Can NOT get trailer lights to work

[tstex]

@tstex

The parts guy at Tasca is sort of correct.... I believe the harness has 5 or 6 direct connect plugs on it... (see pic below)... I believe 2 connect on the right rear & 2 on the left rear near the tail connections on the harness... The others 1 or 2 connect to the truck harness on the frame (I believe one is power & the other is for the fuses & brake controller.... but don't hold me to that)

It is not a very long harness all connections should be at the rear of the truck... when you get the new one & lay it out it is pretty straight forward. I installed one on my brothers truck yrs back... so I'm going on memory...


View attachment 731316

Thanks again, dlc ! the A part number is much, much shorter than the B part #. the B one looks like intestines bc it has so many diff wires...If you have installed in on a F-150 before and all is good, then I will go w that, order and then post back if I need help. if I knew the total length of the harness, I could just work backwards from where the receptacle goes in to see where the 4-pin rectangular plug goes.

Finally, another challenge I am going to have is "rounding out the hole" in the bracket where the 7-4 pin receptacle goes. When my back end hit the ground/street, it made the round hole oblong. The round portion of the receptacle was also "oblonged" and made it impossibe to install the 7-pin adapter. I am going to have to "re-round" it out w a round rat-tail file or similar. Once it's good, I am going to tape-off the area and spray paint it black w 4-5 coats. That bracket is welded on there pretty good so I am not going to remove it unless it's imperative.....thanks again !

 

[pmsmechanic]

What I did on my trucks is move all the trailer plugs inside the box. That way I have no more issues with corrosion from road salt and gravel beating everything to pieces. A short trailer extension cord and I can hook onto my trailers without issues and I haven't repaired a trailer plug of my own for years.

 

[sandytrail]

My 2003 Ford F250 was acting the same way with my trailer. I eventually discovered the problem was with a connector under the rear of the truck. That is where the pigtails that go to the trailer connectors on or near the back bumper connect to the main power going to all the trailer connectors. That connector was loose and I pushed it back together and everything worked. Just a thought.

 

[Dftodd]

At the risk of exposing myself (as incompetent in wiring)...

I recently went to sell a dump trailer. The plug wiring had been chewed off by my dog a couple years prior and I'd wired a new one in its place. As I never used this trailer on the road (just on-farm, behind a tractor) I had never tested it. And wouldn't you know it, I went to test it out hooked up to my truck only to find out that I didn't have lights (some, but they were all messed up). Checked all the wiring on the trailer and all checked out. Verified that the plug was wired up correctly (7 pin). Was totally stumped as my utility trailer worked when hooked up to my truck. After more time than I'd like to admit, I finally figured that I'd wired the receptacle (truck) incorrectly (years ago). And if you're following along you'll understand that this means that my utility trailer is ALSO incorrectly wired (still on my TODO list).

Don't assume that plugs and receptacles are wired correctly. From one perspective they can appear to be, but it's easy to get flipped when looking at schematics and then reversing things as you wire into the back side.

There are wire color changes going from a four flat to a 7 pin rv plug that is standard on most trucks.

The yellow wire on a 4 flat is left turn, green is right turn, brown is parking lights.

Once you get up to the 7 pin, yellow is aux power, brown is right turn, green is parking lights, red is actually for left turn.

What many people make mistakes with (me included) is when someone is used to wiring up 4 flat plugs all the time.

If you convert a trailer thats wired for a 4 flat to a 7 pin and hook color for color the lights start acting really trippy left brake light will stay on all the time. Parking lights turn on the right turn signal. And right turn will flash both parking lights.

And your sitting there wondering what the F is going on with your blasted trailer lights

Every newbie mechanic I get through my shop runs into this issue the first time they install a 7 pin rv plug on trailer wired for a 4 flat

 

[newbury]

My 2003 Ford F250 was acting the same way with my trailer. I eventually discovered the problem was with a connector under the rear of the truck. That is where the pigtails that go to the trailer connectors on or near the back bumper connect to the main power going to all the trailer connectors. That connector was loose and I pushed it back together and everything worked. Just a thought.

I had a similar frustrating problem with me F350. The truck side of the trailer was "flaky". All lights and brakes would work. Then they might not. Traced it down to the plug in the truck had a corroded connection inside. When pushed hard together (like when plugging in a 7 pin tester), all worked. With the trailer wired up it might not.
Replaced the plug.

