7. Towing Electrical & Wiring
Download Chapter 7 (784 KB)
Answered in this chapter:
Before you can legally tow your trailer on public roads, you need to have working trailer lights that are plugged into your vehicle and operating in sync with your vehicle's lights. Not only will working taillights, brake lights and turn signals help you avoid a ticket, but more importantly, they will communicate your actions to other drivers on the road and promote safe towing.
In this chapter, we will discuss the various wiring and electrical aspects of towing a trailer. We will cover the different types of vehicle wiring systems, how to wire your vehicle for towing, how to connect opposing wiring systems and connectors, as well as how to rewire your trailer.
When towing, your trailer's wiring system needs to be connected to your vehicle's wiring system. This requires a plug and socket, as well as a converter in many cases. Before we get into the trailer wiring components, let's look at some of the different wiring systems used in vehicles today.
The two-wire system is the simplest form of vehicle and trailer wiring and is still used by some vehicles today. This system sends the stop or brake signal and the turn signals along one wire, and the taillight signal along a second wire.
The three-wire system is the most common in the automotive industry. It sends the stop, tail and turn signals along three separate wires. Vehicles with a three-wire system usually require a converter to be able to tow a trailer.
Pulse Width Modulation (PWM) System
More and more vehicles today use a PWM wiring system. Sometimes called a 'multiplex' system, this type of wiring is able to control multiple lighting functions through a single wire by varying the signal intensity. PWM systems can use incandescent or LED lights, and they generally fall into one of two categories: ST systems and STT systems.
The ST system (stop / tail) is a PWM system that uses a single wire to control the stop and taillight signals. Separate wires are used to control the left and right turn signals.
The STT system (stop / turn / tail) is a PWM system that uses a single wire to control all three lighting functions: the stop or brake lights, turn signals and taillights.
If your vehicle is not equipped with a factory-installed connector, custom wiring is the ideal solution. A custom wiring harness or 'T-connector' is a vehicle-specific harness that connects to the vehicle's electrical system and provides a vehicle-to-trailer wiring socket, usually in the form of a 4-way flat.
Some custom wiring harnesses use an integrated converter while others do not. Because each harness is made for a specific vehicle, a converter will only be included if the vehicle requires it.
Two Types of Custom Wiring
There are two basic types of custom wiring: custom wiring harnesses and custom wiring connectors. The difference between them can seem subtle at first glance, but in terms of function, the two are not interchangeable.
Custom Wiring Harnesses
A custom wiring harness has multiple plugs that are used to 'T' into the vehicle's taillight assembly, drawing power directly from the taillights or from a direct batter connection. While custom wiring harnesses usually require two or more connection points, splicing and soldering are rarely ever needed.
Custom Wiring Connectors
While some vehicles may not be equipped with a trailer wiring connector, they may have a special socket specifically intended for hooking up wiring, provided by the vehicle manufacturer. A custom wiring connector uses a single plug to plug into the special factory socket and provide a standard trailer wiring connector.
To find custom wiring for your vehicle, you can use CURT's Quick Hitch Lookup tool, found at the top of every page at curtmfg.com, or the CURT Application Guide.
If custom wiring is not available for your particular make and model, an electrical converter may be required to equip your vehicle with the proper vehicle-to-trailer wiring connection.
An electrical converter or taillight converter is a device designed to convert your vehicle's "complex" wiring system to be compatible with your trailer's "simple" wiring system. As previously mentioned, many vehicles are equipped with a three-wire or PWM system. Most trailers, on the other hand, typically use a two-wire system. An electrical converter acts as a bridge between the two, allowing you to properly connect your vehicle and trailer through a standard plug and socket.
Differences Between Plugs & Sockets
Plugs and sockets are the electrical components that actually connect the vehicle wiring system to the trailer wiring system. 'Socket' is used to refer to the vehicle side of the connection, and 'plug' is used to refer to the trailer side. They can use anywhere from two to seven wires, depending on the trailer's electrical requirements.
The tables below show the electrical functions of each plug and socket type. While most plugs and sockets come with standard color-coded wires, the colors illustrated below may not reflect those found on all vehicles and trailers.
Common Plug Ends by Trailer Type
Trailers are equipped with different plug types based on their electrical components. The chart below provides examples of common trailers and the types of plugs they typically use.
Towing Tip: A bad ground connection is actually the most common cause for trailer wiring problems. A bad ground can show up as an overall lighting failure, even when the voltmeter says you have a charge. It can also create intermittent failure, causing your lights to flicker randomly. When mounting a ground wire, choose an existing factory ground connection where other ground wires are connected or use a bolt to mount the wire directly to your vehicle's chassis.
With so many different types of connectors, it is possible that your vehicle will have a different connector type than your trailer. For example, your vehicle may have a 7-way RV blade, while your trailer has a simple 4-way flat.
An adapter is an electrical device that allows a connection to be made between a mismatched trailer plug and vehicle socket. In most cases, an adapter is plug-and-play, creating an instant connection between the two opposing connector types. However, sometimes an adapter requires extra wires to be grounded or spliced into existing vehicle wiring.
Use the chart below to identify which CURT adapter is needed for your vehicle-trailer setup, based on the plug and socket types.
Depending on the type of trailer, installing new wiring typically requires only a few basic tools and a good wiring kit. If you don't feel equipped to do the job yourself, you can also take your trailer in to a professional.
The following installation is based on the CURT #53540 trailer light kit. The steps may not reflect those involved with rewiring your own trailer.
Installing the Wiring
Step 1: Remove the old wiring. If the old taillights and wiring are still attached, remove them and take note of the layout of the wires on the frame.
Step 2: Lay out the new wiring. Route the new wiring through the frame to the back of the trailer, with the yellow and brown wires on the driver side and the green and brown on the passenger side (these may be different for different connector types).
Step 3: Mount the ground wire. With an equal length of wiring on each side and plenty of length on the front end to connect to the vehicle, attach the ground wire at the front of the trailer frame.
Mounting the Lights
Step 4: Install the marker lights. Mount the marker lights with the provided bolts and then splice in the marker light wires using snap locks.
Step 5: Attach the wires to the frame. Use the clips already in place or those provided in the kit. Make sure the trailer wiring stays fairly taut and sheltered by the trailer frame to protect them from moving parts and road debris.
Step 6: Install the taillights. Mount the taillights to the trailer frame and connect all wires to the appropriate connection points.
Step 7: Test your trailer lights. Hook up your vehicle before you tow and make sure your trailer's lights are functioning in sync with your vehicle's lights.
No matter what or far your are planning to tow, it is always a good idea to test your trailer lights.
If you have additional questions about trailer wiring or electrical issues, contact the CURT Tech Support team toll-free at 800.798.0813.
In the next chapter, we will cover the ins and outs of using trailer brakes and how towing with trailer brakes can change the way you drive.