I have a 1990 f150 4.9 4x4 5 speed. Im having fuel pump issues. I’ll give a recent history of the truck. It sat for the last 7 years. Last year (2018) I put a new front tank and fuel pump in it. Then the computer failed causing the fuel pump to constantly run. Replaced that. A couple hundred miles later my #6 injector got plugged up. So this winter (2019) I pulled it all apart replaced injectors and fuel filter. Also put a fixed the egr while I had it apart. It ran great I let it idle a bit while I was getting it road ready for summer.

That’s where the problem starts. I was taking it out for the first drive of the year started driving down the driveway and the truck died. I checked the relay and nothing. The next morning I got a new relay put it in and it started up. So I took it into town to get some gas and a mile down the road it quit again. Same issue no fuel pump. So the next morning I went to start it and it ran but died. Again same issue. So I started looking for broken wires checking all of them with a multimeter and they are good. Wires to the fuel pump are good I manually checked those with the battery and they worked. Then I checked the connection to the computer and those are good. I have 12 volts coming to the relay from the yellow wire (always hot) and 12 coming up the red wire (key on). But even with this the relay won’t click. I’ve checked the relay multiple times and it’s good.

I attempted to pull codes but nothing happens. Which is the first clue that it could be a computer issue. Also I notice that some of the time the check engine light will glow dim with the key on and once I shut the key off it stays illuminated for a few seconds until I hear an audible click under the hood and it goes away. The last odd thing it does is when i turn the key a few seconds later the speakers pop, the tack jumps and there is an audible click from the distributor area. It only does it when the fuel pumps don’t run.

That’s what I have so far. I’m unsure if I’m missing a ground somewhere and I don’t want to get another remanufactured computer if isn’t that. Kind of puzzled at this point.

 

When the fuel pump does not run short pin #2 to pin #6 of the self-test plug and turn on the key. The fuel pump should run until the key is turned off.
If it does the computer is more than likely bad.

If it does not run then short pin #6 of the self-test plug to a ground point and turn on the key. If the fuel pump runs now you have a bad ground through the computer to the body ground point.

 

Forget all the hook ups below. I am only showing you where pin 2 and 6 are in case you don't know. Sandy

 

I jumped 2 and 6, no fuel pump. Then 6 to ground and the pump ran. Shame on me for not understanding the test connector better.

Im looking through my Haynes manual now and it shows that pin 20 runs into an pink/orange wire connecting to a black wire that grounds.

It also shows pin 6 going to a black wire then to ground. Are both of these grounds for the computer?

 

Just to update I checked pin 6 and the ground is good. So I checked the other pin. And there is some resistance in that wire but I’m still unsure if that’s an actual ground.

 

For 1990 and newer Trucks the Computer ground is the black wire with a white stripe in the body ground photo below.
1989 an older Trucks used a black wire with a light green stripe at that point for the Computer Ground.
The large cable is from the NEG post of the battery in the photo below.
It grounds pin #40 & #60 in the computer plug.

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Why is the computer grounded up by the head light, shouldn't grounds be as short as possible?

 

It is the best place (Main Body Ground) for a good ground for the computer.
All Ford Trucks from the late 80's or before to the 1997 Ford Trucks or beyond use the Main Body Ground for the Computer Ground.
The Main Body Ground is the shorted route to the NEG battery post of the battery for the Computer.

 

Ok, that was the issue. The main ground wire from the core support to the battery has been disconnected as long as I owned the truck (didn’t know what it was for). The two grounding wires for the computer where held on with a sheet metal screw that had backed out. I’m going to go back through and properly connect everything the way it was from the factory. I wish I would understood the grounding for the computer better, it would have saved me three days of tearing though the harness. Thank you to everyone for your help!