removed the dash switch and tested for continuity, One tab showed nothing with the switch in either position, so assumed the switch was the problem.
Turns out that switch is no longer made and a search of junkyards and such was fruitless. I did find one on Ebay but has a different model number and the seller is gone until October 6th.
So I tried repairing the switch I had but it's toast. I then had the bright idea that it's only 4 wires----2 that tested hot and 2 what I assumed are the feed to the valves. So I bought a generic on--off--on switch and wired her up. That promptly blew the fuse--permanently. It was badly corroded in the fuse block and is no longer usable. There is no spare spots.
I found the 6 port valve and it has 4 wires only running to it---so can't understand why the fuse went----unless the valve itself is screwed.
I can't see the valve as the culprit of the original problem as the gas guage is wired separately from the valve--so am assuming the front tank pump and sending unit is the problem as the guage dropped to empty when I switched to the front tank.
There has been rewiring and splices done throughout the wiring so suspect there has been problems before.

I have 2 options as I see it. I am planning to tap into a switched power wire at the fuse block, fuse it and run a new power wire and tap into the power wire for the rear fuel pump. I'll switch it using the switch I bought. That should get me running at least on the rear tank but unsure if the gas guage will function. If not I'll watch the odometer.
I can do the same for the front pump---but may need to drop the tank and install a new pump if it was indeed shot.
I'll then need to power and switch the valve system to change tanks.

OR---I could rip the valve out-----splice the hoses if I can sort them out correctly, and wire 2 inline electric fuel pumps and go without the gas guage.
Trying to track down the wiring through the relays, oil switch etc is next to impossible due to rewiring, splices and finding the damn things.

I am leaning towards the inline fuel pump option and getting rid of the valve system. It's an old RV with new motor and tranny but not worth sinking possibly a grand just to fix this one problem as I'm sure others will pop up as I get mobile. If I get a year out of it I'll be happy.

Any ideas or advice on what my best option is? I'm gonna try powering up the rear pump like I described temporarily tomorrow just to see if it will function.

Some pics----it's in good shape----but 25 year old wiring and systems will be maintenance intensive.
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You'd better get yourself a wiring diagram before you burn something else up for good. The scheme you suggested isn't always the case, as it may be switching a single circuit in a normally-open/normally-closed fashion. And fuel level sender switching is often just switching grounds. You can't just assume on this stuff.

I have a diagram--but it's from an 86 truck with dual tank system. It's the only one I could find online. The haynes manual I bought is useless and doesn't get into the dual tank system.
Like you said----the switch requires the identical unit because it controlled the fuel guage as well as the valve unit. It also tied into a start relay and oil pressure relay. Going by the diagram I assumed the switch wires were 2 hot and 2 load wires to the valve unit and wired it that way. But that isn't the case, as well the wiring has been played with before at some point, so I am uncertain of what the setup is at this point.
I am going with new wire properly fused---I'll solder a new extended fuse holder to the bus on the back of the fuse block, run 2 inline fuel pumps from the on-off-on switch and remove the valve altogether. I have read in a few forums of folks having similar problems using this method and seems to work.
At least this way I know it's wired safely and hopefully have both tanks working.
I'm doing the wiring today and will plumb it tomorrow if I can find 2 pumps for a decent price. I'll update on how it went.

Is the valve your talking about mounted on the frame inside the rail. If it is and the truck has fuel injection, thats actually a valve and pump. Each tank has a pump and it also has that pump. In other words its a three pump system. If when switching to the front tank it still runs and just reads empty that means the front tank pump is working as the truck wont run at all on the front pump if the front tank pump is bad. Really depends tho on what motor it has in it as some had fuel injection that year and some didnt. If its carburated rig away. If its fuel injection i strongly suggest any rigging at all as your going to have to deal with fuel return issues also with fuel injection. I think the 302s, and 351ms had injection that year. Im thinking the 460s didnt.