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 CANBUS system like recent BMWs. Wiring in a trailer directly on a CANBUS-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 CANBUS 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.
So, what if the trailer you are considering has led lights?
Kevin,
Good question. LED lights largely resolve the issue of adding extra load to signal circuits. You still have the issue of how the bike’s wiring will be affected if a problem develops in the trailer’s wiring. By wiring direct, you’re adding a lot of extra “exposure”.
If the bike is relatively simple with respect to electronic controls and sensors, the result of a short in the trailer’s wiring or a problem at the plug will probably just pop a fuse in the affected circuit(s). However, more touring bikes are moving to monitored circuit architectures (CAN-BUS) which are much more sensitive to unexpected variations in bike circuits. If the trailer (or more likely, the plug) develops an intermittent problem that isn’t enough to cause an all-out short, it can still be something that an active circuit monitoring system would pick up and flag as a “fault.” That might mean something as harmless as an unexplained trouble light on the console, or if the control module thought it was serious enough, it might disable the bike to avoid further damage. A relay system would electrically isolate the trailer so even if load isn’t an issue, a potential problem in the trailer would not affect the bike.
Again, older model bikes, not so sensitive to this. Bikes coming on line now and in the future, far more sensitive to issues like this.
What is the standard pinout for the 5 pin round trailer connecter? I would like to wire to a standard rather than just wing it.
Well…there really is no published standard. There’s a defacto standard among manufacturers, but I wouldn’t plug my trailer into someone else’s five-pin receptacle without checking the pins first. If you have a magnifying glass and good light, you may be able to make out marks on plastic beside each screw terminal that will give you some guidance. “T” is for the tail or running light, “B” for brake, “R” for right turn, “L” for left and “G” for the ground wire. If they aren’t marked with letters, they may be numbered. In that case, it really is up to you to decide the order.