Troubleshooting trailer lighting in tow vehicle

In summary, the lights on my trailer are not working, but it's not clear why. I would first try the wiring diagram and multimeter or test light, but if the problem is intermittent, I might try the etrailer harness.
  • #1
DaveC426913
Gold Member
22,617
6,265
TL;DR Summary
The lights on my trailer are misbehaving. (Actually the electrical powering my trailer is misbehaving)
I've got one of these. It's not even a year old.

1686254972994.png

The problem is certainly in my tow vehicle (2013 Dodge Journey)'s wiring harness .

The trailer's port tail light is on all the time. The starboard light never comes on.
Neither braking, signaling, nor anything else - including whether the car is on or off - makes any diff.
I guess this started recently, since there was a time when the light worked OK.

The lights on my Dodge work fine, trailer or no.

(Most troubleshooting guides online concentrate on lights not working at all).I've been assuming my future holds a multimeter and a wiring diagram of my car's fusebox, but is it possible that these days they just "flash" the computer?
 
Engineering news on Phys.org
  • #3
berkeman said:
Which electrical connector does it use? You can get little inexpensive in-line LED adapters like this one to help you debug the connections:

View attachment 327630
https://www.amazon.com/dp/B07G8RJT3T/?tag=pfamazon01-20
I guess I could pick one of those up...

But it's kind of after-the-fact, ain't it? I don't know how it would tell me anything I didn't already know.
(OK, I guess it will tell me if my starboard taillight has power but is blown.)
 
Last edited:
  • #4
DaveC426913 said:
TL;DR Summary: The lights on my trailer are misbehaving. (Actually the electrical powering my trailer is misbehaving)

The trailer's port tail light is on all the time.
DaveC426913 said:
I've been assuming my future holds a multimeter and a wiring diagram of my car's fusebox, but is it possible that these days they just "flash" the computer?
If lights are off all the time, or work intermittently, I would first try the wiring diagram and multimeter or test light. But the light on all the time makes me wonder. The supply wire for that light shorted to an always hot wire? The electronics that control that wire failed on? A software failure?

In older cars, the trailer lights were tapped into the rear light wiring harness, which made it easy to diagnose trailer lights that worked differently from the rear lights. The newest vehicles have wiring that is too light to support trailer lights. That is why my 2022 truck has a black box powered by an always hot 12 volt connection, and controlled by the rear lights, to drive the trailer lights. The trailer hitch and wiring was not available as an option when I bought my truck, so I got a kit from etrailer.com. I see that etrailer lists a similar kit, a plug in harness with a black box and power wire to the battery, for your vehicle. If the factory wiring has a similar setup, I'd suspect the black box.

One option is to just get the etrailer harness. Something to consider if the troubleshooting gets too difficult.
 
  • Like
Likes DaveE and berkeman
  • #5
jrmichler said:
One option is to just get the etrailer harness. Something to consider if the troubleshooting gets too difficult.
Can I do this? Can I just replace the harness?
 
  • #6
DaveC426913 said:
But it's kind of after-the-fact, ain't it? I don't know how it would tell me anything I didn't already know.
For me it was helpful in figuring out if the issue was in my trailer wiring or in the cable from my tow vehicle.
 
  • #7
Would I be correct in guessing that your new trailer uses LEDs rather than filament globes?

One year is sufficient time, to corrode a connection, or fracture a wire.

The most common problem with cross functioning trailer lights is a poor or missing chassis common connection somewhere. Isolate the problem to the tow vehicle or the trailer, by substitution. Then look for poor contacts in the connectors, or the broken wire(s) in the cable.
 
  • #8
I would about guarantee your problem is on the trailer. You say 'tail light' on one side is on all the time and the other is not. If the trailer lights are incandescent, there are two filaments on each bulb. One for turn signal and brakes, the other for regular tail/clearance lights. Standard trailer wiring will supply a single wire single wire for the tail/clearance, and a wire for each turn signal.
 
  • #9
Averagesupernova said:
I would about guarantee your problem is on the trailer. You say 'tail light' on one side is on all the time and the other is not. If the trailer lights are incandescent, there are two filaments on each bulb. One for turn signal and brakes, the other for regular tail/clearance lights. Standard trailer wiring will supply a single wire single wire for the tail/clearance, and a wire for each turn signal.
But if the power comes from the tow vehicle, and is normally off, how can the port tail light be on all the time? Unless the circuit in the tow vehicle is on all the time (which would be a fault)?
 
  • #10
What does the trailer connector look like?
 
  • #11
4-pin flat. I bought a circuit tester for it (see berkeman's post (but 4 pins) which I will try tonight.
 
  • #12
DaveC426913 said:
4-pin flat. I bought a circuit tester for it (see berkeman's post (but 4 pins) which I will try tonight.
Yeah, that implies there should NEVER be a constant hot on a flat four. However, if you have a round trailer connector with an adapter to flat four, I'd be suspicious of that. They often have a constant hot. Don't hate me for saying 'thats whatchya get for owning a Chrysler product'. Lol. I was warned when I bought a Dodge some years back. I didn't listen.
 
