Ok guys so I have a 1990 f150 xlt lariat with a 5.0 302. I am fixing to start rebuilding the motor but I need to get all of the electrical problems fixed first. So the truck has been sitting for about 3-4 months and ran fine when I parked it, but now it just doesn't want to start. When I turn the key the eec relay clicks, but the fuel pump relay does not, therefore the fuel pump does not prime. I have replaced the fuel pump relay and the connector, the eec relay and the connector, I have checked the inertia switch, its good, I have checked the fuse link, I replaced the ignition switch and I have checked to see if I get spark and I do. What gets me though is I can pop the cover off of the fuel pump relay and manually push it down and the fuel pump will kick on. I do not know what to do next, I need to know ASAP so I can get started on this rebuild, any help will be GREATLY appreciated!! Thanks guys!

 

What happens when you turn the key?

 

Try replacing the EEC relay as that is what powers up the pump relay also check the ground on the coil on the pump relay.

 

I have already replaced the eec relay and the connector, but still no luck. When I turn the key the eec relay clicks on and off quickly, but no fuel pump prime. The check engine light is on but dim. The truck will start with starting fluid but not run by itself.

 

Like I said when you turn on the key it energizes the coil on the ECM relay which closes the contacts on it sending power to energize the pump relay coil which in turn closes the contacts in it sending power to the pumps.

So either you are not getting power out of the ECM relay to energize the pump relay or you are not getting power out of the pump relay. Since you can manually make the contacts and the pumps run you know everything from the relays on is ok so either you have lost power at some point between relays or lost the ground to keep the relays from working

 

Sounds like you have power to the relay on one part of it. It's ready to close and transfer power to the pump circuit. But there is a separate power source to activate the relay (actually make it close). That's a different circuit.
A relay is not a fuse. It's a remote control switch. So if you've got power ready at the switch but the remote control part isn't delivering power to the activating side, the switch won't do anything.
Go over all the circuits with a test light especially the contacts the relay plugs into. Also sometimes the wire underneath those relay contacts can go bad. You have to unclip the relay box to take it apart and turn it over to look at those.

 

Alright well I got up this morning and got to work on it, I found a problem. There is a tan/green wire going to the fuel pump relay, in my Chilton manual it says that wire goes through the self test plug then straight to the eec. Well I turned my ignition on and touch a test light to that wire and the relay made the connection and the fuel pump ran. The truck started and ran like normal. BUT it will not run unless I use a jumper wire to ground it out. So with that wire grounded, I turn the key and the fuel pump comes on and stays on, it is not priming like it should so I do not know the cause of this. How can I get it to prime like it should? when I turn the key the check engine light comes on and stays on but when the motor is running the light it not there. Any input will be appreciated!

 

The self test plug has a place to manually force the pump to run. That's what you're activating, and everything works. Sounds like you're not getting power through the circuit that's supposed to activate the relay. Bad wire, fuse, connector ? Or worst case, the computer itself, but You want to check everything else first.
I know that I've personally experienced a bad connection on the underside of the relay box where the wire joins to the connector that the relay plugs in to.
But it could be anywhere. Follow the circuit - power to here, move along power to there, move on etc. until you find the interruption point. Your problem is not uncommon, but your truck is unique - so we can't pinpoint the problem for you, you just have to play Sherlock Holmes

 

I did what you said and followed the circuit and there is no break, it ended up being the computer itself, I really appreciate every ones help! I think it was due for a new computer anyway because there was a high altitude eec installed by my grandfather but I never knew about because he passed away before my dad gave me the truck! Im only 18 years old but I learn a lot from yall! Again thank you everyone