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.

A 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.


This plug-and-play isolation harness is for a four-wire trailer and a Harley.

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.

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.

9 Ways To Buy a New Trailer For Less

Photo Courtesy of Alan Cleaver

Thinking about buying a trailer, but need to get it past the home budget czar? Really like a particular trailer but find it’s just a little out of your range? Here’s a list of ways you can save money when buying a trailer. Depending on your circumstances, it is entirely possible to cut your cost for a trailer by more than half. So, I hope this helps you get the trailer you want, and a price you can afford!

Buy Off-Season

This may be the most obvious of suggestions, but making a trailer purchase in the off-season will save you money. In October, we’re still mourning the loss of fair riding weather for the year, and haven’t started looking forward to next year’s rides. Very few people are thinking about trailers. As we begin to approach riding season on the calendar we suddenly remember – Oh yeah, I guess I’d better get that trailer ordered!

Do yourself a favor and budget for your trailer purchase between November and January. There’s a good chance you will find special shipping deals, discounts on leftover models, and incentives on orders for the new year.

Buy a Complete Package

Do you need a hitch and wiring for your bike along with the trailer? You may be able to get a package deal and save money on the extras when you buy everything all at once.

Buy a Trailer Timeshare

Have you and a buddy talked about buying trailers for your future trips? If you have similar tastes in bikes, you may be able to save by splitting the cost of a trailer with your friend. Think of it as a “trailer timeshare.” People buy timeshares of beach condos, exotic sports cars, and airplanes, so why not a trailer?

When traveling together, you’ll find that a 25 cubic foot cargo trailer can easily handle the gear of two couples. You’ll also have the use of a trailer when you go off on solo trips. You’ll need to agree on a common color choice, how the trailer will be stowed, and how to allocate its use when you are not riding together. But if you find you can agree on these things, sharing a trailer means you’ll get all the trailer you want for half the money.

Make a Group Buy

Maybe you and your friend like to travel together, but you know a trailer sharing agreement would never work.  You can also save by buying with a friend or with a group if you purchase trailers from the same place at the same time. A package deal will allow a willing dealer to create some incentives for a group purchase.  Shipping two or more units to the same location can also help reduce your freight costs by as much as a third.

Terminal Pickup

Speaking of shipping, it’s possible to save a big chunk on freight by picking up your trailer at the freight terminal of the truck company handling your delivery. Often, delivery to your door is expensive because the shipping company will need to run a special truck to your house. If you have a pickup or a flatbed trailer, or access to either, picking up your trailer at the terminal will often save you $150 or more.

Make It a Vacation

I’ve sold trailers to many riders who avoid crating and shipping fees altogether by making a vacation out of picking up their trailer. Escapades are manufactured in the Virginia mountains while American Legends are made in the midwestern heartland. Mini Mates come from Pennsylvania and WAGS are in Iowa. All are great riding destinations.

Cash Talks

Dealers who accept cash or personal checks for payment may be willing to give you a discount.  They would be paying the discount to a credit card company, so why not give it to you instead? Card discount rates usually run 1.5 to 2.5 percent. Potential savings: A cash discount on a $4,000 trailer would put $100 in your pocket.

Go Color Neutral

If you’d like to have your trailer color-matched but want to keep your costs down, consider buying a neutral color. Most trailers are available at no extra cost in stock colors. Fiberglass trailers are usually available in a white or black gel coat.  Gel coat is a hard finish used to give fiberglass a finished look.  It is not as glossy as paint, but if a black or white trailer would look good with your bike, even if just for a year, you can get it painted later and spread the cost of the trailer over time.

Escapade LE Motorcycle TrailerGet Only the Critical Stuff

Finally, take a really hard look at what you absolutely need and order only the most critical things with the trailer. Most trailers have a long list of options you can bolt on to the trailer.  Many of these are cosmetic enhancements that may add to the appeal of the trailer, but don’t necessarily change its performance. Many options such as coolers, luggage racks, spoilers, chrome trim, extra lights, can all be added at a later date.  There are a few options that make sense to have installed at the factory.  For example, if the trailer you choose offers interior carpet as an option, have that installed by the factory.  It would be difficult, time-consuming, and probably more expensive to have carpet added later to your trailer.

Let’s take a real example. Let’s say you’re interested in an Escapade LE trailer (25 cubic feet, air suspension). You really want it color matched to your bike but you need to keep the price down. On an Escapade, as I mentioned above, the only critical factory-installed option is the carpeting. That’s it. Don’t let another dealer tell you otherwise. Everything else can be bolted on later. Chrome wheels? Chrome tongue? Luggage rack? These can all be added later with no problem. Stone shield? Skip that and have a car detailer install a clear stone guard on the front. Cooler? Bolt on.  Garmet bag? Bolt on. Spoiler? Bolt on. Not as easy as adding some of the other things, but it’s still a bolt on.

When you take off all the things you can add later, guess what?  A color-matched Escapade LE that would have cost you $4590 with all the popular options is now $2,865.  Later, if they’re important, you can add your accessories. But for now, the important thing is, you have a color-matched trailer for almost half the price of a full-loaded trailer.

How Do You Save?

I’ve shared a handful of ideas for saving money on a trailer purchase. What’s your strategy?