Trailer Wiring and Brake Control Wiring

Tail Light Converters
Brake Control Wiring
Vehicles Towed Behind a Motorhome
Wiring Diagram for Common Plugs
Breakaway Switches

Electrical 6-Pronged PlugSpecial light and wiring systems need to be installed on your tow vehicle before you can tow any trailer. The trailer lighting system must not be directly spliced into your tow vehicle lighting system. Your qualified hitch installer will wire up your taillights, signals, and brake light functions into a special plug or connector in order to access the trailer's light systems.

Basically, when you flip on your turn signal or push your brake pedal, the lights on the trailer must also signal your intentions to the driver behind you. The law says that any trailer that protrudes 4 or more feet behind your vehicle or visually blocks your taillights must be equipped with it's own taillights, and your vehicle must have a connection to the trailer's taillights. Hence the need for wiring your vehicle.

Most trailers are wired to use a single red light for both the brake and turn signals (1 bulb per side). This is known as a 4-wire system. Most import vehicles and newer domestic vehicles have both a red brake signal and a separate amber turn signal (2 bulbs per side). This is a 5-wire system. Tail light converters are required to connect a vehicle with a 5-wire system to a trailer with a 4-wire system. Eyers Trailer Hitch Center sells and installs tail light converters and trailer light power modules to accomplish this.

Tail Light Converters


tail light converters drawing

All trailer light power is routed through the tow vehicle's light circuits.

Trailer Light Power Module

trailer light power module drawing

All trailer light power is routed directly from the tow vehicle's battery via a separate power line. Required for some vehicles to avoid vehicle light circuit damage.


We are also seeing more sophisticated computer systems being installed on vehicles nowadays. In many cases, we must bypass the computer system when we wire a vehicle for towing so that the towing wiring doesn't confuse the vehicle's computer into thinking something is wrong with it's lighting system.

Again, special converter units and wiring harnesses must be installed to alleviate this problem. That can get expensive. So, if you are a do-it-yourself wire person, please ask us for advice before doing your own wiring. Improper wiring can lead to blown fuses, and in extreme cases, overheating of wires which can result in a fire in your vehicle.

Wiring gets a little more complicated when you tow larger or more sophisticated trailers. Instead of needing simple wiring to run lights only, some trailers require extra wiring to run electric brakes, 12-volt battery charge lines, back-up lights and refrigerator power as well. Your hitch installer can determine your wiring needs based on the type of trailer you are towing as well as what kind of tow vehicle you have.

Brake Control Wiring

Pull TrailerIf a trailer is longer than 15 feet or weighs more than 1500 lbs., it must, by law, come equipped with a brake system.

There are two types of brake systems used today. Most boat or water sport trailers are equipped with hydraulic brakes, sometimes called surge brakes. Surge brakes are an independent system activated by a master cylinder at the junction of the hitch and trailer tongue. The master cylinder is mounted on the front of the trailer so any tow vehicle can tow the trailer. Simply put, when the inertia force of the tow vehicle's braking action activates the master cylinder in the surge coupler, the trailer brakes are activated.

The other type of brake system is electronically controlled electric brake control wiring. The trailer brakes are activated electronically by the use of a brake control box mounted under the dash in the tow vehicle. There is additional wiring involved in tying your braking system and battery power to the rear plug, which activates the trailer brakes when you depress your brake pedal. The brake control box regulates the amount of voltage going back into the plug. The brake control box also allows you to activate the trailer brakes by hand in emergency situations, or when you simply want extra braking action independently of your putting your foot on your brakes.

Both of these braking systems must conform to Federal, State and Local regulations. The law states that if you are towing a trailer equipped with electric brakes, you must have a brake control unit installed and working in your tow vehicle.

Some of the older brake control units utilize a pendulum inertia type technology to achieve activation of trailer brakes. The problem with this type of system is that the box must be mounted exactly in only one position in order for the internal pendulum to properly activate the system. Sometimes if you are going up and down hills, the pendulum is fooled into activating the brakes improperly.

