I have a 1998 F-150 with factoy flat 4 wiring harness. My boat trailer turn signals and brake lights do not work. (They work fine with my Windstar). I have checked both 20 amp fuses, replaced the wiring harness but still no luck. I have put a DVM on it and have no power indicated for the turn signals, brake lights or hazard lights. It has 12 volts DC with the running lights on. Does anybody have any ideas?

Thanks

 

Have you used your harness before. If I remember correctly, my 1999 5.4L F150 truck came with a towing relay that I had to install to use my harness.

 

Q1. Is is safe to assume that the parking lamps / turn signals work correctly on the truck ?

Q2. Did the flat 4 ever work ?

Q3. Do you also have the 7 pin round connector on your truck ?

Q4. Which 2 20 A fuses did you check in which fuse panel ?
4.1. Battery Junction box , Fuse 1 runs the trailer tow lamp relay
4.2. Battery Junction box , Fuse 16 runs the tailer tow battery charge relay ( for the 7 pin )

Q5. What harness did you replace ( from where to what connector ) ?

Q6. Where are you getting 12 volts @ with the running lamps ( trailer or Adapter ? )

My '01 I thought I had to install 2 relays, 1 for the reversing circuit, the other I cannot recall if it was for the reversing ckt, or the trailer lamps.

Using the 4 pin adapter on the truck, and the white on the connector as ground, not hooked up to the trailer :

Q7. Dark Green wire : Is there power ( pulsing ) with the 4 ways on ?
7.1. RH side turn / brake lamps

Q8. Yellow Wire : Is there Power ( pulsing ) with the 4 ways on ?
8.1. LH side turn / brake lamps

Q9. Brown Wire : Is there power with the parking lamps on
9.1. Trailer parking lamps.

There are some old and new Body style info in the trailer tow section, sticky at the top.
https://www.f150online.com/forums/sh...d.php?t=184757

This shows the 2000 MY info, but I think this is still close to the '98 ( could be different ).
You can see where the trailer tow running lamp relay is on the LH side of the drawing ( not high lighted ) just below the washer pump relay. This is a half size ISO relay.

Q10. Is the trailer tow running lamp relay installed ?

Sorry for so many questions, just trying to get enough information to get you an answer ASAP.

Let us know.

 

Whoa SCULLY, im impressed!

 

WTF was that, War & Peace ??

 

I dont know if you mention this in your original post in your process of trying different things..

However have you tried actually turning on your truck's headlights and then checking to see if your boat trailer lights work?

I have a utility trailer I pull around and I guess with the way my topper 3rd break light is wired in to my tail lights and that being wired to my trailer light plug ins (having both kinds trailer connections) I have to turn my headlights ON to get my utility trailer lights to work.

I havnt pulled anything else besides that so I dont know if this issue would carry over to any other trailer.

Just a suggestion.