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I am having a trailer lights problem that I have finally narrowed down. I did have a thread up, and had received lots of help, which is what got me to this point (Thanks agian).
Recap: No trailer turns/brakes, fuse good, trailer lights/wiring good.
So I have it narrowed down to the relays. The only wire that gets any power is the truck light wire (Which I am assuming is the control switch). None of the other wires are getting power. I was under the impression that the relay had constant power going to it and when the "switch" was activated, it in turn activated the relay. I was going to test the relay and quickly realized that I have no clue how to do it. The relay clicks, but I am assuming that does not nessecarily mean its a good relay. I had replaced with other relays at one time with no change, but am still stumped.
I have triple / quadruple checked the fuse and even replaced with a known good one. I checked continuity in the wire from firewall to rear of truck and all of that is good. Its somewhere between where my harness plugs in on the L front fender well to the battery.
You should have power all the time to the relays like you thought. You need to find out why you do not have power there feeding the relay contacts. Sounds like the switch and the coil of the relay are working, and since you swapped out relays, the relay is probably not the problem.
I agree it's probably not a relay, but look closely where the relay plugs in. Both the relay and the socket can get corrosion, enough to not make contact. It's usually green if corroded. Sometimes the connector/socket even melts and I've had melted plastic keeping it from making contact.
OK, I just checked and did not see any corrosion. I decided to test agian and came up with something that baffles me. Both relays have one wire color in common so I assumed that its the constant power that feeds the relay for the turn/brake activation. But that was not the case. I got low voltage readings on the wires that correspond with the color of wire that is at the trailer plug.
Best I can do for an image.
L side top L (as seen) green w/ Orange - Truck lights (??Switched power??)
top R Black
Middle Yellow - Low voltage of .22
Bottom Dark Green w/ Orange - No voltage
R Side top L - Orange with L Blue - Truck Lights
top R - Blk
Middle - Dark Green - Low Voltage of .14
Bottom - Green w/ Orange - No Voltage
With my VERY limited electrical knowledge, I would assume that the Dark Green with orange would be the constant power as it would not matter which side (R or L) would matter. This is of course under the hood, and all the wires go into a common harness with NO room to work, and I can only guess that it comes from the fuse box.
 Oh Yeah, The wires that are labeled as truck lights I do have good power to, and have followed those thru the harness under the truck.
Any more help you guys can give me would be appreciated.
I knew that was somewhere. Thanks Franklin2. I thought it was saved on my computer but couldnt find it.
Also, A few of my ford buddies here have told me about the "big fuse". Seems that they had no trailer functions (Not sure if it was all or partial) until they installed a maxi fuse that apparently did not come from the factory. Two of them were SD, and one Excursion. What would that fuse be listed as? I am pretty sure all of the ones in my diagram are there...
 After looking at the diagram, I looks as if its labeled Trailer Turn Relay/battery charge. It also appears as if the wires that I am supposed to be getting constant power is DG/OG. Franklin, would you please double check me to ensure I am reading the diagram correctly?
Yes, you are correct. Be careful going by other friends experiences. If their trucks are different years, they are probably wired different. The trailer tow wiring seems to be something the Ford engineers never were consistent with year to year.
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