Dual Tank Switching System Diagnosis and Fix
(Conversion to an after-market switching valve + some general  fuel system troubleshooting tips)
1986 FORD F150 5.0 (302ci) EFI

Note that this conversion should work for '85 to '89 F150-F350 with EFI and dual tanks and the troublesome non electric tank switching valve. Ford calls the valve a Dual Function Reservoir (DFR). Systems with only one tank have a Single Function Reservoir (SFR). The reservoir part provides some extra backup fuel during extreme 4x4 maneuvers or possibly long haul uphill runs where the in-tank pumps may momentarily draw air or not be able to keep up with the required flow. On my truck I have eliminated the reservoir and have not had any problems. The extra fuel filters I added do have some volume maybe 1/4 to 1/3 the volume of the original reservoir.  A new non switching SFR could be added to the system if any fuel delivery problems arise. (Whining noise from high pressure pump and/or bucking under high demand).

Some of the non EFI engines had an electric solenoid switching valve. This info may work with some electrical modifications. (You will have to use a new dash switch).
Some models already have the motorized switching valve very similar to the one which is described below.

After about '90 Ford went to a single high pressure pump in the fuel tanks and dropped the frame rail high pressure pump and switching valve. Tank switching is done by a simple check-valve system built into the pumps.

Click here to jump to general  Gauge, Sender and Pump Troubleshooting

Symptoms and causes of a bad fuel selector valve and/or problems with in-tank pump(s).

Fuel is mysteriously transfered from one tank to the other:
Fuel gauge shows level of wrong tank:
(And you run out of fuel on the freeway in rush hour traffic - BTDT)

Bucking at higher speeds, unable to go faster than ~50MPH:
High pressure pump whines loudly:

My fuel system troubles:
Several years ago on one long trip I started having trouble. I could not go over 50 mph and the engine would cut out and 'buck'. I had the system checked an the mechanic replaced the fuel filter in the switching valve / reservoir DFR thing. Seemed to fix the problem for a couple of years.
After some time the gauge on my rear tank began to be erratic. The fuel gauge just never seemed to be right. One day I stalled on the freeway with 1/2 tank showing on my rear tank. I knew the front tank was empty. After that realized that the rear tank was not switching in. The high pressure pump would make a loud whining sound when the rear tank was selected.

I limped along for a couple of more years filling only the front tank. Fuel in my rear tank was probably over 5 years old at that time.
If you're like me it's pain to have to fill the one tank every 150 miles or so especially on a long road trip

Eventually I pulled the rear tank and found that the pump was stuck though it was attempting to run. There is supposed to be a rubber connecting hose going from the pump to the metal sender feed tube.  That hose was totally disintegrated  into a pile of sludge on the bottom of the tank. I am thinking that as the hose began to come apart the pump sent bits and pieces of the tube and other debris through the fuel system.  Probably for a time the rear tank was working if the fuel level was higher than the broken part but after awhile the bits and pieces jambed the pump motor.

I attempted to clean the tank but gave up and bought a new tank, sender unit with new pump.

Dual Fuel Tank Switching Valve Overhaul:
I attempted to take apart and clean the valve. I did manage to get it functioning on my bench but was not convinced that it would work for very long.  Note that the OEM selector valve is a dealer only part.

How the OEM DFR system works:

The switching valve looks like this and is located on the driver side frame rail. If you have 4x4 you  may need to take the skid plate off to gain access.
OEM Selector
On the EFI engine fuel pressure to the fuel rail and injectors is regulated by the fuel pressure regulator. The excess fuel from the regulator action is returned back to the selected tank. If the valve is not functioning properly then fuel may be returned to the wrong tank.

The switching valve is non electric and operated by fuel pressure provided by the selected in-tank pump.
Inside the upper portion of the selector valve there is a diaphragm and 2 valves:
One for the fuel feed and one for fuel return from the EFI  system.
The fuel pressure from the selected tank will move the 2 valves so that the return fuel is routed to the correct tank.

The bottom reservoir portion contains a replaceable fuel filter. Note that later models do not have a filter and the body of the DFR looks different (more square). The bottom housing just screws off with a strap wrench.
The filter according to Ford is supposed to be a life time filter but I had to replace mine at least 2x before I decided to 'can' it.

    Inside view showing the 2 valves (1 removed).

Bench Testing:

The bottom reservoir part must be installed.
Connect a piece of hose to the engine supply port on the left and blow air through.  Note which port the air comes out then press the valve pin with your finger. the air should change to the other port. Repeat with the engine return ports.
Note there are 'O' rings, springs and such in each valve.

OEM Electrical Description:
The dash switch is a DPDT switch. Half of the switch is wired to switch power from one tank pump to the other. The other half is wired to select the corresponding tank sending unit to the fuel gauge circuit. This is why you get the wrong reading if the valve is stuck.
Note some models do not have a filter on the high pressure side of the high pressure pump labeled Main Fuel Filter in the diagram below.

