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I know this is an old post, but if you're reading it, you likely have a T3 that you're wiring up for a controller. I just did this 10 days ago in preparation for (finally!) hooking a travel trailer to our Touareg and camping for a weekend. Mine is a 2016 TDI LUX, and one of the last TDI Touaregs brought into the USA. It was held at the dealer brand-new as it was shipped just as the TDI nonsense hit the press.

I found the under dash connector previously, and got the harness and P3, but left them (of course) on a shelf for a year and a half waiting to install in my spare time. That time came in the form of "I've rented a trailer for TOMORROW (!!!), and still don't have an installed brake controller." I'm sure I'm not the only procrastinator here. Anyway, with time working against me, I re-found this thread, and had a look at the following post:

https://forums.tdiclub.com/showthread.php?t=444314

Look at response #12 in regards to the relay. Doesn't sound like it's needed from 2012 on.

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I decided NOT to do the relay, and hooked it up. As it turns out, the issue referenced here deals with display flickering. I didn't know it right off because the TDI is a HOSS, but YOU DO NEED THE RELAY ON 2009+ JUST AS TEKONSHA STATES!!! The brakes will drag slightly without. I know, the harnesses are all over Ebay etc without the relay. Your car, your trailer. If you want it to be right YOU NEED THE RELAY PER TEKONSHA TB0021.


I read that post before wiring up my TReg as well. I called Tekonsha before install, and they said that after more testing, the voltage bleed from the TReg varies based on the electrical use of items bundled in the same loom. This has to do with electromagnetic lines of flux and the types of shielding VW uses. This can cause the voltage reading to the brake controller to be greater than 4.3 volts (which has been programed into the controller to as the max bleed voltage), therefore causing the brakes on the trailer to activate.

I figured spending the $7.00 was a minor expense to alleviate this issue.

Some may be wondering about the battery drain when leaving the brake controller connected. I left it in the car for five days and no appreciable drain was detected. It seems to go to a standby (practically off) when the car is turned off.

For those who think it forgets the settings when unplugged, the P3 does not. It retains the last setting when plugged back in, meaning it has nonvolatile memory. It also has three programs that can be saved. So if you have three trailers, you can program and save the settings for each. If you have a horse trailer, you can set it for empty and full.
I'll be taking the harness out to add the relay this weekend. Interestingly, we towed a 19' bunkhouse trailer 300 miles last weekend, and I still got 13+ MPG on hilly terrain with the trailer brakes partially applied!!!

This is for the people searching for the post: YOU DO NEED THE TRAILER BRAKE RELAY ON A 2009+ TOUAREG PER TEKONSHA TB0021.
 

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I installed the Redarc Pro Tow and I assume that if the light does not indicate that the brakes are activated then the controller does not send current to the trailer brakes.
I also have used the P3 with my T&C and it shows the current going to the trailer when the brakes are applied and I also assume if the P3 is installed in the Touareg it would indicate if any current is applied to the brakes as well.
In my opinion the if there is no current is sent to the brakes you are OK.
If current is sent to the brakes the yes you need the brakes.
By the way the trailer module sends the 12 volts to the brake controller socket and at least in my 2012 touareg if there is no trailer plugged in there is no + 12 volts from the car to the brake controller.
Also on my Touareg there is 0 volts with no brake activation and 12 with. (with trailer plugged in, of course).
I had to check this out as when I installed the brake controller I couldn't get the 12 volts and it took a while to hook up a trailer to figure out the lack of the 12 volt brake signal, tracing it back to the module was a PITA too.
 

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I installed the Redarc Pro Tow and I assume that if the light does not indicate that the brakes are activated then the controller does not send current to the trailer brakes.
I also have used the P3 with my T&C and it shows the current going to the trailer when the brakes are applied and I also assume if the P3 is installed in the Touareg it would indicate if any current is applied to the brakes as well.
In my opinion the if there is no current is sent to the brakes you are OK.
If current is sent to the brakes the yes you need the brakes.
By the way the trailer module sends the 12 volts to the brake controller socket and at least in my 2012 touareg if there is no trailer plugged in there is no + 12 volts from the car to the brake controller.
Also on my Touareg there is 0 volts with no brake activation and 12 with. (with trailer plugged in, of course).
I had to check this out as when I installed the brake controller I couldn't get the 12 volts and it took a while to hook up a trailer to figure out the lack of the 12 volt brake signal, tracing it back to the module was a PITA too.
Its an induced voltage that shows up after the P3, so yes, the P3 will read zero. I wrote my post to state a fact. On a very late T3 Touareg with a Tekonsha P3, you WILL have stray voltage present enough to drag the trailer brakes unless you use a relay. This is the root of the issue. The P3 reads zero, and folks think they are fine, but THE BRAKES ARE DRAGGING slightly.
 

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I don't buy the induce voltage on the brake line from the controller through the blue wire to brake coils for two reasons.
1. DC voltage/ current will not be induced only an AC signal will be.
2. any induced voltage from ac or pulsating dc will be very very small and insignificant as far as the low impedance brake coils.
With the brake circuits some other VWs with the bulb out detectors a very fast pulse is sent down the brake light wires to detect the presence of the bulb load. This pulse is too fast to actually light the bulb, but could flash LEDs and be detected by a brake controller that tied into the brake light circuit for it's signal.
As far as I can tell the line from the trailer module in the Touareg goes nowhere else other than to the brake controller and there is no pulses on it when the brakes are not engaged.
This line from the trailer module is turned on when the ESP or the foot brake is active.
I doubt that any induced voltage/current on the blue wire from the controller to the brake coils would be able to counteract the springs in the brake mechanism.
Since the added relay contact is in the wire coming from the trailer module and not in the blue wire from the controller to the brake coils I don't see how that applies to any induced current in that circuit.
If the induced voltage talked about is in the wire from the trailer module to the brake controller any current should show up in the output to the coils and in the reading.
I don't have the P3 in the Touareg (Redarc Tow Pro instead) but I may well test the system next time I hook up the trailer.
 
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