2017 A3 Cab Trailer Hitch and Wiring

[JimVonBaden]

So, I always add hitches to my cars. In part to haul stuff, and in part to allow me to carry my bicycles.

Most recent was my BMW 128i Cab.



Of course my 2017 A3 Cab was next up. This was MUCH easier than the BMW. No need to remove the lights and bumper cover and bumper. Just bolt it on, mostly.

Curt hitch, with wiring kit, $215. Not bad.





The kit comes with the appropriate bolts, flange supports, and even fish tools to install it.











Install the fish wire on the bolt and flange, fish in the wire, grab it with a curved pic, and the bolts are ready. Be careful, you do not want to push the bolt out of the hole installing the hitch. I used a shop stool with a piston riser to lift the hitch slowly into position.




Found the two inner bolts on the rear bumper restricted movement, so I removed them, then carefully lifted up the hitch, gently added the retaining nuts while holding the bolts in place with a hook pick, and got all four bolts started without pushing the bolts up into the holes. Once that was done, it was easy to tighten them down to 110NM. Basically the hitch is now done.








The hitch fits up high and tight, with it showing, but only if you are looking for it. Nice kit!

Total install time, doing it alone, 45 minutes. With two people, maybe half an hour. If all you want is to haul bicycles around, you are done.




For me, I also haul a trailer with stuff in it, so I need to wire up the car for the trailer. Problem is, I am not yet sure what wire is brake, left and right signal, and running lights. Ideas?

[shadpuff]

Nice job. The way I found out with my F31 BMW 3 series was take off the rear light. You can then check each wire but putting on brakes, signals, etc. and you will quickly fine out what colored wire does what. Hope this helps!

[JimVonBaden]

Nice job. The way I found out with my F31 BMW 3 series was take off the rear light. You can then check each wire but putting on brakes, signals, etc. and you will quickly fine out what colored wire does what. Hope this helps!

That is what I am attempting. The LEDs are canbus, so a bit different than my BMW 128 was.

[JimVonBaden]

OK, wiring for the LED tail light done. I can confidently say that the colors I identified, except the 12V hot, will be the same on any late model A3 with LED tail lights. My 12V hot is from the convertible top motor, so you will have to find your own.

Wire colors to the trailer control unit:

Left Turn, Solid Blue to Yellow from unit (Also functions as the brake light)
Tail light (running lights), left side, Blue White to Brown from unit
Rt Turn, Brown/green to Green from unit (Also functions as the brake light)
Unit Red to Ground
Unit White to Ground
Unit Black to 12V hot Red/Yellow on the convertible

The reversed ground and hot colors are a mystery, and the instructions are not super clear what wire is hot (12V0, it is black, which is usually ground. The red is for the brake light on some cars, but on the two wire cars it goes to ground. Go figure.

The wiring kit, does not include the black Posi-Taps):

Toss the junk fold taps, they will just mess up your wires, and are hard to use. I prefer Posi-Taps, though I ended up using a different folding tap because I ran out of Posi-Taps.

Pull the side, bottom, back and floor panels for wiring access. I left the sides partly attached due to the convertible top. It made it a bit harder to work in the space, but was worth it.

Hot wire on the convertible goes to the top main motor. It is always hot. Red/Yellow for the convertible.



Then pull loose the convertible top motor to access the left tail light wires.


Left Turn, Solid Blue to Yellow from unit (Also functions as the brake light)
Tail light (running lights), left side, Blue White to Brown from unit

Then the right side:

Rt Turn, Brown/green to Green from unit (Also functions as the brake light)

Now position the unit, on the convertible Premier, there is plenty of space. I suspect most will have room:





I routed the trailer wires under the spare tire cover and left it there. There is space on the convertible, because there is no spare tire.

Tested. This little test unit is $5 at Harbor Freight, and good at twice the price to be sure the wiring works.
Tail light with left signal flashing (it will flash on the tester):


Tail light with brake applied.


If you do this with the signal on, the appropriate one will flash.

Put it all back together and take pride in your work. Wiring took about 2 hours, but had I not needed to test for what wires to use, it could be done in an hour.

[JimVonBaden]

Cleaned up, charged, and ready to go:





Batteries off, in the back seat.

[bakedcorn]

Kudos for the write and nicely done. When I was younger I always would enjoy wiring up my vehicles more so with audio systems - new amp, speakers etc. I'd always look for ways to hide the wires and to ensure it looked factory. Nowadays in my age I've got no patience and to be honest these cars scare the begesus out of me with their CANBUS system. Again kudos to you.


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[JimVonBaden]

Kudos for the write and nicely done. When I was younger I always would enjoy wiring up my vehicles more so with audio systems - new amp, speakers etc. I'd always look for ways to hide the wires and to ensure it looked factory. Nowadays in my age I've got no patience and to be honest these cars scare the begesus out of me with their CANBUS system. Again kudos to you.


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Thanks. It can definitely be intimidating, especially with the risk of blowing an expensive module.

[Eli0073]

I'm about to do this install. I was wondering if you could tell me which harness you bought to power the trailer lights? And why you bought that specific one?

I saw this one and I'm wondering if this will work..?

CURT 56130 Non-Powered 3-to-2-Wire Splice-in Trailer Tail Light Converter, 4-Pin Wiring Harness https://a.co/d/5AhbFtc

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[JimVonBaden]

I'm about to do this install. I was wondering if you could tell me which harness you bought to power the trailer lights? And why you bought that specific one?

I saw this one and I'm wondering if this will work..?

CURT 56130 Non-Powered 3-to-2-Wire Splice-in Trailer Tail Light Converter, 4-Pin Wiring Harness https://a.co/d/5AhbFtc

Sent from my IN2017 using Audizine Forum mobile app

I would think not. That one is designed for powering off the bulbs. The canbus would just shut down the lights. I would go with this one like I used on my A3 and Q5. https://www.ebay.com/itm/13421204285...Bk9SR7ag0MXrYA

[Eli0073]

Thanks for the link mate. Hopefully I won't have any fitment issues as my car is a S3. I'll be doing the install over the weekend. Since I was running a bolt on maxton design diffuser..I will be removing that...and replace it with Ecs gloss black diffuser...since there's no way I could keep my setup with the hitch.IMG_20210402_200714__01__01.jpg

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[Eli0073]

Install was a super easy mate. All do it was easier for me to remove part of my AWE exhaust. The hitch has plenty of clearance. Since I was there I also replaced my rear diffuser with a gloss black.IMG_20220924_173731.jpgIMG_20220924_215911.jpgIMG_20220924_220049.jpg

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[Eli0073]

I got this custom made third brake light. It finally arrived today. So I'll be doing the wiring harness today. I ended up getting CURT 56146 Powered 3-to-2-Wire Splice-in Trailer Tail Light Converter, 4-Pin Wiring Harness.IMG_20220928_141303.jpgIMG_20220928_141317.jpg

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