Understanding 4 wire vs. 5 wire Motorcycle Trailer Wiring


Electrical systems can be difficult to understand and trying to explain it over the phone can be like trying to get my wife to pack light for a trip. It's seemingly impossible! In this article, with the use of some visuals, I will attempt to help you better understand the differences and the compatibility between 4 wire and 5 wire trailer systems. You can start by watching this video to see how a 4 wire and 5 wire lighting system operates, and then read more below for additional explanation.

Trailer Wiring Colors:

There is a universal standard in trailer wiring color codes which is in the order as follows:

White - Ground

Brown - Running Lights

Yellow - Left Turn (& brake on a 4 wire system)

Green - Right Turn (& brake on a 4 wire system)

Blue (or Red) - Brake Lights (only used on 5 wire systems)

*Warning: Even though this color code is a universal standard, some motorcycle manufacturers have opted to use different color codes for their trailers. Why? Just to be difficult I guess!

A 4 wire system will not use the 5th blue (or red) wire. There are many types of plugs used in the motorcycle trailer industry, but cars almost exclusively use a flat 4 plug. The pictures below show the 3 most common plugs for motorcycle trailers. A flat 4 plug for 4 wire trailers, a flat 5 plug and a round 5 plug for 5 wire trailers.

Trailer Wiring: 4 wire systems

4 wire trailers are probably the most common type of trailer and almost exclusively used when it pertains to non commercial flatbed or cargo type trailers. A 4 wire trailer will generally only have two function lights, one on the left side and one on the right side. These lights act as running lights (on all the time) and brighten for BOTH turn signals and brake lights. Below are two examples of a 4 wire trailer. A typical flatbed utility trailer pulled by small cars and trucks and a small cargo trailer for motorcycles.

As you can see, both of these trailers only have 2 function lights in the rear that will work as both your brake lights and your corresponding turn signal indicator lights. While this is perfectly legal, it minimizes the functions of the lights and decreases safety. For example, when you apply the brakes with the left turn signal on, the left light will flash while the right side light will stay lit indicating braking. You only have 1 brake light, 1 turn light, and no remaining running lights. This type of trailer is like shooting for a D on a test just to pass the class, as it only meets the bear minimum requirements for trailer lighting.

Trailer Wiring: 5 wire systems

5 wire trailer systems are scarce in the car/truck industry, but are common when it pertains to motorcycle trailers. The main reasons behind this is that it provides more lighting (which increases safety) and maintains the FULL functionality of tow vehicle lighting. A 5 wire trailer will have 3-4 function lights on the back of the trailer as opposed to just 2. Two of these lights will act as left and right turn signal indicator lights while the other one or two lights will be designated for brake lights. This means that the brake lights will work independently from the turn signal lights. Using the same example as above, when you apply the brakes with the left turn signal on, the left turn light will flash, the right turn light will remain as a running light, and the brake light(s) will stay lit while the brakes are applied.

Below are pictures of our Route 66 cargo trailer and a Mini Mate Camper, which both utilize a 5 wire lighting system.

The top two lights on our trailers are designated as brake lights, while the bottom two are left and right turn signal lights. All of 4 the lights are dual brilliance LED's which means that they are running lights (on all the time) and then brighten even more by adding lit diodes when signaled for turn or brake lights.

While adding more lights and function to the trailer costs more for the manufacturer, it is clear that they add a far greater amount of lighting and safety. Now lets take a look at vehicle wiring and understanding which types of trailer wiring you can use for your tow vehicle and trailer.

Tow Vehicle Wiring: Motorcycles

Virtually all motorcycles have a 5 wire function lighting system. This means that the motorcycle has a left and right turn signal and a separate brake light. This is great for towing 5 wire trailers, as you can maintain all the function of your motorcycle lighting to your trailer. If you are towing a 4 wire trailer with your motorcycle, you will need a converter which will convert 5 wires down to 4 wires by combining the brake function to both turn signal functions. Below you will see an example of a converter. As you can see, you have 5 wires to the left of the converter that will go to the 5 functions on your motorcycle (or car) and then the converter combines the brake and turn functions and has only 4 wires going out on the opposite side to the flat 4 plug.

You no longer have the independent brake light wire (red), as your brake lights will now run through your turn signal wires (yellow and green). This has minimized the function on your trailer lighting so you do not have independent brake lights from turn signals, which is legal but not optimal.

Tow Vehicle Wiring: Cars

As with motorcycles, nearly all modern cars also have a 5 wire function lighting system. Unfortunately, when it comes to purchasing trailer wiring for a vehicle it is standard to have a converter in-line, making it a 4 wire system. The car industry assumes that cars will be towing some kind of flatbed trailer which are almost all 4 wire systems. This is when towing a 5 wire motorcycle trailer with a car often becomes confusing for customers and they ask "How can my car which is converted to a 4 wire system, tow a 5 wire system motorcycle trailer?" There are options! Let's take a look at them.

1. If the turn signal lights on your trailer are red, the easiest and most common option is to just run the trailer as a 4 wire system. This means that you will simply bypass the 5th brake wire by not hooking it up to the trailer plug. Your top two lights on your trailer which are designated as brake lights will now just remain as running lights. Your bottom two lights designated as turn signal lights will work both as turn signals AND brake lights. The one positive thing about doing this with a car is that there is a good chance that other vehicles will still be able to see your vehicle brake lights because a motorcycle trailer is so small. Once again, even though it does not fully utilize all of the trailer lighting, it is completely legal. A benefit of this option, is that you will be able to tow any 4 wire or 5 wire trailer that is equipped with a flat plug.

2. If the turn signal lights on your trailer are yellow, it is more complicated. Since brake lights must be red, (while turn signal lights can be yellow or red) you cannot just simply run the trailer as a 4 wire system because the brake lighting would run through your yellow turn signal lights which is not legal. In this case, you can change the turn signal lights to red lights, or you can rewire the trailer turn signals to work through the red brake lights instead. Another option is to hook up your vehicle with a wiring harness that does not have a converter. These can be difficult to find, so you could also just cut out the converter from your vehicle wiring and attach a flat 5 plug directly in line so that you can maintain the independent brake light function as the trailer was designed to have. This is a little more work, but it gives you more functional lighting for your trailer. A drawback to this is that you will NOT be able to tow a 4 wire trailer with this vehicle. If you would also like to tow a flatbed trailer or other 4 wire trailer with this vehicle, you should keep it equipped with a flat 4 plug and change the trailer instead.

If your tow vehicle has a flat 5 plug and your trailer is a 4 wire system, you will just need to add a converter to your vehicle wiring. The simplest style of converter for flat plugs is one that will plug directly into your 5 way plug and then convert it to a flat 4 plug, which will enable you to hook up your flat 4 trailer plug directly.

I hope that all this information has given you a better understanding of the different types of trailer and vehicle wiring. If you have additional questions, please send us an email to kompactkamp@gmail.com.

For more information on our motorcycle trailers, visit www.kktrailers.com. Happy Travels!

Picture source: Google images. No copyright infringement is intended.

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