Tag Archives: CAN-BUS

Harley CAN-BUS Trailer Wiring

Oh boy. If water cooling weren’t enough on new Harley models, now they’ve gone and added another BMW-like feature. CAN-BUS wiring. (Can an electrically adjustable windshield and integrated caviar cooler be far behind?)

In any case, the introduction of CAN-BUS wiring to the 2014 Harley touring bike line-up has created a lot of concern and questions.


You can still wire up your Harley for a trailer. And it won’t blow up your bike, it won’t trigger fault codes, it won’t transmit data behind your back to executives in Milwaukee who have an itchy finger on the “Delete Warranty” button.

Now, there is a twist. As I’ve written about before on this blog, and probably will again, there are two basic types of wiring setups for trailers, four wire systems and five wire systems. These refer to the number of wires used for your signal circuits and nothing else. A four wire system has one set of lights that function as both brake lights and turn signals. A five wire system has brake lights that operate separately from the turns.

For decades, motorcycles have been set up as five wire systems. Your brake light is a separate circuit from your turn signals. And so, the majority of motorcycle trailers made here in the US have been five wire systems.

Here’s the twist. In 2014, Harley eliminated the separate turn signals on the rear fender of MOST (not all…more on that), of MOST of their touring bike line-up. In the process of doing that, they combined the turn signals and brakes, making most 2014-to-present Harleys a FOUR wire system.

If you’ve purchased one of those little import trailers, you’re in good shape, because most of those trailers have a “flat four” plug and are four wire systems. Install a Harley plug-and-play wiring kit and it will, by default, mimic your bike’s four-wire system and drive your trailer lights without any problem.

Now, what if you have, or are planning to purchase a trailer that has separate brake lights and turn signals? In your case, what you need is a “four to five” converter — pretty much the opposite of what every auto parts store sells. You need to split out the brake light from the turn signals.

Luckily, the  wiring kits I sell have an extra CAN-BUS module option that does just this. It installs along with the other plug-and-play components and converts the Harley four wire system into a five wire system for your trailer. No muss, no fuss.

Now, I mentioned that most 2014+ Harleys have gone to this new setup. But there’s an exception. Any CVO model touring bike that has a set of separate turn signals down around the brake light are still set up as five wire systems. That means, when you want to connect to a five wire trailer, you need to do nothing other than buy the standard wiring kit. It will work, by default, with your five wire trailer. To connect to a four wire trailer, you’ll need a five to four converter, just like all the previous Harley models.

I realize that’s all a lot of information and some of it may be confusing. So don’t hesitate to ask for help to get the right kit for your bike. In the end, the thing that’s most important to know is that despite everything you may hear or read about the difficulty of adding a trailer to a CAN-BUS system, there’s really nothing to it. You just need the right kit.

Trailer Wiring – Going Direct

Your trailer’s lighting must be attached to your bike just as you connect a trailer to a car. A trailer depends on the vehicle’s system for power and to synchronize the trailer lights with the vehicle. It was once a common practice to wire the trailer harness directly to the bike’s wiring. This is still a common practice today, but there are a growing number of drawbacks to this approach.

Harley pass-thru connector for direct wiring.

Adding trailer lights increases the load on a circuit originally designed to drive one set of lights. Years ago, that wasn’t an issue. Light circuits used heavier gauge wire and could handle higher loads. Today’s bikes use thinner wire for weight and cost savings. Thin wire has more resistance per foot than thicker wire, so you can expect your trailer lights to receive a lower voltage.

Doubling up the lights probably won’t cause an outright failure, but the wiring harness will simply be unable to deliver the full power both sets of lights require to operate at maximum brightness. When lights experience a voltage drop of as little as half a volt, this can lead to the loss of as much as 20% of an incandescent lamp’s output. That’s a visible difference.

There can be other consequences as well. An increasing number of bikes are sensitive to changes in the load placed on circuits, especially bikes using the new CAN-BUS system like recent BMWs. Wiring in a trailer directly on a CAN-BUS equipped bike can trigger failure conditions in the bike’s monitor systems, even if everything is wired properly. Sometimes it becomes necessary to add countermeasures such as “load equalizers” to make the circuit appear to function normally to the bike’s control sensors. Expect CAN-BUS and other load sensitive monitoring systems to appear on an ever-widening range of bikes over the next few years.

If that weren’t enough, there’s another reason to avoid direct wiring. Even though your trailer may be well designed, the wiring running from the bike to the trailer is exposed to the elements, possible damage, and the potential for shorting. Short circuits and wiring problems are not common, but they can develop over time, particularly at the point where the trailer and bike are plugged together. When a short develops in a direct-wired harness, the problem will affect the bike’s lights as well as the trailer’s.

In the next installment, I’ll discuss a better method for powering your trailer lights using a isolation harness with relays.