Problem in wiring on truck, for trailer connection.

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
Just to get this thread rolling, I am going to describe the problem I am having...

Later, I will do a test using a light probe, to find out what is actually happening inside the connector that is on the truck.

Hopefully if I describe the problem, someone else has had this problem, and will already know the answer...


Connect the trailer up.

Turn on the running lights and every light on the trailer lights up.

Perfect.


Step on the brakes. Left light gets brighter, as it should.

Right light goes out. Completely out.


Left turn signal works.

Right signal does not work.

Hazard flashers works on left, but not on right.



This trailer works perfectly on other trucks. It is the wiring in my truck that is the problem.

And all of these lights work exactly as they are supposed to, on the truck itself.

Any ideas?
 

Oldred

Registered User
Joined
Jun 1, 2022
Posts
19
Reaction score
11
Location
Sunbury NC
Bet right tail light bulb on truck. Or socket wired wrong. Have seen turn signal switch do similar on a semi truck
 

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
I'll check but I'm pretty sure it's not the bulb.

That light and entire system works just fine, on the truck. It's only on the trailer that it doesn't work.

Pretty sure it IS the wiring at the connector, or somewhere just before it. Just hoping someone else has had the same sort of problem, and I won't have to go doing all the troubleshooting, pulling wires, and all that. LOL
 

Cubey

Full timer RVer
Joined
Oct 18, 2017
Posts
3,604
Reaction score
1,310
Location
USA
The factory "tee" connector that splits off for the 4 wire trailer connection went bad on my F250, totally disabling my tail and brake lights. So if you have one of those, check it. It's near the bumper under the bed out in the open. You can unplug it from the two connectors that go to it and connect the two connections together to bypass the tee.
 

franklin2

Full Access Member
Joined
Feb 24, 2009
Posts
4,674
Reaction score
986
Location
Va
Bad ground on the right taillight. When you turn the marker lights on, it's using the brake circuit as a ground. When you press on the brake, the voltage coming from the brake circuit bumps into the voltage running up backwards from the light marker light circuit, and that makes the light go out.
 

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
The factory "tee" connector that splits off for the 4 wire trailer connection went bad on my F250, totally disabling my tail and brake lights. So if you have one of those, check it. It's near the bumper under the bed out in the open. You can unplug it from the two connectors that go to it and connect the two connections together to bypass the tee.
Thank you. I will look for the tee. I believe the problem will be there, or between there and the trailer connector itself.
Bad ground on the right taillight. When you turn the marker lights on, it's using the brake circuit as a ground. When you press on the brake, the voltage coming from the brake circuit bumps into the voltage running up backwards from the light marker light circuit, and that makes the light go out.
That sure as heck sounds like it! Thank you.

I'll check the ground to that light.

Deal is, the light on the truck works correctly, in every way. Whether the running lights are on, or not. There is absolutely no problem with the light on the truck. Running lights, brake light, turn signal, all work the way they are supposed to.

It is only on the trailer that it is wonky.
 

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
This looks just like my connector.
You must be registered for see images attach

It's a 7 round. And a 4 flat built in, on the side. So I can use either kind of connector.

I don't know what it looks like behind the bumper. I haven't checked yet.

Tomorrow I will have to build some kind of rain cover, then I will crawl under there and see what I find.

I am 99% sure that the problem is in the connector, or the wiring going to it. Not in the truck.

As I have said several times now, the lights on the truck all work exactly as they are supposed to. No problem whatsoever.

And the trailer works perfectly on other trucks. So it's not the trailer.

The problem has to lie somewhere between the truck wiring, and that connector that the trailer gets plugged into.
 

chillman88

Supporting Member
Supporting Member
Joined
Feb 22, 2017
Posts
5,420
Reaction score
5,067
Location
Central NY
I understand your thoughts on the truck lights working, but it's entirely possible there's sufficient ground for everything until the added load of the trailer lighting. It could be a poor ground, not necessarily a broken ground. I'd definitely check the dedicated trailer harness ground very well since you're in there.

How are the pins on the trailer plug? I was checking mine the other day for an inop right signal, turns out if I push the trailer plug towards the driver side of the truck the turn signal works fine. Must be my trailer plug is more loose than I thought so it's not maintaining a good connection. Food for thought there too.
 

chillman88

Supporting Member
Supporting Member
Joined
Feb 22, 2017
Posts
5,420
Reaction score
5,067
Location
Central NY
And just for kicks take a good look at that particular light on the trailer too. Just because it's only showing up on this particular truck don't ignore the trailer. I've seen some odd nonsense when electrical gets involved of things that shouldn't happen happening anyway.
 

Nero

Full Access Member
Joined
Jan 3, 2022
Posts
453
Reaction score
348
Location
OR
Do other trailers do this on said truck? I usually get a test light and check the connector itself, or if you have access to it, one of those plug in dongles that has led's on them to show if the signal wire is working.

How is your trailer pigtail wired on your truck? Self spliced in? Or one of those inline adapters that you just connect to the oem harness? Was it something you installed, or a previous owner/business?
 

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
Excellent thoughts.

I've decided not to work on it today because it is just raining too hard. I don't have a garage or carport. I could rig a tarp, somehow, but it's raining hard enough that I'd still be getting wet.

I'm retireded. I don't HAVE to get this done today, so I'm not gunna! LOL

I'll be using a test light, first thing. I did do a quick run with one the other day, and BOTH, the spot on the 7 round, and the spot on the 4 flat were dead, when the right turn signal was operating. I believe it also didn't come on with the brakes.

I'll do it all again, and write everything down as I test it. And I will probably look for one of those test dongles as well.

I have no idea how it is spliced in. It was on the truck when the truck was given to me.


Once I have tested with the test light, I will be moving underneath the truck and having a look at the actual wiring. The two things I will be checking for first are the ground condition, and whether any wires have gotten cut or broken, or abraded through to a short condition.
 

Nero

Full Access Member
Joined
Jan 3, 2022
Posts
453
Reaction score
348
Location
OR
If other lights are working, chances are the ground is fine. Sounds like the right light wire got cut somewhere, should be an easy find. So long as there isn't decades old electrical tape that was poorly wrapped up in there to take off first.
 

Selahdoor

How can I help you, or make you laugh, today?
Supporting Member
Joined
Oct 26, 2015
Posts
2,198
Reaction score
1,876
Location
Index Wa
That is what I hope to find.

But the fact that putting on the brakes actually turns the right light completely off, makes me think there is something else going on.
 

Nero

Full Access Member
Joined
Jan 3, 2022
Posts
453
Reaction score
348
Location
OR
Sometines when actuating brakes, the running lights turn off, that may be what you're seeing. It's to prevent the bulb/socket from getting too hot at a stop light.
 
Top