BMW CAN-BUS and Trailer Wiring

Question: I’d like to add a trailer to my BMW. I have a 2007 R1200RT. I’m having trouble finding someone who will wire it up. In fact, no one wants to touch it! They say that anything I add to the wiring will cause it to fault and they don’t want to be responsible, etc., etc. Do you have any thoughts or ideas?

Answer: BMW makes a great bike, but in this situation, the wiring system they use for their bikes is a little too smart for its own good. However, it is possible to add trailer wiring to your recent model Beemer. And, as other manufacturers adopt smarter wiring systems, this situation is going to occur with greater frequency.

Most bike signal circuits are wired the way they have been for decades. 12 volt power runs throughout the system and it is applied/removed from lights and turn signals based on the position of various switches, e.g. the ignition switch, kickstand, and kill switch. For the most part, the bike doesn’t monitor the power drain on the system in any way. It either works, or it doesn’t.

BMW began adopting the Controller Area Network BUS (CAN-BUS) for their motorcycles somewhere around the mid 2000′s. It makes sense, and it’s a move that other motorcycle makers will eventually adopt. CAN-BUS is essentially a “smart” wiring harness. It simplifies wiring, which reduces production costs and improves reliability. On the other hand, it makes it difficult to bolt-on goodies that have to interface with the bike in any way beyond tapping the battery. For example, adding a harness to drive the lights on a trailer.

The problem is most apparent when you try to figure out how to make your CAN-BUS bike drive the brake and tail lights of a trailer.

In the department of brake and tail lights, most bikes use a bulb with two filaments, one that operates the tail light and one that operates the brake light. Extending these functions to the trailer is easy. Tap the wiring harness for each light separately and run them to the trailer. Do the same for each turn signal and you’re done.

When you pull off the tail light module of a CAN-BUS Beemer, the first thing you’ll notice is the brake/tail lights use bulbs with just one filament. And just two wires, a hot wire and a ground. How do you get tail lights and brake lights out of a bulb with one filament and just one hot wire? The CAN-BUS system. This intelligent wiring system runs the bulb at a low 5 volts of power, enough to make it glow as a tail light. When you apply the brake, CAN-BUS bumps up the voltage which brightens the bulbs. Voila! Brake light!

This presents two problems for the would-be trailer puller. To begin with, a trailer has separate circuits for brake and running light. Second, CAN-BUS systems are very sensitive to the amount of load on the circuit. Extending the circuit by adding lights can make the bike’s computer think there is a problem, shutting down the circuit. Like I said, a little too smart for its own good, in this situation.

The good news is, there is a simple fix. First, make sure your trailer’s light are drawing the smallest amount of power possible. If your trailer is equipped with incandescent bulbs, replace them with LEDs. Plug-in equivalents that fit standard light sockets are inexpensive. LED lights draw a fraction of the power that incandescents require, which will prevent the bike’s computer from detecting a fault.

Now, since the bike is driving the lights at low voltage, then high, your trailer’s LED lights will mimic the action of the bike. All you need to do is hook up the trailer’s running light circuit to the bike’s combined running light/brake circuit. When first turned on, all your running lights will operate at low brightness, just like they should. And when you grab the brake, your running lights will brighten, just like brakes.

5 Responses to “BMW CAN-BUS and Trailer Wiring”

  1. Don Smith 26 January 2011 at 8:57 pm Permalink

    What you really need is an isolation wiring harness so that your motorcycle lighting system ‘triggers’ a low current relay (mechanical or solid state style) that routes power directly from your motorcycle battery to the trailer light. Most newer motorcycle wiring systems are not designed to take the extra load of additional lights like from a trailer installation, and switching your trailer lights from incandescent to LED style is probably not practical, and not inexpensive. You can get universal or made-to-fit trailer wiring isolation harnesses from several online sources.

  2. Dale Coyner 26 January 2011 at 10:00 pm Permalink

    Don, you’re right. I forgot to mention that this was done in conjunction with a relay package. The relay alone, however, did not eliminate the fault. I had to use the relay package AND an LED conversion to get it to work properly. I disagree that it’s not practical to change trailer lights to LEDs. Making the change is very easy when a trailer uses standard 1156 or 1157 bulbs, most of which do. LED bulbs in that format are easy to find and are not terribly expensive.

    The relay kit from Electrical Connection you mention is the one we sell.

  3. Mike 6 March 2011 at 6:29 pm Permalink

    I have been looking for a simple way to run lights to my single wheel motorcycle trailer. I changed the brake light stoplight bulb to a LED, $10.99 at Auto zone for a 2 pack of bulbs. I ran the wire directly from the trailer brake stop light to on on my bike, a 2007 BMW R1200RT. It works fine and it does not trigger a fault. The LED draws very little power, no relay involved.
    I am going to do the same with the directional lights on the trailer, switch them to LEDs. The LEDs last longer than bulbs, and they have come way down on price.
    I don’t think anything else is needed. With an isolation relay harness the relays would be drawing SOME additional current, so there is some extra to be had without triggering a “computer fault” that would open that circuit.


  4. Mike 4 May 2011 at 9:13 am Permalink

    The isolation relay harness will draw some power through the can-bus system. So there is some “extra” draw that will not trigger the computer fault or over current warning. I used a flat 4 wire trailer connector, and LED lights on my tail light/stop/light and directional lights. I connected the harness inside the taillight on my 2007 R1200RT. LEDs are not expensive anymore. The brake light on the trailer works just like the one on the bike. Common ground for all, 1 wire, taillight/brake light, 1 wire, each directional light , 1 wire each, total of 4. Simple to hook up.
    I have been using it this way and it works.

  5. Dale Coyner 4 May 2011 at 2:36 pm Permalink

    The trigger in the solid state relay is on the same order as an LED bulb, so it is fractional. The setup you describe doesn’t isolate the trailer wiring from the bike so if something happens to the exposed wiring (the plug location is most vulnerable), you’ll take out the lights on the bike. Yes, it’s easy to wire up something direct, but I’d still prefer a relay package. $100 to protect an $18K+ bike seems like a small investment.