WallyD Posted July 8, 2021 Share Posted July 8, 2021 Shifting the fuel tank selector switch on my 1986 F250 with 6.9 liter diesel no longer results in changing from the rear to the front tank. The truck runs fine from the rear tank. Multiple things could be causing this. Any suggestions on what is the most likely problem. Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted July 9, 2021 Administrators Share Posted July 9, 2021 WallyD...Is this a factory tank and switchover system? Or is the rear tank aftermarket? If aftermarket, please provide details on the system. Moses Quote Link to comment Share on other sites More sharing options...
WallyD Posted July 9, 2021 Author Share Posted July 9, 2021 This is a factory system. Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted July 10, 2021 Administrators Share Posted July 10, 2021 WallyD, your F250 should have a six-port switchover valve. If so, troubleshooting is fairly easy. Here is the factory procedure for checking the valve's function. The flow chart is easy to follow. Read down the left column and follow the steps. When done, the results/recommendations at the right column show whether the 6-Port Valve needs replacing: img20210709_18511704.pdf Often, the electrical contacts become dirty or oxidized. Before replacing the valve, I would clean the switch contacts with a spray electrical contact cleaner and a soft (nylon) brush. (Do not use a screwdriver or sharp object to clean the contacts.) The tests with jumpers should answer questions and pinpoint whether the valve is defective or not. Note that the additional information on the PDF is related to the sender and gauge assembly. Disregard those details. You're only interested in the 6-port valve at this point. Let us know how this works out. Moses Quote Link to comment Share on other sites More sharing options...
WallyD Posted July 12, 2021 Author Share Posted July 12, 2021 Wonderful Moses. Thank you. Quote Link to comment Share on other sites More sharing options...
WallyD Posted July 13, 2021 Author Share Posted July 13, 2021 Moses, I crawled around under the truck and could not find a device that looked like the picture in the pdf you sent. Attached is a picture of what's on top of the tank. In reality there's only one tank with a partition separating it into front and rear. Do you know where the selector valve is located? Wally Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted July 14, 2021 Administrators Share Posted July 14, 2021 WallyD...The illustration I provided was from the 1985 (not '86) Ford F-truck FSM. There may be a difference between your 1986 diesel fuel tank switchover valve and the 6-port valve in the 1985 illustration. I have the 1993 factory service manual and copied the details on the diesel dual-tank switching system, including the troubleshooting flow chart. (See photos below.) Excuse the photo orientation, you'll need to rotate each photo to read the information. 1985 and 1993 are the only Ford F-truck FSMs I have in my library from that period. I have earlier books but nothing between these two model years. Your single tank would still need a switch valve if the tanks can be operated separately. Unless the switch valve is within the tank (which would be highly unlikely), there should be a fuel pipe coming from each end of the tank that would lead to a switching valve. If the two pipes shown in your photo each go to separate ends of the tank internally (again, highly unlikely), they would still need to merge at a multi-port switching valve if your truck has such a switch. Follow the two pipes and see whether they meet at a switch valve. If not, one pipe may be a return line from the low pressure (pickup) fuel pump. In the two shop manuals that I have, there are no references to a single fuel tank with a separation chamber within the tank. If you have a single tank with two (separate) chambers, fuel gauge senders would need to read each chamber separately. There would be two fuel gauge senders. Try following the wiring to see whether that leads to a switching valve. Note that by 1993, perhaps earlier, Ford made the diesel fuel tank 6-port switch valve a "motor" driven unit. The earlier (1985) 6-port valve looks more like a solenoid type valve. Zoom into the illustration below to see the design of the 1993 switch valve with motor that is mounted along the frame rail. Without a 1986 F-truck factory service manual, I am unclear which valve was used on 1986 models. A Ford dealership's parts schematic would be helpful here. A visit to your local Ford dealership parts department could turn up the details you need. If your truck uses a switchover valve, the parts department may have an illustration of where the 6-port valve mounts on the 1986 F250 chassis: If this does not solve the riddle, I would find a 1986 Ford F-Truck factory shop manual that covers the fuel supply system on the diesel models. These original manuals are found at eBay when available. There are also automotive used book sources. You might try an online search using the keywords "picture 1986 Ford F250 fuel switch valve" to see whether photos details are available. To save time, a Ford dealership parts department would be my first information resource. They have illustrated schematics of parts by model year that include the parts location on the chassis. Try your local Ford dealership's friendly parts department. Let us know what you find... Moses Quote Link to comment Share on other sites More sharing options...
WallyD Posted July 14, 2021 Author Share Posted July 14, 2021 Thank you Moses. Wally Quote Link to comment Share on other sites More sharing options...
Administrators Moses Ludel Posted July 14, 2021 Administrators Share Posted July 14, 2021 You're welcome, Wally...Let us know what you discover... Moses Quote Link to comment Share on other sites More sharing options...
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