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Dieseljunky2
Dieseljunky2, Ford Mechanic
Category: Ford
Satisfied Customers: 3943
Experience:  ASE Certified, Ford Certified, Cummins Certified, & 20+ years in field experience
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1990 F-150: dual tanks, fuel pump issues

Resolved Question:

1990 F-150, 5.0, dual tanks, fuel pump issues.
Over several days, when I would start the truck, it would start well kand 15-30 seconds into warm up it would stop. I would turn the key and it would start up again and run fine. This last time, it didn't start. I changed the fuel relay, check the reset switch and finally dropped the tank and pulled the pump. Since the back tank had not worked in a while, I dropped it too and pulled the pump. Here is the rub. If run a wire directly from the battery to the pumps, they work fine. If I connect them (they are out of the tanks) and turn the key....nothing. The relay clicks when I turn on the key but no power seems to get to the couplings on either tank. I get power to the sensor wire but nothing to the wires that start the fuel pumps running. Please help?
Submitted: 3 years ago.
Category: Ford
Expert:  Dieseljunky2 replied 3 years ago.

Dieseljunky2 :

Hello, what you have sounds like a failing fuel tank selector switch. The year of your truck falls into the year range that is known for melting the switch wiring on the back of the switch itself. Here is the wiring diagram for your truck. I would recommend to start by pulling the dash switch and inspecting the connector and switch for signs of overheating. Thanks

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Customer :

I did that also and it seems to be okay. I had already purchased a replacement switch since that one that was on the truck was generic and did not have any labeling. I just went to the EEC-IV and grounded A. The relay kicks in but the pump will still not turn on.

Customer :

Also, I pulled the reset switch and bypassed it to make sure that it was not that switch.

Customer :

When I turn the key on, the red engine light and battery light are illuminated and do not go off. Does that possibly have something to do with the fuel pump not turning on even after bypassing the fuel pump reset switch and grounding A on the EEC-IV plug?

Dieseljunky2 :

Does the fuel pump relay click when you ground the test wire? If it does, then leave it grounded and see if you have power on the brown wire of the inertia switch.

Customer :

??Is the inertia switch the same as the reset switch? If not, where is the inertia switch and what does it look like?

Dieseljunky2 :

Yes, the factory term for the reset is the inertia switch, sorry about the confusion. Thanks

Customer :

I am not near the truck at the moment but I will check tomorrow am and get back to you? If I remember correctly, the wire colors are gr/b and y. Should the key be in the on or off postion when I check the switch or is that irrelevent?

Dieseljunky2 :

It should be on, and you will have the fuel pump wire in the data connector grounded as you were doing earlier. Thanks

Customer :

Got it.

Customer :

Yes there is power.

Customer :

So I have now reconnected the ineritia switch and checked the relay again by grounding the EEC. Still no power to the fuel pump plug.

Dieseljunky2 :

Check for power on the selector switch. It comes from the inertia swicth to the selector switch on the red with yellow stripe wire. It goes to the fuel pumps on the red wire for the front tank and the brown with white stripe for hte rear tank. See if it has power at these points with the key on and the relay grounded in the data connector. Thanks

Customer :

I bypassed the inertia switch with a piece of copper wire and check for voltage. Same voltage at the swithch as at the battery.

Customer :

All of this with the key on and the EEC grounded.

Dieseljunky2 :

The only thing left to do is to trace the wires back to the tank from the switch in the dash. The power goes from the selector switch to the fuel pumps. The wire colors are the same as at the switch all the way to the tanks. The wires go through the connector on the driver side floor by your left foot. Check that plug to make sure it is not burnt or corroded. THanks

Customer :

Sorry, I did not get to finish before I hit enter. There is no power to the switch. I checked for voltage between the inertia switch and the tank selector (red wire with yellow stripe). I checked it at switch and then back about 6 inches and the harness. I assume I need to trace the wires from the inertia switch to the tank red wire with the yellow stripe. Is that correct?

Dieseljunky2 :

Yes. the inertia switch to the tank switching switch. Thanks

Customer :

Still working on it. Pulling apart the dash to trace and see it that is it. Is it possible that there is another fuse that I missed?

Dieseljunky2 :

If you have power on the brown wire at the inertia switch, the fuse link is good. There are no other circuit prtectors or fuses for the fuel pumps.

Customer :

Hello Dj, Sorry haven't been back to release the funds. I was travelling. Last question (s) i think. Do both tanks have to be connected (I did not put the rear tank back in) and is there a need for me to ground the tank other than the connection itself?

Customer :

Hello DJ,

Customer :

Did you see the last questions? Quick answer on those and I will accept regardless of whether I am able to fix the issue. Thanks.

Dieseljunky2 :

No, you only need one tank hooked up and the ground is through the harness connector. Thanks

Customer :

It's finally fixed. There is a wiring harness about 6" long before the tank switch....bad wire. But I think that both tanks need to be hooked up. I grounded the EEC, and jumped the connector at the end of the harness mentioned before. With the rear pump connected but the rear tank not installed and the rear tank fuel lines disconnected, I jumped between the red/y which was hot and the red/w which was neutral. The front pump came on and gas started shooting out of the rear tanks disconnected fuel lines. Then I jumped the red/y with the Br/w and the rear pump turned on.

Customer :

Last I jumped the sending unit connection on the wiring harness and the front gauge immediatly registered. Based on everything you said and that I read it should have been the brown wire for the front tank. I have accepted, but it you don't mind, could you explain why the fuel was pumped from the from tank into the lines in the back tank. Thanks

Dieseljunky2, Ford Mechanic
Category: Ford
Satisfied Customers: 3943
Experience: ASE Certified, Ford Certified, Cummins Certified, & 20+ years in field experience
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