The only way I could get my truck to start was to tie it into the red wire?

89F150L6

MEMBER
1989 FORD F-150
4.9L
6 CYL
4WD
MANUAL
149,915 MILES

My fuel pump relay was having some issues seating in the harness (the relay as well as the EEC relay are new as of a few months ago). I was working on seating it properly and a yellow wire came disconnected clean out of the harness leading to the fuel pump relay. With it disconnected I could not start my truck despite it cranking over. When I went online to find a picture of the harness to see where the wire goes, the wiring is correct leading into the harness. The only way I could get my truck to start was to tie it into the red wire. My question is, where does that yellow wire (that seems to lead to fuse link w according to haynes that I also cannot find) go? There is another yellow wire that leads in, as well as red, tan/lt green and brown. I am an airplane mechanic, and usually do pretty well at wire chasing, but this one stumps me.

5 Replies

MHPAUTOS

MECHANIC
31,937 POSTS

I was working on seating it properly and a yellow wire came disconnected clean out of the harness leading to the fuel pump relay. Did the wire come out of the back of the relay / fuse box?

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Monday, February 15th, 2021 AT 9:54 AM

89F150L6

MEMBER
3 POSTS

There are only two relays. The EEC relay and the fuel pump relay. The wire can reach the fuel pump relay only, the only fuse box I have is a kick panel box on the cabin side of the firewall. If I tie the wire into the blank spot of the relay harness (my first thought, which according to all pictures and diagrams is supposed to be blank) the pump will kick in ONLY when the key is flipped from "on/run" to "off." It does nothing when I tie it into the brown wire, which leads from the relay to the pump itself. Finally when I tied it into the red wire, the pumps primed the system and the engine started. I did not check the tan/green wire.
Here's one of the pictures I used, as well as a picture I found on eBay that was pretty clear, all in conjunction with the wiring diagram. I also did some poking around and found no loose wires that lead to the EECM or towards the fuse box. The only thing I can think of is if it came from the fuel pump relay, and if it did, which socket did it come from?