OEM wiring

I gave up on the OEM valve and decided to use an electric motorized valve.

Tank Switching Valve Replacement:

Before taking on a project such as this make sure you are comfortable with and have a thorough understanding of the following.
Parts needed:

I got the Pollak electric motor driven valve off ebay for about $60 with connector and switch. May be sold under different names or even OEM branded. Be sure you are getting the kit with the 'pigtail' connector included. If you need the switch then check for that as well.

Pollak 6 port model (For Fuel Injected Engines)
    42-159 Valve only
    42-302 Valve and Connector
    42-300 Valve, Connector and Switch
Pollak 3 port model (For Carburetor Engines)
   42-151 Valve only
   42-308 Kit (description does not specify what's in the kit)

Note that at first glance the 3 port version looks just like the 6 port version. The difference is that the 3 port version has 3 of the 6 ports plugged off. Be sure you get the correct model.

  Pollak switching valve compared to the OEM non electric switching valve:

Dash Switch - OEM or New
I choose to use the existing OEM dash switch. 

OEM Switch Modifications: (Refer to the New Wiring Diagram below)
  1. Cut the Y/LB and Y/W wires about 3" from the switch connector.
  2. Rearrange the connections in the switch as shown on the diagrams below. Use a small screwdriver to bend the lock tangs back and pull the lug and wires out of the connector and reinsert them as shown in my diagram.
  3. Splice the short Y/LB to the DB/Y and the Y/W to PK/BK as shown below. I used 'vampire' splicers but there are better choices.
  4. Splice the Y/W (fuel gauge) and Y/LB wires together as shown.
I also choose to get the ground connection from the harness by using the DB/Y wire as shown below. This is made possible because the new valve is doing the sender switching instead of the dash switch. This means you will end up with an unused wire (DB/Y) between the valve area and the dash switch area. (Only one wire is now needed between the valve, (connection B) all the way to the fuel gauge). I used the Y/LB wire for the sender wire.
If you don't want to make the extra splice at the harness under the truck, then you could instead ground the switch wires DB/Y and Y/LB directly to a ground point somewhere in the dash area.

If using the New Switch
If you choose to use the included toggle switch, (or buy a different DPDT switch), then you will need to find a place to mount the switch. The existing OEM switch will either  need to be removed or left in with connector pulled out.
Note that the sender wires will need to be spliced as shown (Y/W to Y/LB). (A short jumper could be made and plugged into the connector)

Here is the wiring diagram/ instruction sheet that comes with the valve:
Pollak 6 port Diagram and Instructions.
Important! The new valve (switch) should be powered from the fuel pump circuit AFTER the inertia switch as shown in my diagram.  Do NOT connect to the accessory (key on) power source as is incorrectly shown in the Pollak diagram.

If NOT wired into the fuel pump circuit AFTER the inertia switch then you will have these problems:

How the Dash Switch Works:
(Refer to new diagram below)
The dash switch is a DPDT type switch.
Switch handle up will select front tank and down will select the rear tank.

When the Front Tank is selected:
The front pump wire (R) is connected to the +12 power (PK/BK) via the (Y/W) crossover jumper. At the same time the rear pump wire (BR/W) is grounded.
When the Rear Tank is selected:
The rear pump wire
(BR/W) is connected to the +12 power (PK/BK). At the same time the front pump wire (Red) is grounded via the (Y/LB) crossover jumper.

Note that the crossover jumpers on the switch are necessary to make the motor on the switching valve operate. Switching the polarity at the connector pins D & E is what makes the valve operate.

Gauge / Sender Function;
Inside the Pollak valve there is an electrical switch which is thrown at the same time that the tank valves are switched.
When the front tank is selected:
Valve pins B & A are connected which connects the gauge wire (Y/W) to front tank sender wire (DB/Y)
When the rear tank is selected:
Valve pins B & C are connected which connects the
gauge wire (Y/W) to rear tank sender wire (Y/LB)

Note that both the wiring AND fuel lines must be connected to the valve as shown in order to function correctly

Installing the New Tank Select Valve:
See the photo below for approximate location.
The valve has a mounting flange with 5/16"-18 nuts which I did not use. I just tie-wrapped the pump to the frame.

See wiring diagram below
For the electrical splice I removed several inches of the outer tape. I used a screw type terminal block. Just something I had laying around in my junk box. You could use any type of water proof splice connectors and/or dielectric grease to keep water out of the connections.
Note that the supplied pigtail wires were not long enough so I had to splice
more wire on.

Installation photo

Fuel Lines and Filters

I used rubber fuel line and hose clamps as the Ford fuel line connectors do not fit on the new valve or fuel filters. Standard rubber fuel line with hose clamps work fine on either connection type. 3/8" ID for the fuel lines and 5/16" ID for the return lines. You can get the proper splice-in Ford type couplings from an auto-parts store if you prefer.

On my rear tank I used all new rubber fuel lines as I was replacing that tank at the same time.

