Why are trailer lights always a problem with seemingly no answer? Purchased a used RV from a guy. Drove over three hours one way to get it because it was what I wanted and a good deal. Plus I'm retired and I find road trips like these enjoyable. So I get there I find everything to be as advertised give the guy his money and hook up the RV. None of the lights work. Guys says he just hooked it up to his truck the night before to make sure it all worked and it was fine. So we messed with it for about 20 minutes trying the round and flat connectors and we couldn't get the lights to come on.

I keep getting messages on my dash "Check Left Trailer Bulb" "Check Right Trailer Bulb" Check Trailer Light Module See Your Manual". I have a 2016 Ford Expedition. So the guy backs his Jeep up plugs the lights in and they work fine! Ugh! So I drive it home three and a half hours with no trailer lights. It was during the day so that helped and eighty percent of the people on the road don't use their signals even though they work so I fit right in.

I do know that 90% of trailer light problems are grounding. So I redid the ground on the trailer (white wire) which looked pretty suspect but it did no good. I'm still getting all the messages on my dash. As an added bonus I got to listen to the chime when all these messages kept rotating through my message center all the way home. A really fun one was when the message center displayed "Trailer Disconnected" while going 70mph down the highway! The problem has to be with my truck. I have pulled other trailers and sometimes I get these messages but the trailer lights are working fine. Other times I hook up and no messages and everything works. I did a quick search and didn't find anything that really helped. Anyone have any ideas? Thanks in advance.

It's been a while, but going with what is left of my memory. The connector at the back of the factory trailer plug would back out or have some corrosion issues.
There is a TSB on it (couple years newer than yours), may still apply though.

https://ford.oemdtc.com/3893/t...17-2018-ford-lincoln

Worth checking, if I recall the connecter is round and never seated completely.

Wow, this is a timely rant!

Just this week I noticed the left side lights on my trailer weren't working. Again.

I just worked on this stinkin' thing a month ago!

I'm of a mind to just tear out all the wiring and start fresh. I'm even nerdy enough to run the new wire in conduit, to keep it from getting caught on stuff or pinched by strap hooks, or otherwise abused.

Years ago I had issues that were similar. The problem was the plug in my truck, there was wet crud, muck, wedged in my plug.

I forget if I just cleaned & dried it, or replaced anything. It seems to be an issue with your truck, likely the wiring or connection in or near your plug in.

That area is susceptible to wetness and abuse.

I cleaned all the plugs and connections with spray electronics cleaner so all that is clean. The connector on the trailer is the 4 wire flat connector. I have both round and flat on my truck. I have the adapter for the round to flat connectors. That adapter has translucent housing that has a light indicator. When it is plugged into my truck and I put the hazard lights on you can see the light blinking indicating it is working to that point but yet the trailer lights aren't working! Argh!!

The connector in question is the one that goes from the truck to the truck trailer plug, underneath/ behind the factory trailer plug/connector.
Large and round connector, once it gets the full electrical load of the trailer on it, it fails. Very hard to pull apart when dust/dirt gets packed in the two pieces.

Over the years I have sometimes found that driving down the road for a few miles the lights come on. (horse trailers, car haulers and boat trailers). My theory is that the ground was established through the polished ball, socket and hitch.

Works with his truck, works with his Jeep, doesn’t work with your tow vehicle. I’d start with the trailer wiring commection on your vehicle.

It sounds to me like the problem is in your vehicle, not necessarily the trailer lights.

It's a known problem, especially with a TSB on it. The round connector behind the trailer plug. Pic of it here:
https://www.etrailer.com/Custo...?VehicleID=201645673

If there is another connection in that harness, check it too.

quote:

Originally posted by rizzle:
It's a known problem, especially with a TSB on it. The round connector behind the trailer plug. Pic of it here:
https://www.etrailer.com/Custo...?VehicleID=201645673


If there is another connection in that harness, check it too.

That pic helps rizzle thanks! What's weird is I show power at the output of the round connector below the bumper as evidenced by the light flashing in the connection going to the trailer. So if that plug underneath the truck were the problem I wouldn't be getting a light flashing in the connector going to the trailer right? It's like I told the guy I bought it from. I don't care if trailer lights worked yesterday, an hour ago or five minutes ago it's an adventure every time you hook them up and need to go somewhere!

