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Question about the dual fuel tanks on the 85' F250 with 460

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Old 04-20-2017, 10:02 AM
V8SHO V8SHO is offline
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Question about the dual fuel tanks on the 85' F250 with 460

Hello,

My '85 F-250 lariat 4wd with 460 engine (4bbl carburetor) and automatic transmission has dual fuel tanks with electric fuel pumps in each tank. It has a fuel tank switch next to the heater controls to switch between front and rear tanks. Currently, only the rear tank works, when switched to the front it will not run or start, been this way since I got it. So I plan on replacing the fuel pump and sending unit assembly in the near future.

Before I do so, I thought it would be a good idea to test the connector going to the pump just to make sure it's getting power. Wondering if anyone happens to have a wiring diagram for the duel fuel tanks or know which wires to test? I found some diagrams on google on the newer trucks after 1988 but nothing earlier.

Just as a side note slightly off topic, I was reading here on a forum post on a 93 truck with dual tanks having problems with one tank filling the other tank. Does that problem occur on the 1985 as well?

Thank you kindly in advance!
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Old 04-20-2017, 01:44 PM
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That one tank filling the other seems to happen more with the later switching valves that use the pump pressure to do the switching. Your valve is a Pollack style and has a little motor in the valve that switches the valve spool and also switches the sending unit electrical. It's most common failure mode is sticking in one position.

Here's a diagram showing the wiring if you have two relays over near the brake booster, one is a selector relay. Some did not have this selector relay, instead they used the dash switch to select the pump. There is another diagram for that if you have that system.

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Old 04-20-2017, 01:55 PM
V8SHO V8SHO is offline
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Perfect! Thank you kindly!
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Old 04-20-2017, 04:55 PM
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Gary Lewis Gary Lewis is online now
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Big Blue has the selector relay and the style of valve that has a motor. And when I pulled the rear tank and ran on the front tank it created a puddle where the rear tank had been, so the valve was leaking, although I don't know if it was from the return or supply. I plugged them up and the leak stopped.
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Old 04-20-2017, 06:36 PM
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Thank you Franklin2, I'm figuring out my truck and have a Vacuum and Electrical Troubleshooting Manual, but it does not show the transfer relay. I have the manuals also on CD, and fortunately I was able to locate a diagram showing it after seeing your post.
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Old 04-21-2017, 10:33 AM
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Here's the EVTM stuff for a 86 460 truck from Gary's site. Look at page 105 in this link

Electric Fuel Pump Control - ???Gary's Garagemahal

And page 97 in this link

Fuel Tank Selector & Gauges - ???Gary's Garagemahal

Notice in his diagrams they do not use a selector relay, they use the dash switch to select the pumps.
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Old 04-21-2017, 11:33 AM
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Thanks. That's the manual I have. The manual on CD shows the tank selector relay, saying it is for the 7.5 and hot fuel handling. I do have the two relays near the brake booster and the fuel vapor/shut off valve. Your post got me looking deeper.
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Old 04-21-2017, 01:08 PM
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I never have been able to figure out what determined when they used the relay and when they used just the switch. It must have been a running assembly line change. The setup that used the switch only, would have had to use a more complicated and expensive switch.
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Old 04-21-2017, 05:56 PM
SubieDriver SubieDriver is offline
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Can some of this information transfer to a 1986 F-350 with a 7.5L motor? I'm a complete newbie here, just signed up, and my sister owns the truck. We're having issues using the big tank, as it won't pump gas to the motor, even with a new fuel pump installed. Ideas would be helpful.

-Gregory
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Old 04-21-2017, 06:02 PM
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Quote:
Originally Posted by SubieDriver View Post
Can some of this information transfer to a 1986 F-350 with a 7.5L motor? I'm a complete newbie here, just signed up, and my sister owns the truck. We're having issues using the big tank, as it won't pump gas to the motor, even with a new fuel pump installed. Ideas would be helpful.

-Gregory
Welcome to FTE. And yes, the stuff does apply. Use the links to the 1986 EVTM above and scrol to the pages for the 7.5L.
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Old 04-22-2017, 03:52 PM
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Quote:
Originally Posted by SubieDriver View Post
Can some of this information transfer to a 1986 F-350 with a 7.5L motor? I'm a complete newbie here, just signed up, and my sister owns the truck. We're having issues using the big tank, as it won't pump gas to the motor, even with a new fuel pump installed. Ideas would be helpful.

-Gregory
page 96 mechanical pump & 97 electric pumps ;

- power to the electric pumps comes from the tank selector switch.

- electric pump needs a good ground.

- fuel flow comes via tank selector valve, either pump type.

- power for the tank selector valve comes from tank selector switch.
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Old 04-23-2017, 12:45 AM
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Here's my 1986 manual with the selector relay.





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Old 04-23-2017, 06:42 AM
ArdWrknTrk ArdWrknTrk is offline
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Here are the diagrams in the 1986 EVTM on Gary's site





​​​​​​​
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Old 04-23-2017, 09:10 PM
SubieDriver SubieDriver is offline
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Thank you all for the helpful information! I'm just trying to get the truck running well so we can start the process of registering it and driving.

The truck is a stake bed "shorty" dually.

-Gregory
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Old 04-23-2017, 09:21 PM
ArdWrknTrk ArdWrknTrk is offline
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250 dually seems an unusual combination.

Can you explain what you mean by "the big tank"?
​​​​​​​Both tanks should be 19 gallons.
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