I have a 1986 f150 supercab shortbed that i bought to keep someone from sending it to be crushed. It has the 5.0 fuel injected engine and a c6 auto transmission. I am having trouble diagnosing a fuel pump relay issue as I've read several conflicting explanations as to how this thing works. I've got 12v on the red wire when key is switched on, and i understand the brown wire is the output to the fuel pump via the inertia switch (mine is on transmission hump just under passenger side dash) but the yellow wire, which some have explained as hot all the time and some saying only when ignition is on, has no power going to it. I've tried to trace it back to the fusable link at the solenoid on passenger fender and found no breaks, but have yet to try to make a jumper wire and wanted to make sure that is the origin of the yellow wire. Also, the tan wire with a green stripe I have read goes to ground from the computer. Is the tan w/green a positive until the computer switches it to ground? I've got the truck to run through some tinkering with the previous owners wire hacking (they ran a switch power straight to the fuel pump from some plug under the dash that i can't figure out what goes to) was hoping to get truck going so i can start work on the body and interior then maybe five years or so do an ecoboost swap on it. my son is 9 months old, was hoping this may be his one day but don't wanna keep is getting it going for time being is going to be too much of a hassle. sorry about the long post.

 

Search and you should be able to find a schematic for the fuel pump relay. It is quite a simple circuit and there is a test pin to allow the pump to run. Subford has a great drawing in this post;

https://www.ford-trucks.com/forums/8...p-problem.html

I had a strange fuel pump relay problem last fall. Fixed it as described here;

https://www.ford-trucks.com/forums/1...y-control.html

 

Here's another schematic.

Yellow is the hot all the time wire. When the relay is energized, the power passes from the yellow to the brown and goes to the fuel pump.

The red is the key-on power from the EEC relay. When the key is on, it energizes the EEC relay, which then puts power on the red. So power is at the red all the time the key is on. The red goes to one side of the coil of the fuel pump relay.

But the relay will not energize, since it does not have a ground on the other side of the coil. The other side of the coil is the tan/lightgreen wire.

When the key is first turned on, the computer will ground the tan/lightgreen for a determined amount of time with a timer. It then ungrounds this wire and shuts the relay off. It then looks at the pulses from the dist and if it see's the pulses, it will turn the relay back on by grounding this wire, since it's verified the engine is turning or running, and will allow the fuel pump to run.

 

ok. had the day off so I figured I would go out and do a little testing. here's the first big problem I've found. The yellow wire going to the fuel pump relay is showing ground. I've tried tracing it back towards the solenoid on the passenger fender and have found no breaks thus far in the line but I'm sure it's showing as a ground. The truck runs through some rigging the previous owner did bypassing the fuel pump relay and inertia switch and I don't like it because it's unsafe in my opinion. And on top of that once it warms up I think I hear a knock in the bottom end of the motor. If I were to give up on this how much shame would it be to send it to a pick a part yard a friend owns so some others could get some use out of the pieces on the truck? would love to completely rebuild but most of my leisure budget goes to my tournament fishing habit

 

Aha! The dreaded "previous owner rewired it" problem.

All bets are off and this is extremely important info that should have been in the first post.

If you want to fix the truck and I think you should, you need to undo all the add on wiring and restore the factory wiring, repairing as needed. No amount of on line help will fix that because we have absolutely no chance of observing what the previous owner did.

As to scrapping the truck, well, it's your choice and your priorities. It is not a particularly rare object, so the world will not be lost if you scrap it. Could I suggest offering it up for sale rather than sending it to the scrapyard?

 

been out there trying to go through some of the wiring. I have power on the yellow wire of the eec relay and the number 1 pin (yellow wire) on the computer. guess I'm going to try and pull stuff off the top of the motor so I can access the whole harness and trace the wire. went to the pick it yard to try and find a harness and those trucks are getting scarce even there, no luck on a harness. here in south Georgia its almost impossible to come by a supercab shortbed at any reasonable price though I agree with you it's not like its a collector item. do you know if the yellow wire on the fuel pump relay leaves the eec relay and then continues on to the fuel pump relay. would greatly help in my trying to find the break in this wire. could be just pinched and shorted as there are not too many wires cut that I have found

 

The schematic above shows the yellow wire that the eec relay is the same wire or goes to the same place as the yellow wire to the fuel pump. One works and one doesn't?

 

Franklin2, thanks so much for the schematic. it really helped in finding the problem. I went back out to my shop and removed anything off the top of the engine in the way of the wiring harness and started tracing wires. found the yellow from fuel pump relay all the way back to solenoid on passenger fender, most wires untouched just the loom gone and a few splices that seem to be in the correct place here and there. anyway, the fusable link at the solenoid had been replaced and put on the negative side. fixed that and now I have correct power at fuel pump relay, inertia switch, and fuel pump! fixed wires to fuel pump and all is well. do think I hear a knock in the bottom of motor so if anyone knows where I may find one that isn't shot in south ga or north fla please let me know. This thing may turn out to me my son's first truck yet. 15yrs and 3 mos until he turns 16. lol.

 

How far away is Ocala. Florida? This place has a rebuilt engine for about $1000 on ebay. Maybe you can go pick it up and save $250.

http://www.ebay.com/itm/Ford-5-0-302-Remanufactured-Long-Block-Roller-Cam-/271708588569?pt=Motors_Car_Truck_Parts_Accessories&hash=item3f43180a19&vxp=mtr

 

I think Shaun Smith lives there....

 

may have to check that out. ocala is driving distance.

 

wanted to thank the people that replied to this thread. got most of the problems solved and truck running pretty much perfectly until i got brave and drove to work. now i have a whole other list of problems (and another thread to go with them) but the info you gave was invaluable in me getting the truck on the road.