Hello, new here. A member (ymeski56) recommended I come here for help. I'll try to keep it as short as possible.

Just got this truck, (1990 F150 with 5.0 engine) a few weeks ago. I'm dealing with several issues that I inherited from the previous owner. What I'm working on right now is the fuel system. It don't work. To get it to work he ran a wire from the battery to a switch, then from there to the front tank fuel pump. So the truck runs, but has bypassed a safety feature and I would like to correct this.

I checked and both the ECC and fuel pump relay click at key on. I have battery voltage going in to the FP relay, and with the relay engaged I have battery voltage out. I tested voltage at the inertia switch, unplugged it and tested and had 3.6v at key on, with key on, and 4.6 v with the engine running.

I then tested voltage at the tank selector switch, Unplugged and tested right at the plug. with key on I had 3.53 on the red/with yellow stripe wire, that's #2 on the switch, and 7.32 on the yellow/ with white stripe, that's #5 on switch. I also checked the tank selector switch and it seemed to work correctly.

I have not had a chance to check voltage at the fuel pump with key on, sorry. I'm not one for doing swapnostics, I prefer testing and finding the problem. Any insight on this would be great. Thanks.

 

At the fuel pump relay, what voltage do you have on the Yellow wire?

Here is the diagram:


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I have battery voltage at the yellow wire

 

At the fuel pump relay, what voltage do you have on the Brown wire with pin #6 of the EEC self-test connector jumped to to pin #2 or grounded with the key on?



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I don't have a brown wire, looks like a green wire with a brown stripe, I have battery voltage.

 

Looks like someone has replaced the Fuel Pump Relay Socket.
If you do not have battery voltage on either wire at the Inertia switch then it sounds like the splice they did when they replaced the socket is bad.



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That may explain why the wire color isn't matching up. It's a bit confusing. I know nothing about the history of this truck, so I'm working threw each issue. When I get back today I'll try to trace the wires and see what I find. Thanks for the help.

 

By the way the wire colors you have are like the 1991 F-series.
Are you sure you have a 1990?



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You know, that matches what I have. I had thought perhaps I had the wrong year, so I went and checked the title, it says 1990 on it. Is it possible this is a late build 1990?

 

I think I may have just found it. That new diagram matches what I have. I tested voltage on green with yellow stripe and I have .04v with key on and when I jump #6 on the test plug. Shouldn't I have battery voltage? Bad fuel pump relay?

 

If the fuel pump relay clicks closed and you have battery voltage on the yellow wire and you do not have battery voltage on the Dark Green wire with a yellow stripe then yes you have a bad relay or relay socket.

 

Great, looks like that's it. Next week I'll pick up a new relay. I was told you would find the problem. Any thoughts why a 1991 diagram matches my 1990 truck? I checked the title as well as the owners manual. Both say 1990. I guess someone may have swapped engines. I have no information on the trucks history. Again. Thanks.

 

Check the VIN number for the year and not the title or the manual that came with it.
Ford could have ran out of wiring harness at the plant that made it and started using the next your harness also.

 

That's my guess, a late year build. Really made tracking this down hard because others were giving me wire colors that didn't match.