I did not drop the front tank so I left the hard nylon fuel lines attached to the tank sender.
For the front tank main fuel line I just forced the nylon line onto the fuel filter. For the front tank return line, I was able to splice the nylon and the rubber line together by inserting the nylon line into the rubber fuel line a couple of inches.  I then put a hose clamp over that. Works very well but I will replace all with new rubber line when/ if I redo the front tank.
Used new rubber 3/8" hose between the new select valve and high pressure frame rail pump and then spliced rubber 5/16" to nylon for the engine return line just like I did for the front tank.
I used 2 generic in-line fuel filters (Autozone part # FF3419) between the new valve and tanks


Need to tie wrap the filters so they don't bounce around.

New Wiring Diagram

This is how I wired the system on my truck. Note that there is more than one way to wire this up and make it work. For the most part I followed the Pollak diagram except for the power sourcing to the switch. See warning in switch section above.
Also note that you must connect the fuel lines to the valve as shown. If not right you will get fuel returned to the wrong tank etc.

Wiring Diagram

One minor quirk with this system:
It's important to understand this when trying to hook up and test your new system or when trying to diagnose no start issues.
When engine is running the tanks can be switched at anytime with no problems, however the gauge can show the wrong tank if you attempt to change tanks when the engine is NOT running with the key in ACC or ON positions.
When the key is first turned to the ON position, the EEC (Computer) primes the system by running the pumps for 2 or 3 seconds then shuts them off via the relay. During this short on time, the new switching valve will operate one time only.  After the timeout, (with key still ON), if you then flip the dash select switch, the valve will not switch over as there is no power present.
The gauge circuit is always active with key turned to ON or ACC, but since the valve does not switch, the gauge will continue to display the last tank used (switched to).
Also the tank select valve will NEVER switch at all when the key is turned to the accessory position but gauge will be active.

 In order be sure you have the correct tank reading on the gauge:
Troubleshooting Fuel Gauge, Senders and Tank Pump Circuits

If not getting proper gauge readings or tank switching not working or other problems, then be prepared to do some diagnostics.
There could be problems with the gauge, sender(s), tank select switch, or any wiring in-between.
You will need a multimeter or simple test light.
Follow the wiring diagram and trace through step by step. A good place to start is at the tank switch as most everything except for grounding can be tested there.
If you are getting odd behavior and things don't seem to be testing or measuring as they should, before replacing anything, first check the grounding. Check for good ground from HP pump connector and sender ground pins to the frame with the ohm meter or continuity tester if your meter has that function.

Note: For testing purposes there is a pin in the test (OBD1) connector which can be temporarily shorted to ground in order to force the pumps to stay on. If not done then you will need to have someone switch the ignition key on and off in order keep the pumps running while taking measurements etc.

Test Connector
OBDI (self test connectors). Located in engine bay near passenger side front tire fender.

Fuel Gauge:
The fuel gauge is powered by the Instrument Voltage Regulator, (IVR) circuit in the instrument cluster. The IVR pulses 0 to about 5 Volts DC. The pulse rate is about every half a second. The IVR also provides power to the temperature and oil pressure gauges. So if the fuel gauge is not working first check and see if the other 2 gauges are working. If those are also not working then there are problems with power to the cluster or the IVR itself is bad.

This pulsing voltage can best be seen on an analog type volt meter, but a digital meter works OK, just a bit erratic on the display. Some of the better digital meters have a bar graph display which will show the pulsing nicely.
The gauge output can be measured at the tank select switch or at the tank connector(s). Refer to diagram above for tank connections

Instrument diagram

The sender is a variable resistor circuit which is operated by a float on a pivot arm. The resistance is approximately 10 Ohms full tank and 73 Ohms empty tank.
I was having trouble taking an Ohm reading of the senders at the switch. There were some parasitic voltages ~10mV DC present on the senders that threw off my ohm meter. Basically if you are getting the pulsed sender voltage at the tank connector and no or erratic gauge reading then most likely the sender is bad or the float is saturated.

Note the short piece of black rubber hose connecting the pump to the sender tube.  On my rear tank that hose was completely missing... dissolved away.

If you order a sender be sure to specify with or without pump. Depending on the vendor, you may need to order the pump separately.

  After-market sender with pump installed

Pumps (In-Tank) (Pump shown in above photo)
The pump motor resistance is about 10 Ohms. Can be measured at the tank select switch or on the sender terminals. 
Getting a 10 Ohm reading however only tells you that the pump motor is not burned out (open). It does not tell you if the motor is jambed or connecting tube is broken etc. The best way to tell if the pump is working is to disconnect the tank fuel supply line at the select valve. Get a container and run the fuel tank motor and measure the flow. The Ford shop manual states a flow rate of 16 oz in 10 seconds, but you can pretty much tell if they are working or not.
Note there is a pin in the test (OBD1) connector which can be temporarily shorted to ground in order to force the pumps to stay on. If not done then you will need to have someone switch the ignition key on and off in order keep the pumps running.

Last update 11/13/2012

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