Your not putting a full electrical load on the connection, it my be good enough for the tester but not the trailer. It's not a very good design, there may be two more factory connections at the corner of the frame that are zip tied in place.
Check them all, some WD 40 will help reconnecting them, make sure they are fully seated. Other than being a pain in the ass to mess with, not gonna cost anything to check those.

quote:

It sounds to me like the problem is in your vehicle

All of this "smart" crap on vehicles is certainly part of the problem. I don't need my vehicle deciding when a trailer is connected or not, I just need it to send the appropriate power down those wires when I tell it to.

Had just about the same issue with my Expedition. I found the ground wire from the plug to the vehicle was faulty. Unbolted the wire from the frame cleaned it to bare metal reattached and shot it with a good coat of paint

Figured it out by attaching a wire to the trailer frame and jumping it to the truck frame, and screwed it down. Everything worked fine. This is not to say the rest of your plug isn’t bad but it’s an easy job to clean that ground attachment.

I remember it being on left side a couple feet forward of the bumper.

Happens with my F150, no message center so thankfully no chime, but the 4 plug connector will not always work, suspect sand, dirt corrosion however the large round plug will work so I have to carry that adapter for the 4 plug trailer harnesses.

Since you've had issues with other trailers sounds like the problem is on the truck side...

Thinking about getting a set of these wireless trailer lights as a backup, I saw them on Tee Vee.. But if you have harness issues, you'll still have a problem.

Link

I pulled a boat trailer regularly for many years, and other trailers occasionally. I’ve never had problems with trailer lights. In general, make sure all contact points are clean, and keep all plugs covered when not in use to make sure they stay clean. Use dielectric grease to make sure they stay corrosion free. Use dielectric grease in the bulb sockets as well.

One other thing- many vehicles that have sensors that tell you when a bulb is out use the current draw of the bulb for that determination. If you use (or trailer has) led bulbs instead of standard incandescent bulbs, your “smart” car will think the bulb is out because the led pulls so little current. I would see if the new trailer has led bulbs, and try replacing with standard bulbs if so.

There is a fuse in my Tundra that partially backs out and causes problems with trailer lights and brakes. It messages that no trailer is hooked up...reseat the fuse and all is well.

Gotta be a ground connection on my truck. Went out yesterday morning to start the truck. Going to tow the RV up to the lake place where it will live all summer and serve as a bunkhouse for extra guests. For some reason I decided to switch the trailer connector from the round plug with the adapter on my truck to the 4 prong flat connector on my truck. Hit the hazard flashers on the truck and lo and behold the lights worked! This is about the tenth time I've tried the flat connector and it never worked before.The wife followed behind me all the way up to the lake and said all the blinkers and brake lights worked just fine!

But I was still getting messages on my dash but not quite so many. I'm about due for an oil change and I take my truck to a friend's exhaust shop and he changes the oil. I'll get a good look at things when it's on the lift. Thanks for all the advice here gentlemen!

A few years back I was purchasing a trailer. When I went to pick it up the dealer was working on a Ford truck. He told me he always had problems with Ford trucks and their trailer wiring harnesses.

quote:

Originally posted by lastmanstanding:
I cleaned all the plugs and connections with spray electronics cleaner so all that is clean. The connector on the trailer is the 4 wire flat connector. I have both round and flat on my truck. I have the adapter for the round to flat connectors. That adapter has translucent housing that has a light indicator. When it is plugged into my truck and I put the hazard lights on you can see the light blinking indicating it is working to that point but yet the trailer lights aren't working! Argh!!

I have a 2019 Expedition. I had a similar issue. The night time running lights weren't working. There are several fuses for the trailer light functions 1. turn signals 2. trailer running lights 3. trailer stop lights. Pull out your owners manual and see which ones they are and test them with a multimeter. Also a search on Expedition forum is a great help as some years Expeditions didn't even have the fuses installed etc.......some had issues with ground etc.