Trailer Wiring -- Using an Isolation Harness

Trailer Wiring – Using an Isolation Harness

If you’re starting to think you should forget the idea of trailering to save your bike’s wiring, fear not. Rather than wiring a harness directly, use an isolation harness, also commonly referred to as a relay package. An isolation harness is a simple solution that will not only power your trailer’s lights at full power, it will also electrically separate the trailer’s lights from the bike’s light circuits, preventing any possibility of damage to the bike.

An isolation harness contains a set of relays; one each for the brake, turn signals, and running lights. Your bike is full of relays that control the lights, horn, and many other circuits. In short, a relay is an electrically-controlled switch.

When installed on your bike, the isolation harness uses your signals circuits as the triggers, but it draws power for the trailer lights directly from the battery. This means your trailer lights are running at full power and they are separated from the rest of the bike. Even if a short developed and rendered the trailer’s lights inoperable, it would not affect the bike’s lights.

(You'll find our motorcycle trailer wiring kits here.)


An isolation harness can be installed on any 12 volt motorcycle. A popular relay package I often use has three components: 1) the relay package consisting of a weather-sealed epoxy package with a passel of wires, 2) a 5-to-4 converter package for use with four-wire trailers, and 3) a subharness.

The subharness, consisting of four wires terminating in a four-pin plug, is installed first. If you own an ’88 to present Gold Wing or Harley touring bike you can use a plug-and-play style subharness that plugs into your bike. All other bikes require a universal subharness which contains a set of quick taps. We’ll assume you have something other than a Harley or Wing and need to use the universal subharness because your installation will require a little more effort.

Job one is to identify which wires run your bike’s running lights, left and right turn signals, and brake. If you have added a brake light modulator or other aftermarket accessories, you may already know. It’s times like these that it pays to have the shop manual for your bike or access to an online owner’s forum.

Remove the seat from your bike and look for a wiring harness that runs toward the back of your bike. On many bikes there is usually only one wiring harness running to the lights in the back. Without the shop manual you can still figure out which of those dozens of wires are the right ones to tap. Remove your lights from the back of the bike and make a note of the wire colors for each light. All of your lights should have one color in common and that common color should be the ground side of your wiring. Locate the other color wires in the wire bundle running under the seat. You will need to pull away some of the wrapping around the harness so you can get at enough length of the wires to tap into them.

Before you start tapping, I’d advise that you take an extra step just to make sure you’ve flagged the right wires. Using a pair of pliers, push a straightpin into one of the wires until you’ve punctured the insulation. Attach a test light or voltmeter (set to a range of 20 to 50 volts) with the positive lead on the pin and the negative lead on the negative battery post. Turn on the bike to check the light you’re testing. If you’re checking a turn signal you can expect to see the test light flash on and off just light the turn signal. On a voltmeter, the needle will swing as the voltage on the circuit rises and drops. Do this for each wire and you can feel confident you’ve identified the right wires.

The subharness comes with a set of quick taps. If you haven’t used quick taps before, they’re an easy way to splice in a wire into your harness without actually cutting into your harness. A quick tap has two slots that hold the wire you are tapping and the wire you are adding. Using a pair of pliers, push the metal tab down until it penetrates the insulating jacket of both wires, then fold the plastic tab over the body of the tap to clip it shut. Do this for each of the four signal wires and you have just completed the most difficult part of the trailer harness installation.

After installing the sub-harness, you’ll plug in the 5-to-4 converter if required. You’re probably wondering, “How do I know if I need this?” The easiest answer is to check your owner’s manual or look at your wiring harness. If you have a separate wire for brake lights and each turn signal, you have a five-wire system. If the wiring diagram in your owner’s manual shows your brake lights combined with the turn signals, you have a four-wire system. This is similar to the wiring scheme used on boat trailers and motorcycle haulers.

If your trailer is a four-wire system, you need to install the converter. Your bike has separate lights for brake and turn signals, so they can operate independently. The converter adapts the bike’s wiring so the trailer’s lights can operate properly, allowing the turn signals to override the brake lights on the trailer. Without a converter, your turn signals will only work when the brake is not applied.

With the 5-to-4 converter in place (or not), you’re now ready for the relay package. The relay has a four-pin connector that plugs into the sub-harness (or 5-to-4 converter), two power leads, and five output wires. Find a spot where you can tuck the relay package. It’s built to be weather resistant but you can usually find one little spot under the seat if you look around.

Next, run the power leads. The relay package uses power directly from the battery to operate the trailer lights. The relay package contains a set of crimp connections and a fuse to wire to the hot lead of the relay package. Always wire in the fuse! The connectors on the package require a crimp connection. To ensure you get a good crimp, use a real crimping tool to attach the terminals, not a pair of pliers.

With the fuse and terminals wired onto the power leads, connect the relay package to your battery. Disconnect the negative lead entirely from the battery before connecting the positive lead. This avoids a potentially dangerous situation. When working around the positive terminal, you can’t accidentally short out the battery if the ground lead is disconnected. After disconnecting the ground, connect the positive lead to the positive side of the battery, then connect the negative side, reconnecting the main ground lead.

Before you go further, it’s a good idea to check your work at this stage while everything is easily accessible. Attach a voltmeter or test light to the outputs of the relay package. The ground lead of your test device connects to the ground wire on the relay package. Connect the positive lead of your device to the wire designated for the running lights. Key the bike to the “on” position and check your test device. A test light should simply light up while a voltmeter will read a steady 12 volts on the scale. Check the other lights including brake and turn signals. Make sure the leads of these wires do not contact the ground lead while you are testing.

How did that turn out? Just fine, I’ll bet. Now you are ready to wire on a plug and button up the bike. I'll discuss this step in a future installment.