kgstevens

I have a 2007 Q7. I have been struggling for many months trying to figure out the trailer lights.

1- This is a US model. I have confirmed that the coding on all modules in the car are correct, including the trailer control model (69). I do own a VAG-COM and am very comfortable with it.

2- Plugging the trailer into the Q7 causes multiple errors on the Q7 dash display - Left Turn Signal, Right Turn Signal, Brake Lights, etc.

3- HOWEVER, the TAIL lights work just fine. And if the trailer is towed by more conventional vehicles (e.g. trucks) there are no issues. So I know it is not the trailer.

3- I have measured the voltage, current and pulse width of the signal on each of the Right and Left Trailer Plug signals from the Q7. Results: I have an approximate 6 volt pulse that is on 58% of the time at 0.32 ma of current on EACH wire regardless of what lights are supposed to be on, including them being off.

4- I assume that this low amperage pulse signal is for the light check controls to check if the bulbs are there and if they are burned out.

5- In fact, if I connect a bulb to one side or the other the Q7 recognizes a trailer and starts the slew of error messages.

6- A normal bulb at the rear of the Q7 wired in with jumper wires to the Q7 trailer plug has a resistance of 3.2 ohms. Because I get an error thrown I assume that 3.2 ohms is too SMALL for the Q7 to recognize and it thinks it is shorted out.

7- My trailer shows a resistance of 2,500ohms on the right or left lights. This also throws a code on the Q7. I assume that the 2,500 ohms is too LARGE for the Q7 to recognize and it thinks it is burned out/open.

So (sorry for the long background), what is up with this? Does anyone out there know what the tolerance range is for the Q7 to properly recognize a bulb in a trailer so that it will turn it on?

So this brings up another question: I get no proper voltage/current on either wire from the Q7 whether the light is actually on/off. Does the Q7 avoid turning on the circuit if the bulb check circuit fails in order to protect the Q7 from shorting? Or do I have a bad/defective trailer control module?

Anyone have any suggestions? Thanks.

Jusforfun

Do you have EU or US spec Q7 and same question for trailer lights?
Are they standard lights or LED?
Yes the Q7 runs that test whether lights are on or off.
And yes it only supplies voltage when the plug is in as discovered by others trying to set up electric trailer brakes.
Have you carried out all adaptions? pcoliver's post

kgstevens

My Q7 is US spec.
All lights are standard (not LED)
All adaptations have been performed.

After the Q7 determines a trailer is connected, which mine does, will it send signals if it detects an error on one of the lights? For example the left turn signal. If the bulb warning is active, will the Q7 still send the 12volts on that line? For the life of me I can not get any signal on either the left or right lights no matter what I do, other than the bulb diagnostic low amp pulse signal. Trailer brakes, tail lights, and tow adaptation all work just fine. Just no signal or brake lights?

Is there by chance a set of fuses in the Q7 that feed these circuits that I don't know about?

Thanks for any help.

kgstevens

OK, I resolved my problem. Fuse Number 10 in Fuse Block F was blown.

The fuses are marked #9 Trailer; #10 Left Trailer; #11 Right Trailer.

This is not necessarily correct, at least for US Spec cars. My Fuse #10 caused BOTH Left and Right to not work.

Also of note. The trailer is detected when ANY bulb is detected. This includes tail lights, right turn signal/brake, left turn signal/brake. The Q7 is using the bulb check circuit to determine if a trailer is connected. Any bulb present on the trailer plug will cause the Q7 to go into trailer towing mode.

Also of interest (which is what cost me a bunch of time to figure out): The bulb detection circuit is totally INDEPENDENT of the bulb driving circuit. I could measure the bulb check on the wires, but they still wouldn't work and the Q7 would throw codes for the good bulbs. This means that the FUSE(s) is also part of the bulb check circuit for the trailer.

Interesting.

Cpool324

Ok I’ve tried following every VAGCOM post, and tried the fuses to ensure those are fine... I CAN NOT get my running lights to work period and I still get the errors on the rear trailer brake lights left and right...

i even purchased all LED bulbs to try converting and still no F’ing luck... this is beyond frustrating with a car that originally cost $60k!

i have a 2015 Q7 TDI with the factory tow pkg. I got the brake control harness from Kevin (lugnuts9) and the 7pin pollack connector from amazon.

all the settings are set correctly in the VAGCOM according to the posts and Ross tech docs I’ve found... the camper has incandescent bulbs all around and will not work with this vehicle. So beyond frustrated and I have no idea what to do now... I picked this up specifically for towing and what a nightmare. Audi dealers are useless they don’t have a clue when it come to actually hooking the vehicle up to a trailer, unreal.

Anyone have any thoughts or info as to how I can get this damn thing working short of replacing every damn light fixture on the camper with an LED package?

i know that it works perfectly with trailers that are outfitted with led’s I tested it on a different trailer and all lights worked (markets and brake lights etc)...

HELP!!!

dreadlocks

Sounds like your camper might not be wired right or has a short thats draining, if it works on other trailers but not this.. You got an incandescent test light? they only a few bucks.. if that works at the trailer plug but your trailer dont, something is wrong w/trailer.

Cpool324

Thanks for the quick response Dred, the trailer has worked fine on every other vehicle until this Audi, it’s a new camper as well (2018). The o my trailer it worked on that I tried was one that was all LED from the factory.

Im assuming changing out the bulbs to led wasn’t enough as somehow it’s still calling for more voltage or something?

really frustrating... I’m willing to get whatever tester you mentioned though and try it... at this point I don’t know what to do.

dreadlocks

You put in LED conversion bulbs?

Typically what happens is LED's dont have enough resistance for the vehicle to detect the trailer.. but Incandescent bulbs work fine.. some trailers might have resistors wired in to make em work with such vehicles, others need adapters.. your situation is just *** backwards, a multi-meter or a test lamp probing the wiring plug coming out of the back of the Audi will verify its sending the right signals.. if it is then probe inside the the trailer lights and see if they are also getting the signals.. just going to have to check all the wiring, make sure its fine.. a fault on the trailer might have gone ignored by other vehicles that dont monitor this stuff but now its an issue.

FWIW, I've towed 5 different trailers w/my 2014 TDI and never had any issues, mixture of incandescent and LED.. mostly LED tho.

frqflyer

I would ohm out the connector just to be sure you don't have a loose wire, short or bad ground on the trailer.

If you have another vehicle hook it up once more and verify all lights work as expected or just supply 12v to test each pin. If it worked before on another vehicle it may not be working now unless you already tried it right after another.

It could be a bad connection at Audi side plug also, I would make sure everything has great contact and zero oxidation. You could verify the connection is fine by putting one probe on the Audi side of the wire and the other on the trailer side. They should read zero or at least near zero ohm.

Kelly Stevens

The bulb check circuits work THROUGH the fuses in the Q7. If one of the fuses is blown or missing it will throw errors. I know you said you have checked all the fuses - HOWEVER, this car is very advanced. I struggled for years not having tail lights until I figured this out. There is NOT a one-to-one mapping of a fuse to a trailer function (at least for the lights). The system is designed to assign fuse circuits according to the load requirements. That is why the fuses are not labeled with trailer functions, only trailer #1, trailer #2, etc. This was done to allow the car to adapt to various trailer electrical loads.

I suspect your problem is:
1- blown fuse (probably not as you have checked them)
2- MISSING fuse. Suggest you look carefully at the fuse panel and put a fuse in every place that has wiring terminals for a fuse (this is what caught me out).

Make sure you are looking at the correct fuse panel too. The one in the right rear of the vehicle.

Best of luck.