Trailer Lights Issue

Neuk_

Neuk_

Honorary Master
Joined
Jan 23, 2018
Messages
13,375
Reaction score
13,229
Location
Johannesburg, South Africa
Hopefully I'll explain this well...

Went to pick up a friends trailer yesterday and drop off a bike rack for him to use. Connected the trailer to my Touareg and there are no lights working, absolutely none. Disconnected and connected it up to his Subaru and all lights except left indicator work. The bike rack lights work perfectly fine on my car, as did the other bike rack the family owns, as did the trailer I normally borrow when I last used it in September. Strangely the bike rack I dropped off works on the Subaru but not as well as on my Touareg, brake lights dim when indicators are on, etc.

So I need to get the trailer lights working before Wednesday when we head to Dullstroom, will clean out the connector and check the left indicator bulb and replace if necessary to see if that solves the issue. What else can I check? I certainly hope the connector pin wiring isn't different in some way...
 
Does the trailer have LED lights? Older cars that have sway control sometimes have difficulty detecting the trailer due to the low power draw of LED lights.

Otherwise do the usual and spray with q20, clean and then use a voltmeter to check voltage, ground resistance on the vehicle side and continuity on the trailer side.
 
B-1 said:
Does the trailer have LED lights? Older cars that have sway control sometimes have difficulty detecting the trailer due to the low power draw of LED lights.

Otherwise do the usual and spray with q20, clean and then use a voltmeter to check voltage, ground resistance on the vehicle side and continuity on the trailer side.
Click to expand...

Old trailer with incandescent bulbs, like the trailer I normally borrow, I can't recall if the bike racks have LED lights though.
 
Neuk_ said:
Old trailer with incandescent bulbs, like the trailer I normally borrow, I can't recall if the bike racks have LED lights though.
Click to expand...

On the trailer check the resistance between the ground pin and the trailer frame and see if that is good.
Otherwise cleaning the connectors and adding some conductive grease may do the trick.
 
B-1 said:
On the trailer check the resistance between the ground pin and the trailer frame and see if that is good.
Otherwise cleaning the connectors and adding some conductive grease may do the trick.
Click to expand...

Found a bulb that had come loose but that still hasn't sorted the issue so getting the volt meter out to test the feed from my car first and I'll go from there. It is just strange that it worked fine on my friends Subaru...
 
Neuk_ said:
Found a bulb that had come loose but that still hasn't sorted the issue so getting the volt meter out to test the feed from my car first and I'll go from there. It is just strange that it worked fine on my friends Subaru...
Click to expand...

Its a pretty simple system but sometimes the pins on one car are a bit looser than another. Some cars also have protection circuits that disconnect if the resistance is out of range to protect the car.
So you just need to go around and check things systematically.
 
Well tomorrow is public holiday and you want to leave on Wednesday. So I suggest rather take it in and get it fixed now. It should not be a big problem or cost too much. Most places that fit tow bars, shocks etc will sort you out quickly
 
Oh and on a previous VW I had there was a problem where when you test the trailer lights while the car is switched off, everything works. Then when you start the car half the trailer lights did not work. There was some controller box or something in the back they replaced.
 
When I got my bike rack (Thule) a few years ago I seem to recall some issues with them and VW that had LED lights.
The light system is not designed for incandescent lights.

ChatGPT.

Why a non-LED trailer “doesn’t work” with your car​


Many modern cars with LED lights use low-current, computer-controlled outputs. They expect to “see” a certain electrical load.


When you plug in a traditional (incandescent) trailer:


  • The car may not supply power at all, or
  • Indicators may not flash, or
  • Brake / tail lights may be dead

This happens because the car:


  • Uses CAN bus / BCM-controlled lighting
  • Detects the trailer load and shuts down the circuit if it doesn’t behave as expected

Ironically, LED trailer lights often work because:


  • They draw less current
  • The car’s trailer module is designed with LEDs in mind

Common real-world causes​


  1. Vehicle requires a trailer module
    • Without it, the car won’t drive external loads properly
  2. Incorrect wiring (shared earth)
    • Old trailers often have poor grounding
  3. Car expects a specific resistance
    • Incandescent bulbs confuse the detection logic

The usual fixes (pick one)​


Best / correct fix


  • Install a proper trailer control module (vehicle-specific if possible)

Quick workaround


  • Fit a bypass relay (powered directly from the battery)

If LEDs work but bulbs don’t


  • Add load resistors or
  • Convert the trailer to LED lights (often simplest)

Quick test you can do​


  • Plug in a known LED trailer → works
  • Plug in bulb trailer → dead or erratic
    → Confirms a load-detection / BCM issue

Bottom line​


Your car is not “dumb-wired” like older vehicles.
It’s actively controlling the lighting outputs, and incandescent trailer lights don’t present the expected electrical behaviour.
 
