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OlyS4 09-12-2011 04:03 PM

Trailer brakes/towing questions
Hi all, I've searched and read all existing posts that I can find regarding towing, and still have a question on the trailer brakes. I'm in the market for an '08 3.0 to replace our '02 3.0, but will want to tow a small ski boat with this one. It seems that I will be able to tow around 3000lbs just fine, and it sounds like the BMW tow hitch is the way to go, although I don't know if it makes a difference if it's installed by BMW or not, as long as it's done correctly. I will mostly be towing short distances, but occasionally will want to take the boat over mountain passes for vacations.

My concern is braking, since the vehicle braking is only rated at around 1600lbs max tow weight with an unbraked trailer. Some previous posts referenced a factory harness that could be connected to trailer brakes, does anyone have any first hand experience towing connected to the trailer brakes this way. When towing and connected this way, does it feed some type of info to the ECU? Is this mis-information, or did I possibly read this incorrectly? I would like to purchase a trailer with the trailer brakes to be safe, but don't want something wired insufficiently or incorrectly. I would like the whole install to be very clean.


JCL 09-12-2011 04:16 PM

That size of trailer will tow very well with a 3.0. I agree that the OE BMW hitch is the way to go. It does not need to be installed by a BMW dealer, but once it is installed coding will be required. That is simply to change a setting in the vehicle so that it recognizes the towing wiring module. That coding will require the dealer, or a shop with specialized tooling. I would get it installed elsewhere, and get the dealer to do the coding, unless the dealer is competitively priced. They usually aren't.

By way of background, there are two types of trailer brakes. Surge brakes have a mechanical application trigger built into the trailer hitch (on the trailer side, not the vehicle side). They work automatically, and no vehicle modifications are required. When the truck slows, the trailer coupling pushes back on the trailer brake trigger. They are not legal in all jurisdictions. They work fine IMO. The second type of trailer brakes are electrically applied. They are controlled by an electric trailer brake controller, which you need to purchase separately from the hitch kit. The controller has two ways of applying the brakes. The first is that when you put your foot on the brakes, the brake lights come on, and that circuit triggers the controller to apply the trailer brakes. The controller is connected to the Lighting Control Module, which controls the brake lights, to accomplish this. The second way is via a manual control that lets the driver apply the trailer brakes separately from the vehicle brakes. In both cases, the controller takes the signal, and feeds electrical power to the trailer brakes to apply them.

You can achieve a clean and safe install with either method. You will likely find that your BMW dealer knows little about trailer brakes; the experts are usually at trailer supply stores, not dealers. BMW has come out with a trailer brake controller for the E70, so it is easier than it used to be. Still, satisfy yourself that they have done the job before.

With that background, the many posts on trailers and trailer brakes may make a little more sense.


OlyS4 09-12-2011 04:37 PM

Thanks for the great info. I didn't realize that BMW has a trailer brake controller for the E70, seems like that would make for a very clean install. Time to go search the "how to's" for that install!

Thanks again, great help.

JCL 09-12-2011 05:15 PM

1 Attachment(s)
If you buy the BMW controller (which is rebranded, they don't make them) it plugs right in. Early hitch instructions for the E70 say that BMW doesn't have a trailer brake controller. That was true at the time. When the controller came out it was advertised as being suitable back to start of production in 2007.

Install instructions attached below, with part numbers.

The alternative is to buy a Tekonsha Prodigy, P3, or whichever model you choose.

Instructions courtesy X5Meister, from this classic thead:



4MoJoe 09-12-2011 05:24 PM

I used the Tekonsha Prodigy RF on my 2011. No under dash wiring necessary – utilizes wireless communication. Works like a charm.

I debated wiring in the regular Prodigy I had but decided to go path of least resistance. The OEM controller as mentioned above is a good option but you give up a cup holder (drivers side) in the process.

ard 09-12-2011 06:54 PM

1. The BMW hitch and BMW wiring harness is fine.

2. Agree with JCL that BMW dealers know jack squat about trailers and braking.

3. Disagree that the BMW 'brake controller' is worth buying and installing... as far as I know, the car is NOT prewired for this and you'll spend a bunch of money hacking this into place. (I'd wire it under the dash, not a cup holder- and Tekonsha makes a clip in mount that you can use instead of that cup holder/garbage trap they include...)

4. The Tekonsha RF is a sweet unit- the control of the brakes is all done via wired connections at the rear of the car, it is just the user interface that is controlled via the RF (think about it, you don't ever 'adjust' anything while you are braking)

5. The BMW Controller is actually a Tekonsha (Prodigy?)...

No first had experience with eh X5, but tow lots of stuff (14k pound gooseneck, bumper pull stuff, cement wagons, cars, etc) with a tekonsha prodigy.

JCL 09-12-2011 07:02 PM

I believe the BMW rebranded unit is a Prodigy. The only advantage to it is that the harness is prewired for it in the back. If it was a significantly different price then I would buy a Prodigy and figure out where the four wires that BMW supply went to. Agree that it isn't prewired to the front in any case. Same as Ard, I wouldn't put it in a cupholder, but rather under the dash.

I've never used the wireless version, but it looks good.

blue dragon 09-12-2011 08:26 PM

X5 does just fine towing! My boat trailer uses surge brakes, the actuator is made by tie down engineering, which controls disc brakes on all 4 wheels. No problems stopping with it

OlyS4 09-13-2011 09:51 AM


Originally Posted by blue dragon (Post 843105)
X5 does just fine towing! My boat trailer uses surge brakes, the actuator is made by tie down engineering, which controls disc brakes on all 4 wheels. No problems stopping with it

Hmmmm, after reading all your posts I am honestly thinking that surge brakes may be the way to go for my application. I am only going to be towing an 18ft Sea Ray, around 3000lbs loaded with the trailer, the reviews I've read on surge brakes seem to be really favorable overall. Do surge brakes have any glaring limitations compared to electronic trailer brakes that I've missed? Are they less reliable under sustained braking conditions like coming down a mountain pass or an extended decent? I am a gadget guy and an electronics buff, but it seems like a BMW hitch with surge brakes on the trailer would be a simple, and most importantly to me, safe set up for my situation. Thoughts?

blue dragon 09-13-2011 10:11 AM

^^ The chief advantage of electric brakes is that you can activate them remotely to control trailer sway. I towed my boat to Crystal Beach on Lake Erie about 200 KM (120 miles) away from where I live at highway speeds up to 120 km/hr and I never experienced any trailer sway. The surge brakes work just fine.

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