The newer technology utilizes an all-electronic solid state system to activate the brakes. The brake control wiring box can be mounted in any position. They are relatively trouble free.

Brake Controllers

Primus Digital Brake Controller
Primus Digital Brake Controller

Introducing Primus Digital trailer brake control, a totally proportional electric trailer brake control.

Tekonsha P3 Brake Controller
Tekonsha P3 Brake Controller

The P3 electronic trailer brake control, for 1 to 4 Axle Trailers, is proportional. Distinct, easy to see LCD display with multiple color and contrast options. Allows you to switch between Electric and Hydraulic brakes.

Tekonsha Prodigy Brake Controller
Tekonsha Prodigy Brake controller

Prodigy trailer brake control is the latest and most intelligent brake control from Tekonsha. This powerful control features new motion sensor technology similar to what is used in the aerospace industry. For 2, 4, 6 & 8 Brake Trailers.

Curt Triflex Brake Controller
Curt Triflex Brake Controller

Triple-axis motion sensing for highly responsive, smooth stops, compact size, automatic leveling and calibration - No setup required, automatically adjusts brake output when going up or down a hill, sensitivity adjustment for varying trailer loads or driving conditions, compatible with anti-lock brakes and cruise control, operates 2-8 brakes

Wiring Diagram For Common Plugs

Here's a handy Wire Diagram for the most common types of electrical plug connectors:

4-Flat Wiring Diagram

6 and 7 Way Plugs Wiring Diagram

RVi3 Braking System

Nowadays, most states and Canada require you to have a braking system installed on your towed vehicle. Canada won't let you in if you don't have one! We love the RVi3 Braking system which is easy to set up and transfers to any another vehicle with ease. It comes with a tablet-controlled monitor for the coach, and breakaway system.

RVibrake 3

RVibrake 3 Breaking System
Towed Battery Charger

RVi Brake 3 Braking System for towed vehicles behind a motorhome.

No wires, no installation Top View braking RVibrake3 Everything is done through the RVibrake3 in the towed vehicle. There is no wiring or installation from the motorhome. RVibrake3 is activated by the same force that draws your chest forward in a braking, called inertia. An accelerometer in the brake measures these changes and knows when to push softer or harder on the brake pedal, keeping your brake pads from having excessive wear. Terrain sensing software keeps RVibrake3 from being more sensitive downhill and less sensitive uphill, giving you accurate braking at all times.

Towed Battery Charger

There are not many things worse than a dead towed battery when flat towing. Many towed vehicles today have a large current draw on the battery when towing. Towed Battery Charger Plus will prevent this unseen parasitic loss on your towed battery. With LEDs that give feedback of your towed battery and charging voltage there is no more need for a charge line that gives you zero feedback.

Breakaway Switches

A breakaway switch uses either a large dry cell battery or the auxiliary 12-volt battery contained within the trailer to automatically engage the trailer brakes if the trailer is accidentally detached from the tow vehicle. The pin is inserted in the breakaway switch to hold the switch open. If the trailer becomes detached, the cable will pull the pin from the switch and the switch will then apply electrical current to the trailer brakes. You should occasionally check the breakaway switch by pulling the pin out and checking the braking action.

Breakaway Switch located on the Trailer Coupler Frame You can test the breakaway switch before each trip as follows:

  1. Hitch the trailer to the tow vehicle.
  2. Pull out the breakaway switch-actuating pin.
  3. Test brakes by attempting to drive away. The breakaway switch is functioning properly if the trailer brakes are activated. (Brakes will be locked up.)
  4. If the brakes are not activated, check that the trailer battery is connected and fully charged, and the trailer brakes are properly adjusted.
  5. Obtain service repair if the trailer brakes do not operate after making these checks.
  6. Reinsert the breakaway switch-actuating pin before towing the trailer.

Note: Never use the breakaway switch as a parking brake when you are parked on a hill. The battery will run down and the breakaway system will not be functioning properly when you really need it.

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