Trailer Wiring and a bit more

George Van Parys

Mar 1, 2000
RO Number
Trailer Wiring and a bit more

To ensure safe towing, you must make certain that your vehicle and trailer are outfitted with WORKING running and signaling lights.
By law in every state, a trailer must have running lights, tail lights, and or marker lights, turn signals, and brake lights. To add a trailer light adapter, you must tap into the tow vehicle's electrical system and transfer it's power to the trailer wiring system.

Warning: Be aware that some manufacturers, such as Cadillac, Lincoln, Dodge, Jeep, and many luxury imports require a special wiring system that is included in the Towing Package. This wiring system has a built in sensor to prevent electrical damage to the onboard computer that monitors the vehicle's standard light system for faults. If you are wiring a vehicle that has such a system, kits are available that connect directly to the vehicles battery, not to the vehicle's wiring. Be careful, computer repair is expensive!

Note: If hooking in trailer lights to the wiring under the vehicle, avoid using 3M style quick pinch connectors. The metal prong pierces the plastic coating of the wire and lets in water and road salt that eventually destroys the wiring loom on the vehicle.

Electrical power is transferred to a boat trailer by using a four-way connector. One pin in the connector transfers power to the running lights, two others power the turn signals and brake lights. The fourth pin is for supply and electrical ground. A tow-vehicle connector is often included when you order a vehicle with the ''Towing Package." Otherwise you'll need to install one. (Most new boat trailers come with the correct plug from the manufacturer of the trailer, and the selling dealer will often install one for you.)

The most common connectors for boat trailers are flat plugs. But,they generally dangle from the rear of a tow vehicle and scrape on the ground. Many boaters wrap the connector wiring around hitch parts, but this causes the wire to crack and create electrical shorts. The best way to correct this is to install clips that hold the plug securely to the tow vehicle without twisting the wire. These are available from many hitch dealers.
Another connector option is a round, plated, four-way plug. It is permanently attached to the vehicle and has a lid to keep the connectors inside clean. However, should the connectors get dirty, they're not as easy to clean as plastic flat plugs are. With a plastic plug; a few in-and-out, twisting motions with the pronged part of the connector will free the open plug of dirt and corrosion. With the round connector use contact cleaner to do the job and to keep the door lid from sticking.

Note: When wiring for the best fit between tow vehicle and trailer, be sure to leave enough slack in the wiring for tight turns. Otherwise, you'll disconnect or tear the wiring. Also, never wrap or drape the wiring over the hitch or ball of the vehicle because the coupler from the trailer could pinch the wire in turns.

Many people tow more than one kind of trailer, so today's towing packages often include a wiring adapter. This eliminates the need for wiring multiple connectors to the tow vehicle. These adapters can also be found at trailer dealers and hitch-installation centers.
Most adapters include four-plug and seven-plug access. A seven-plug adapter is mainly used for travel trailers that need power for refrigerator and electric-brakes. Some even use nine-plug adapters.
If you tow two trailers, each with different wiring systems and connectors you may need an additional adapter. Generally, the main plug is a seven-way connector. To this, you insert a four-way or any other size that is needed for the application.

Many automotive manufacturers require that the factory turn signal flasher be replaced with a heavy-duty "flasher". The flasher controls how the turn-signal lights "flash". The standard flasher that comes with most vehicles is not designed to operate more than vehicle's lights, so it overloads. This overloading causes the tow vehicle and trailer turn signals to flash rapidly and faintly, so they are hard to see by motorists driving behind you. Changing to a heavy-duty flasher will solve the problem. Be sure you get the right heavy-duty replacement, make sure it is designed for trailering applications. All flashers are calibrated to the amperage required and the number a light bulbs on the circuit.
The flasher is usually located under the dashboard. On most new vehicles, it is connected to the fuse box and simply pulls out. There are usually two flasher modules, one flasher for the turn signals and one for the emergency lights. You should only need to replace the turn signal flasher module, although we recommend testing the emergency flashers as well.

Wiring flat, four way plugs is based on a standard color code used by GM and others. (Ford, as well as others, color codes are different) Brown is for taillights and sidemarker lights. Yellow is for the left-turn signal and brake light. Green is for the right-turn signal and brake light. White is for electrical ground.
Round, four-way plugs don't always have a consistent color code, and often come without any wire at all, so you will have to use a 12 volt test light to map the socket lay out and install wires color coded and of sufficient gauge for your application.

Note: Always use a 12 volt test light and not a volt meter, when doing any trailer wiring or troubleshooting. ( Just take my word for it.)

All foreign vehicles, and several American ones, use an "international" lighting system. This means that the turn-signal lights are separate from the stoplights. If the lights on the rear of the vehicle have an amber lens for turn signals and a red lens for stoplights, you need a converter. The "American" lighting system combines the turn signal and stoplight functions into one wire instead of two.
American boat trailers use the "American" lighting system. Consequently, if the tow vehicle has an "international" system, the two separate wires for turn signals and stoplights, on the tow vehicle, must be combined into one. To do this, a converter is necessary.
A converter is a circuit board built into a small, plastic, waterproof box or built directly into a four-way connector. The one built into the converter is preferred, because it reduces the number of wires needed to activate it, and it is not necessary to have a separate box that needs to be mounted elsewhere. Usually, three or four wires from the tow vehicle left and right turn and brake wires go into the converter and two wires come out. The two wires coming out are connected to the left and right turn connectors in the trailer-plug receptacle.


Always check all lights before you leave home. If they don't work, the most likely cause is dirt or oxidation on the contact points. So, be sure that all connector plug prongs and receptacles, light-bulb sockets, wire splices and ground connections to the trailer are clean, oiled, and shielded from moisture. A little light waterproof grease or engine fogging oil spread on the surfaces will act as a barrier against air and moisture, retard oxidation, and keep the lights operating longer.
В· It's best to solder wire-to wire splices, then wrap them tightly with plastic electrical tape, or better still, with heat-shrink tubing, it will help seal out dirt and moisture. Crimp connectors work for a while, but eventually corrosion forces you to cut them out and replace them.

В· Make it a frequent habit to scrape the prongs clean with a pen knife or sandpaper. Try to scrape off any surface deposits in the connector holes with an small file or small piece of sandpaper rolled around a toothpick (be sure the lights are off when you do this, otherwise it could blow a fuse). Then, dab a little grease on the prongs, push the connector together and wrap electrical tape around the crack to keep out dirt and moisture.

В· Between uses, keep both halves of the plug protected from weather and scuffing. To keep dirt from getting into the connecting plugs; wrap the plug with a small. plastic bag slipped over the top of each half. Then wind a rubber band around the open end to seal it. You can also use an old plug, greased and with the wires cut off, to plug into the one on the car as a way to protect the plug from damage.

In either case it is advisable to replace the light adapter, that hangs under the vehicle, every couple of seasons, if the car is driven in northern climates where salt is used on the highways to keep them from icing. The road salt has a nasty habit of permeating the wire itself and electrical resistance can increase dramatically in a very short time.


Always disconnect the trailer wiring and give the lights a chance to cool down before launching or retrieving a boat. The trailer bulbs get very hot, so when the lights touch the cold water, there's a good chance of them popping. Though a bulb change may not seem a major catastrophe to some, if the taillights are sealed units, you will have to replace the entire light assembly. Plus, you won't have any trailer lights when you return home.