Brawler said:
When I got my bike rack (Thule) a few years ago I seem to recall some issues with them and VW that had LED lights.
The light system is not designed for incandescent lights.

ChatGPT.

Why a non-LED trailer “doesn’t work” with your car​


Many modern cars with LED lights use low-current, computer-controlled outputs. They expect to “see” a certain electrical load.


When you plug in a traditional (incandescent) trailer:


  • The car may not supply power at all, or
  • Indicators may not flash, or
  • Brake / tail lights may be dead

This happens because the car:


  • Uses CAN bus / BCM-controlled lighting
  • Detects the trailer load and shuts down the circuit if it doesn’t behave as expected

Ironically, LED trailer lights often work because:


  • They draw less current
  • The car’s trailer module is designed with LEDs in mind

Common real-world causes​


  1. Vehicle requires a trailer module
    • Without it, the car won’t drive external loads properly
  2. Incorrect wiring (shared earth)
    • Old trailers often have poor grounding
  3. Car expects a specific resistance
    • Incandescent bulbs confuse the detection logic

The usual fixes (pick one)​


Best / correct fix


  • Install a proper trailer control module (vehicle-specific if possible)

Quick workaround


  • Fit a bypass relay (powered directly from the battery)

If LEDs work but bulbs don’t


  • Add load resistors or
  • Convert the trailer to LED lights (often simplest)

Quick test you can do​


  • Plug in a known LED trailer → works
  • Plug in bulb trailer → dead or erratic
    → Confirms a load-detection / BCM issue

Bottom line​


Your car is not “dumb-wired” like older vehicles.
It’s actively controlling the lighting outputs, and incandescent trailer lights don’t present the expected electrical behaviour.
Click to expand...

I am sure the other trailer that I use has incandescent bulbs as it is bloody old as well. I am going to recheck the bulbs in this trailer but am sure they are all incandescent. I think even the family's Thule bike racks use incandescent bulbs and I have used both bike racks and the other trailer with no issue before.
 
Jean Claude Vaaldamme said:
Oh and on a previous VW I had there was a problem where when you test the trailer lights while the car is switched off, everything works. Then when you start the car half the trailer lights did not work. There was some controller box or something in the back they replaced.
Click to expand...

Yes, cars like my Touareg do a test on the trailer plug to check if there is a trailer and if it thinks there is no trailer it 'switches' off the feed to the trailer plug.
 
B-1 said:
Its a pretty simple system but sometimes the pins on one car are a bit looser than another. Some cars also have protection circuits that disconnect if the resistance is out of range to protect the car.
So you just need to go around and check things systematically.
Click to expand...

I opened up the split pins and there is noticeable resistance when plugging in the trailer plug. I started my Touareg, put the lights on, tested voltage from ground to LH and RH trailer light pins and it only supplied 1.2V which is strange.
 
Confirmed the 1.2V supply is just the trailer control module not supplying full voltage as it hasn't detected a trailer connected. Opened up the trail plug, two wires aren't even connected so need to figure that out and hope when I connect them the magic smoke stays in the wiring.
 
Nope, no luck, the colour of the wiring in the trailer plug is different from any wiring diagram I have come across and I tried various combinations of connecting the unconnected wires, nothing worked and I am not about to try rewire someone else's trailer so I'll have to make another plan before Wednesday.
 
I can't remember his username on here but a friend tried to help me figure out why the trailer lights wouldn't work with my car, they wouldn't work on my wife's either, so the theory we have is since the trailer is wired with the 'driving lights' bridged the control module isn't picking up there is a trailer connected. So, no trailer for the holiday, we'll make do with a roof box and a few things hauled by my SIL and her boyfriend.

Thanks for all the input everyone!
 
You must log in or register to reply here.
Back
Top
Sign up to the MyBroadband newsletter
X