  • #13
DaveC426913 said:
4-pin flat.
To provide left, right, tail and stop from 4 connector pins, would require a CAN-Bus with chassis and B+.
 
  • Wow
Likes berkeman
  • #14
Averagesupernova said:
Yeah, that implies there should NEVER be a constant hot on a flat four. However, if you have a round trailer connector with an adapter to flat four, I'd be suspicious of that. They often have a constant hot. Don't hate me for saying 'thats whatchya get for owning a Chrysler product'. Lol. I was warned when I bought a Dodge some years back. I didn't listen.
The tow bar and harness are after-market.
 
  • #15
Baluncore said:
To provide left, right, tail and stop from 4 connector pins, would require a CAN-Bus with chassis and B+.
Er ... what if my filbert flange won't mesh with my grapple grommet?
1686341438553.png
 
  • Like
Likes OmCheeto
  • #16
Baluncore said:
To provide left, right, tail and stop from 4 connector pins, would require a CAN-Bus with chassis and B+.
No. Stop and turn signals on a trailer with incandescent bulbs would use the same wire for stop and turn signals. Like since forever. Turn right and the right turn signal wire flashes, left is on as long as your foot is on the brake.
 
  • #17
As I now understand it, only in North America are the turn indicator lights and the brake lights permitted to share the same lamp and red colour.
White: 1. Chassis ground.
Brown: 2. Tail and plate lights.
Green: 3. Right turn and/or brake.
Yellow: 4. Left turn and/or brake.
That requires something like diodes in the tow vehicle circuit to prevent the rear brake lights from feeding to the front indicator lights.
Therein may lie your problem.

The rest of the world requires amber turn signals, a red tail, and a brighter stop light, which requires a minimum of five pins in the connector. Seven pins provide also for reversing lights and electronic brake control on the trailer.

Since newer vehicles now have everything electronically controlled and the current monitored, there is insufficient power at the rear lights to power trailer lights also. That is where Chassis, B+ and the internal CAN-Bus take over powering and controlling all the trailer lights.
 
  • #18
Baluncore said:
That requires something like diodes in the tow vehicle circuit to prevent the rear brake lights from feeding to the front indicator lights.
No. Multiple poles in the turn signal switch.
 
  • #19
Baluncore said:
Since newer vehicles now have everything electronically controlled and the current monitored, there is insufficient power at the rear lights to power trailer lights also. That is where Chassis, B+ and the internal CAN-Bus take over powering and controlling all the trailer lights.
Backwards compatibility is still a thing last I knew. I doubt a can-bus has taken over the lowly trailer light system just yet. Vehicles with LED all around will certainly have to have some kind of module to control trailer lights. Before that, only if the manufacturer chose to.
 
  • #20
OK, fault confirmed. Left taillight has power all the time. This was tested in the morning without even having my car keys, so it's like this 24/7.

P_20230610_112625.jpg


I guess I could test the rest of the states but I'm not sure if there's much point inasmuch as no matter what I have to take the harness in to get it re[paired,placed].
 
  • Like
Likes Averagesupernova and berkeman
  • #21
Where does the 24/7 B+ power come from with all other lights off?
A faulty flasher control unit in the car?
Or a short to B+ always on, in the wiring loom?
 
  • #22
Baluncore said:
Where does the 24/7 B+ power come from with all other lights off?
A faulty flasher control unit in the car?
Or a short to B+ always on, in the wiring loom?
My guess is there is a module specifically for running trailer lights on that particular vehicle. There are likely MOSFETs to drive the trailer lights. Anyone reading this, don't take this as known fact. Based on the failure @DaveC426913 is having and based on other failures I've seen on vehicles over the years, this is my best guess.
-
I've fiddled with non working tail lights and trailer wiring and lights many times over the years and this is the first time I've heard of a line coming off of a trailer connector with constant power that isn't supposed to be.
 
  • #23
Baluncore said:
A faulty flasher control unit in the car?
Averagesupernova said:
My guess is there is a module specifically for running trailer lights on that particular vehicle. There are likely MOSFETs to drive the trailer lights.
I agree. You call it a module, I call it a control unit.

Because only 3 wires are being controlled to the trailer, I expect it will be integrated into a greater flasher-control-module, with the hazard lights and brake signal. The hazard lights require B+ always on, so a failed PNP inverting switch, NPN emitter follower, or P-channel MOSFET in the flasher control module would explain the "always on" fault.

Maybe it is all part of a Body Control Module, BCM.
 
  • #24
From Years of having to trouble shoot trailer lights, in the rain , at the race track , in the dark..90% of the time it is a ground wire problem. Check ground from the tow vehicle battery on back.
 

Similar threads

  • Mechanical Engineering
Replies
10
Views
2K
  • Mechanical Engineering
Replies
3
Views
61K
Replies
5
Views
15K
Replies
7
Views
4K
Replies
16
Views
3K
  • Art, Music, History, and Linguistics
Replies
26
Views
4K
Replies
67
Views
14K
  • General Engineering
Replies
2
Views
4K
Back
Top