My trailer stoplights and turn signals do not work when I tow with my 98 F150. The trailer parking/running lamps work fine and all of the lights work on the truck itself. I checked all the fuses in both the inside and underhood panels.The owners manual does not list a relay for this. Any ideas on which relay if any controls this. If it is a shorted/broken wire, would it be shorted or broken between the 7 pin connector and the tail lights or does the tow wiring originate and run seperate from the wiring for the truck's tail lights ?

 

Hi Azfordfan

I assume you have the owners handbook. The 98 book i have dosent show a relay, just a 5amp fuse, #18.

I aint no elect. whiz, got crooked finger nails to prove it.

You may have already found your problem? Some thoughts, curious, did you use a "test light" on the (rear) 7 pin connector? To see if the stop/turns are hot. when you apply foot brake or use turn signal?

Did you check the ground wire on 7 pin and also ground wire on trailer plug. Maynot apply, but those connectors get dirty etc. Be sure where wires are connected to plugs, there clean etc. Pull off tape or ? so you can get a good look at the connections.(thats if the plug ground aint workin.) Then spray some WD40 or? to clean them out and leave a protective film on contacts.

Maybe someone else here can help more. Sometimes its just dirt & loose connections, but? I think that 7 pin is tapped into the wiring and not a separate line.

Good luck. OT

 

Thanks for the reply. All my fuses are good, a blown fuse would have been the best news. I used a voltmeter to check both turn signal pins and the stop light pin at the 7 position connector. No juice on any of them. I had the brake pedal depressed at the time and the truck's tailights were working fine for brake lights and /or signals. My connector's ground is good because I have parking lamps that work on the trailer and voltage can be measured at the 7 pin connector when the parking lamps are on.( I guess that the stop/turn lights could use a different ground but I would be surprised if that were so. ) It's unlikely to me that all 3 wires (left turn, right turn ,stop ) would go bad at once to the connector. That's why I suspect a fuse/relay problem. Any help is greatly appreciated. Jim

 

Hey Azfordfan

Maybe you already found problem?

If not, more grunt work. If you havent done this, try testing that 7 pin connector from the back side, where the wires are connected to the plug terminals. Peel off tape or what ever covers wires so you can get at the connections.

If some of those wires arent hot when tested, then you know its forward of plug.
If you follow that wiring harness you may run into where its connected to another wiring harness (like on my 99 SC) If you do find that connection, then again, you gotta check that connector (not the one coming from 7 pin) Keep going towards front--

My 99 SC has a "trailer battery charge relay" in the power dist.box under hood, that supplies 12V current to trailer. One of those 7pins is this 12V, so look for blown chg. relay, as you said its not working?
My mistake, there should be a 40amp Maxi fuse for the TT battery charge & also Fuse 1 20A TT runnings lamps, but you said running lights working. Still kinda sounds like a short or bad connection. Tip, on relays you may know, but when working, you can hear the relay making contact, but gotta turn ign. on.(unplug relay & replug,should hear click) I pulled my hair out, cause i had an intermittant fuse (yeah rare)

This stuff as you know is trial & error.

Good luck OT

 

Happy to report that my lights are working. It did turn out to be a fuse problem. The truck has at least 3 fuse box locations (news to me , thought there were only 2). The 2 main locations are; under drivers side dash ,and what is called power dist. box which is under the hood on the drivers side. There is a smaller black box on the power dist.box (towards the firewall) which has relays in it.If you keep going toward the firewall there is another 1 inch by 3 inch black plastic box which holds up to 6 fuses. It looked to small to hold fuses, so I overlooked it , thought it was some sort of single relay or engine sensor. I had to cut off some factory tape/seal to open it ,my truck only had 2 of the possible 6 fuses. The 20 amp fuse was blown. Replaced the fuse and had voltage on the 7 pin connector. Makes you wonder how many other fuse locations exist. Thanks again for the help. Jim

 

Hi Az Glad you got problem fixed

Gonna ck. my truck to see if that other fuse box is there. I only saw one big one with relays & fuses;

Yeah, blown fuse, simple huh!

Tks. for posting how u fixed. OT