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.

Checking your lights should be part of your preflight check every time you hook up the trailer. When you find a light is not in working order, it’s usually a simple to troubleshoot the problem.

If a single light is not working, start at that light and work your way back. Open the lens and remove the bulb to inspect the filament. Some trailer lenses have external screws. Some lenses fit into the body of the trailer, requiring you to loosen the lense from the inside and pop it out.

Your trailer likely uses a standard tail light bulb. Single filament bulbs (1156) are used for turn signals. Dual filament bulbs (1157) are used for running and brake lights. Holding the bulb up, a broken filament is usually self-evident. Replace the bulb with a known good bulb and try the light again before you reassemble the lens.

If replacing the bulb did not solve your problem, begin checking at the plug that runs from your bike. Let’s say your left turn signal is not working. Attach a test light or voltmeter to the pin on the bike-side plug that should be the left turn signal. Activate the signal and check the meter or light. If the voltmeter needle shows no activity or the test light does not blink, the problem is likely on the bike and not in the trailer.

If the voltmeter needle swings or test light blinks, move to the trailer and check for any breaks in the wire between the trailer plug and the turn signal socket. If this wire run shows continuity, check for continuity between the ground side of the light socket and the ground wire on the trailer.  Unless the trailer has sustained damage to the wiring harness underneath, the best candidates for problems are a loose ground wire or a problem at the plug.