First post on this site.Haven't had a gasser since
well,1988.Been diesel ever since but I just bought a 88 F-150 302 EFI supercab.All the other 87 and 88's I've had all had a selector valve for the dual tanks. This thing doesn't.My other trucks were all 250 /350's.My manual says it has a dual function aux fuel res??Can someone explain the operation without a selector vavle.Does the switch control the fuel pump operation and gauge only?Thanks Paul H

 

Are you talking about the Front/Rear rocker switch on the dash? I had an '87 F-150 with dual tanks and had the switch. It not only switches the sending units, but it also switches the fuel pumps in the tanks and sends a signal to a solenoid valve that switches between the two valves.

It's a lot better system than the old International TravelAll (a real beast with a V-345 engine and split rims) my father had. That one had a ****, sorta like the old chokes, that you pulled out for one tank and pushed in for another. As I recall, you twisted the **** one way for the gas gauge to work with one tank and twisted it the other way for the aux. tank. I used to feel so small in that thing.

 

The rocker switch in the truck provides power to both the appropriate gauge and fuel pump. From the understanding I have from my Haynes manual, your truck should have both low pressure (in tank?) pumps and a single high pressure pump.

The dual function reservoir switches (mechanically) the appropriate valves by using the pressure from the fuel pump that is active.

I have heard that some trucks had a problem with the fuel return not being directed to the proper tank at times. Not quite sure which years were affected. You might try a search in the "Pre-97" section if you have any concerns with the system in your truck.


Later trucks used a different setup with high pressure pumps in the tank and a fuel delivery module that incorporated the switches into the fuel tank.

I just had to swap the rear tank on my '95, so I'm fairly familiar with this stuff right about now.....

 

I always thought it was a solenoid controled valve. Now that you mention it, it makes sense to use sort of a check valve affair. Saves a lot of money that way. Boy, if the valve fails, it sounds like it would dump fuel into the other tank like crazy.

 

The FDM's are more then just pumps --they have check valves and shuttle valves, and yes Dennis there is a factory recall because they had problems with the tanks pumping into each others and overfilling...

There is also a circuit for the fuel return...

Each tank has its own sender and the rocker switches this as well as the FDM...

These are fun systems to work on, and diagnose....

 

This system doesn't use a solenoid but uses what they call a dual function resevoir.My original problem was rear always showing empty and I find a 4 ohm reading back from the sending unit even tho the tank is full.Float could be stuck or bad sender but tank did work until yesterday it sputtered like out of gas.I was just wondering how that dual res worked.

 

Hi Paul

Signmaster did a good post on the DF res.

It is an important part that can fail.. Been awhile since i worked on mine. mostly guess work at time...i believe theres a test you can do to see if the dfres. is working properly.

Test also can be done to the fuel pump/sending unit if you can get to wires on top of tank. 88 and maybe later had alot of sending unit problems, seen articles about that over the years.

My 88 F250 had dual tanks with the selector switch mounted in dash.

Front tank pump/sending unit was replaced under warranty. A few years went by and the rear sending unit quite working. No biggie, just learned to adapt i.e. make note how much gas i had.


Then the front sending unit went dead,(one replaced under warranty) so no guage reading but pump was working. Anyhow...didnt pay enough attention, ran out of gas. Dumb only because my wife was driving by herself. Not fun!!!!

Heres the quess work, because first sender went bad and replaced under warranty, figured same thing happened again.

Got lucky, after i installed the pump/sender in tank, before i put it all back up in place, ran check, guage did work....phewww The df res. was probably ok...but?

Because i had fuel system broke down, i went ahead and replaced the dual function res. & the fuel filter.

I did drop the rear tank from the bottom. Fortunately that tank had only 1 or 2 gal. gas in it. Backed the rear end up on ramps. Gives alot more room to work when your on your back.
No problem dropping tank, but on this one theres a heavy rubber hose inside the file pipe. So tank after all hardware removed just didnt drop into my hands. Had to twist, push etc shudda got help. I did because i couldnt get tank back up by myself becasue of that rubber hose has to be feed into the fill pipe at same time your pushing tank up, and not much room under there, very tight.

I got all parts from Ford except the fuel filter. Oh, beaware if you tackle this, there are special type clips used to hold the fuel lines in place. Probably could reuse old ones maybe, i decided to put all new clips on, they arent expensive.

Never done that before, but found out i could do it. Really not complicated.

You maynot do this, but just passing along. Good luck..OT

 

Yes signmaster did a god job explaining the operation and this weekend I'll take another look.My diesel doesn't have these problems,just one big tank.Paul H

 

Mach 1,

His truck is pre - FDM. You're correct the FDM combines all the valves etc into one unit. I expected the FDM to be larger than it was when I pulled the tank.

Before the FDM the shuttle valves and check valves were housed in the external reservoir. Looking at the Haynes manual, this setup would be a PITA to troubleshoot.

As for the TSB, the dealers I checked with said this affected the units without the FDM for the most part. The FDM is designed so that the return valve isn't open unless the pump is energized. It is possible but they said it is very rare. The answer I got was that they wouldn't even check anything unless I had problems with the tanks cross filling. Either way mine is working.

 

Old Timer,

I just replaced my rear tank. Sounds like you should have been there to help me with that PITA hose! LOL

No doubt that sucker was a pain. I unbolted the filler end nozzle and this let me pull out the inner hose. I was still a big pain in the butt to get the other end of the hose over the frame rail. Poor engineering at best. With a little more clearance, or a hose clamp with a right angle tightening screw, you could put the tank up first and then attach the hose. Even a kid couldn't get a hand over that rail and loosen that hose clamp.

As for the clips, they are just a safety item. On my truck the fittings are the same as the fuel filter and the clips just help relieve pressure from the lines. Probably just to avoid damage to the fittings. I ran my truck without the clips on and no problem. I wanted to make sure everything was working and leak free before I wrested with that filler hose.....

 

Well I just filled both tanks and the rear was indeed empty.Gauge still showed empty but I left switch in rear position and took off.Smelled gas and looked in side mirror and fuel is pouring out of front filler neck.Switched to front tank and it stopped.So my rear tank is pumping into the front.I suppose the problem is that dual funtion res but can I get away with just running on front until 1/2 full,switching to rear and let it fill up the front? Paul H

 

Why not. Last i heard gas was gettin cheaper.

Good luck. OT