Duel Tank help
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New User
Joined: Feb 2007
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Duel Tank help
I have a '90 F150 4x4 with a 302 and a five speed. The truck has duel gas tanks and I have just recently started having a problem with the front tank. The truck runs great off of the rear tank, but when I flip the switch in truck to the front tank, the truck dies. I just put a new pump in it today and that didn't change anything. When I turn the key on with the switch set for the rear tank, I can hear the pump run. But, when I turn the key on with the switch set on the front tank, nothing. Are there any possible explanations for this other than the in-cab switch? Are there separate relays for the two pumps? Any help would be appreciated. Thanks
Posting Guru
Joined: Jan 2001
Posts: 2,358
Likes: 3
From: Nebraska
Just one relay for both tanks. It's under the hood by the air filter box. The EEC power relay is also there, so don't confuse them. The computer energizes the fuel pump relay. Power is then sent to the switch, and then to which ever pump is selected. This applies to 90-up trucks which don't have a switching valve.
As for only one tank working, it's either a wiring or switch issue if you know the pump is good. What I'd do is jumper the fuel pump terminal of the self test connector and turn the key on. This will keep power to the fuel pump. Verify this by switching to the good tank. Then check for power starting at the connector of the bad tank, and work your way forward. A good wiring diagram will help.
As for only one tank working, it's either a wiring or switch issue if you know the pump is good. What I'd do is jumper the fuel pump terminal of the self test connector and turn the key on. This will keep power to the fuel pump. Verify this by switching to the good tank. Then check for power starting at the connector of the bad tank, and work your way forward. A good wiring diagram will help.
Freshman User
Joined: Feb 2007
Posts: 38
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If im not mistaken there was a TSB or recall regarding a bad check valve in Fords dual tank setup. What happens if the check valve that seals off the non in use tank does not seal up and as the pump run it diverts the fuel to the other tank starving the engine. If given the chance/time it will overfill the other tank and spill out the cap/vent/charcoal canister or where ever it can go. Where exactly the valve is I dont know.
Elder User
Joined: Feb 2006
Posts: 537
Likes: 1
From: Formerly Nawlins
The valve is inside of the fuel pump assembly. There was also a recall in like 95 that they put an external check valve on both tanks to keep the fuel from backflowing into the not in use pump. I just changed my front tank and am not getting correct pressure out of it ... remove that checkvalve on the front tank and got the same incorrect pressure reading. There is another issue with a in pump assy check valve to keep fuel from returning to the wrong tank, but that one would not apply to the problem you and I are seeing. I will keep working at it, but until I find the problem, I can't give any more.
Someone said to rebuild or clean the dash switch, tried to do this but was afraid I would break it trying to take it appart and then not be able to use the truck without it. I may just jumper it out with wire to test if the switch is bad then order a new switch if it is. The swith is a dual pole dual throw switch.
Someone said to rebuild or clean the dash switch, tried to do this but was afraid I would break it trying to take it appart and then not be able to use the truck without it. I may just jumper it out with wire to test if the switch is bad then order a new switch if it is. The swith is a dual pole dual throw switch.
Fleet Owner
Joined: Aug 2005
Posts: 23,668
Likes: 301
From: Easton,Ks
Originally Posted by motoman
If im not mistaken there was a TSB or recall regarding a bad check valve in Fords dual tank setup. What happens if the check valve that seals off the non in use tank does not seal up and as the pump run it diverts the fuel to the other tank starving the engine. If given the chance/time it will overfill the other tank and spill out the cap/vent/charcoal canister or where ever it can go. Where exactly the valve is I dont know.
A bad in tank hi pressure check valve in the non running tank will allow fuel to go into this tank and the engine will not run or be starved for fuel. This tank will also over flow.
This however would have nothing to do with a non-running pump. If you think you have a bad switch just ground pin #6 of the test (VIP/DLC) connector with the key on. Use a test light to check for power at each wire going to the fuel pumps at the switch. Do not use a DVM as they do not load the circuit but are best for checking the charging circuit.
Test (DLC/VIP) connector:
http://i54.photobucket.com/albums/g91/subford/Fuel/DataLinkConn.jpg
Fuel System:
http://i54.photobucket.com/albums/g9...FielSystem.jpg
1990 Fuel Diagram:
http://i54.photobucket.com/albums/g9...el1990F150.jpg
Last edited by subford; Feb 14, 2007 at 07:56 AM.
