Thread: I am at a loss!!

Never heard of it being a legal requirement
Most trailer tell tail lights rely on current detection to work - ie it looks for current draw in the van lights and a current detector in the line will relay to the tell tale lamp in the dash
I bet your van lights are Led units - much lower current draw, and thus no relay to the dash circuit
Just a hunch

Nope! Old trailer with bulbs.

This is what I found after a couple of hours of Rave diagrams, a multimeter, owner’s manual and Google.

Please note: the L322 went through a transition period where all ECUs were gradually replaced, changing BMW systems for Ford – this occurred between late 2004/2005 and 2007. What I found is on my 2004 TD6, but should be consistant with vehicles pre MY07.

Law: EU requirements state that the vehicle must test and report trailer light faults. There is no requirement under any other authority body that I could find. Hence why all EU built cars have the same issue with LEDs and US, Jap ect don't…

Here goes - The “Trailer Light Module” (another ECU in the rear) sends a 2v test pulse to both indicators, both tail light circuits, and brake lights, 5 total. When a trailer is plugged in and 2 or more circuits are detected, the car will effectively operate in a trailer connected mode, Air suspension locked to standard height, rear parking sensors off, parameters for traction control and ABS changed. From a driver’s point of view the only noticeable changes are the parking sensors and the “Lock” button for the Air suspension is illuminated. (Lock button is the button in the center of the suspension height adjustment rotary dial).

If problems are detected with any of the 5 circuits you will have “Check Trailer Light” message displayed on the dash. A fault with either indicator will also cause the corresponding car indicators to flash rapidly, just like an out bulb on the car itself. There is no trailer indicator lamp like seen on the D2.

If you connect a trailer with LEDs the car will still recognise and diagnose them correctly – (unlike D3 the L322 tests continuity not current draw), but the trailer lights (all of them) will flash like a Christmas tree on the 2v pulses. For LEDs to function correctly the L322 requires a more complicated Pulse Shunt than D3, on the plus side, if done properly it will sill allow the car to test the LEDs (and circuits). I have built 2 now and can share the schematic if needed.

I mentioned 2 tail light circuits – EU trailers have right and left tail lights wired separately (apparently). Referring to the factory Large Round 7 pin plug; The factory tow electrics in Aus (stupidly) have the same. Pin 7 and 5 are for tail lights - you need pin 5 for trailer brakes if fitted. Find the wire going to Pin 5 at the Trailer Light Module, cut it, solder it to the wire going to pin 7 (sorry can’t remember the colours - can take a pic if needed). That way both tail light circuits operate and test the tail lights. Pin 2 will be rear Fog Lights which are not tested- personally I have connected reversing lights to pin 2 for night arrival to camp sites.

I think a valid question is - Do you have the “Trailer Control Module” or has some one just hooked on to the car tail lights?? Sounds to me the latter - that will cause other problems….

Connecting a trailer brake controller is a whole other conversation…

I have a trailer connect module fitted and was just curious as to why a relatively new vehicle does not have indicator tell tail light.

Stupid Question,
Has the BCU been told to operate with trailer function (or basically been turned on)
The system will work but not to full potential.
How to test!
Plug your trailer in & select reverse, do the reverse park sensors keep beeping continuosly?
As you have said you have normal incandescent globes therefore the system should NOT beep if turned on.

Cheers.