 

[DieselBound]

There are wire color changes going from a four flat to a 7 pin rv plug that is standard on most trucks.

The yellow wire on a 4 flat is left turn, green is right turn, brown is parking lights.

Once you get up to the 7 pin, yellow is aux power, brown is right turn, green is parking lights, red is actually for left turn.

What many people make mistakes with (me included) is when someone is used to wiring up 4 flat plugs all the time.

If you convert a trailer thats wired for a 4 flat to a 7 pin and hook color for color the lights start acting really trippy left brake light will stay on all the time. Parking lights turn on the right turn signal. And right turn will flash both parking lights.

And your sitting there wondering what the F is going on with your blasted trailer lights

Every newbie mechanic I get through my shop runs into this issue the first time they install a 7 pin rv plug on trailer wired for a 4 flat

My issue wasn't with changing plug types, but with replacing plugs. Looking at a schematic from the front and then wiring at that back- it's easy to get reversed/flipped. I suppose with a four-pin it's not an issue (flat). A note, however, is to not assume that something is wired correctly in the first place: whoever wired up my truck before me had one hack of a mess (and then someone installed an aftermarket security system and what I found under the dash area was as repulsive of a job as I've ever seen- was running wiring for a new brake controller and was forced to clean things up because I couldn't button things back up in such disorder).

 

[Dftodd]

My issue wasn't with changing plug types, but with replacing plugs. Looking at a schematic from the front and then wiring at that back- it's easy to get reversed/flipped. I suppose with a four-pin it's not an issue (flat). A note, however, is to not assume that something is wired correctly in the first place: whoever wired up my truck before me had one hack of a mess (and then someone installed an aftermarket security system and what I found under the dash area was as repulsive of a job as I've ever seen- was running wiring for a new brake controller and was forced to clean things up because I couldn't button things back up in such disorder).

I was tracing out some trailer light issues on a piece of equipment that had just been shipped in from another location.

I soon came to the conclusion that the person was dyslexic and color blind

Plug was wired completely wrong. Then there were several places in the wiring where they had cut and spliced wires back together. One splice had a green wire hooked to brown. Then yellow was hooked to green, and then they managed to hook the brakes to the turn signals

I came to the conclusion pretty quickly that it would be cheaper and faster just to replace the wiring.

 

[Bearsixty7]

There are wire color changes going from a four flat to a 7 pin rv plug that is standard on most trucks.

The yellow wire on a 4 flat is left turn, green is right turn, brown is parking lights.

Once you get up to the 7 pin, yellow is aux power, brown is right turn, green is parking lights, red is actually for left turn.

WHAT?? What year and model because most 7 way 's have used the same convention as the 4 ways for many years, they just add 3 more,

Aux is Red
Brakes is blue
Backup lights is center pin, usually light green

Many examples of this on the web.

 

[Dftodd]

WHAT?? What year and model because most 7 way 's have used the same convention as the 4 ways for many years, they just add 3 more,

Aux is Red
Brakes is blue
Backup lights is center pin, usually light green

Many examples of this on the web.

Might wanna pick up a 7 pin rv plug from the local parts store that is labeled for color and try it out.

You'll find the diagram you posted is incorrect. Functions labeled in your diagram are correct. The colors are wrong though.

Not sure on the vehicle side. All I test for is function there when diagnosing a trailer issue.

On the trailer side, blue and white are correct. Yellow is labeled as the center plug for aux power. Black is back up lights, red is right turn, brown is left turn, green is right turn.

I haven't seen a replacement 7 pin rv plug thats been labeled for function in about 6 years. They've come labeled for color and wire up as I stated above.

I just replaced two of those plugs today on equipment at work.

I take a plug apart Monday and post a picture.

 

[DieselBound]

WHAT?? What year and model because most 7 way 's have used the same convention as the 4 ways for many years, they just add 3 more,

Aux is Red
Brakes is blue
Backup lights is center pin, usually light green

Many examples of this on the web.

NOT flat pin: I was wiring up plugs and receptacles. And, as I stated in my original posting, I'm not a wiring genius (BUT, I'll at least not make a rats nest out of things ). Just thought that I couldn't be the only one who has gotten things backward: wasn't born knowing these things.