Paul47

I'm not sure if there isn't a better board to put this post on; sorry if it's wrong.

I had a hard time getting my hitch electronic module installed on my A3, and along the line learned a few things I'd like to pass on. It's general, not specific to any model, although the context is my 2015 A3.

The main problem I found was trying to figure out what wires to hook up to. Somebody pointed me at erwin.audiusa.com, which is where I went to get the wiring diagrams (at a cost of $35 for one day). But then the problem is, which wiring diagram should I use? Looking at the door sticker told me (if I interpreted correctly) the date of manufacture of my car was "0614", which I took to mean June of 2014. So, in looking through the chronologically-ordered diagrams, I found one concerning tail and brake lights labeled "from May 2014"; I thought that would be the one, and printed it out. It was #97, pages 19 and 20 in the wiring diagrams.

The module I was using was the Curt 59236 converter, which is what etrailer said was correct for this car (apparently several different ones will work though). This converter supports 4 modes:
"Two wire" (combined stop and turn signal)
"Three wire" (independent stop, turn and tail wires)
"PMM-ST" (combined stop and tail wires, using lower voltage for a tail signal)
"PMM-STT" (combined stop, turn and tail wires)

The first job was figuring out which mode I should wire for. Just by looking at my selected wiring diagram, you could see that the "M22" bulb was called "Right Brake/Tail Lamp Bulb". It was combining brake and tail in one wire, so that meant mode "PMM-ST" in the converter.

The only problem with this is that when I operated the brakes, turn signals and tail lights, and just looked at the rear of the car, I could tell that the same LEDs were NOT being used between brakes and tail lights. Instead, the same LEDs were used between the brakes and turn signals. That meant that the mode for my car was actually "Two wire" rather than "PMM-ST". This told me that the schematic I was using was wrong.

I went back to the Erwin portal and found the next page covering tail, brake and turn signals. It was #111, pages 3 and 4, and described as "from November 2014". In this one, the bulb "M84" was labeled "Left brake lamp/turn signal bulb 2". That meant the brakes and turn signals were shared, which was a "Two wire" mode as my car appeared to be. So I started using that wiring diagram instead.

The wire colors are labeled in German (i.e. "ge" = "gelb" = Yellow); but there is a legend on each page to help with that. I had previously tried an earlier hookup which did not work properly, using what scattered info I could find on the internet. I found that I had to move one connection, basically grounding the red wire from the converter (which is what puts it into "Two wire" mode). This single change got my trailer wires working. I had also previously miswired the brown converter wire to the blue/white wire going to the "M37" side marker bulb; it should have gone to the "M4" tail lamp bulb wire. However, as the side marker apparently works the same as the tail light, I left it as it was and the lights do function correctly.

I also wired my converter power, the black wire, to the +12v socket in the trunk as many have done, rather than going all the way up to the battery. Since this A3 will only tow my small trailer, rather than the big one with many lights, it should be fine.

Bottom line for THIS car, I wired the converter wires as follows:
BLACK: to the +12v socket
GREEN: to the "vi/br" or violet/brown (looks straight violet or purple unless you look closely) wire on the right side of the car
RED: to the ground lug
YELLOW: to the "bl/ge" or blue/yellow wire on the left side of the car
BROWN: to the "bl/ws" or blue/white wire on the left side of the car (should have been to the brown/red wire, but still works)
WHITE: to the ground lug

I hope this description helps others pulling their hair out about how to wire up the trailer converter.