If you are not confident or experienced in vehicle wiring, the hook-up should be done by a licensed automotive mechanic.

Trailer lights can be an almost constant problem because of water damage. The most common causes  are:

• Incorrect wiring hook up between the trailer and towing vehicle
• Bulb failure
• Poor contact between the bulb and its socket and contact(s)
• Poor or no ground between the bulb socket and the trailer chassis
• Damaged wiring
• Poor contacts in the trailer to vehicle connector
• Burnt-out fuse

Most trailers in North America use four-pin connections between the vehicle and trailer, one for ground, one for the license plate and running (parking) lights and one for each stoplight and turn signal. Additional pins may be used for electrical brakes, etc.

Standard Trailer Wire Colouring

White: Ground

Yellow: Left turn/Brake Stop

Green: Right turn/Brake Stop

Black or Brown: Tail (Parking License Plate)

Blue: Ancillary – Electric Brakes

Purple: Reversing lights

Red: Auxiliary power

Some vehicles have a two-light cluster on each side for the turn/brake stop indicator, one bulb for turn and another for the brake stop. In order to connect a trailer to these vehicles, an adapter box is required. It is available at most automotive stores and directions how to connect them normally is provided with the unit.

In addition, some vehicles now have fault indicator lights, which tell the driver if any exterior light on the vehicle has failed. The circuits for these lights operate on resistance sensing, so that if trailer lights are connected directly into the circuit, the fault indicator lights may indicate that all the vehicle lights on those circuits have failed. It is advisable to contact your vehicle dealer, for advice on how a trailer should be connected to your vehicle.

To reduce wiring problems caused by corrosion in the wiring joints and connectors, a junction box should be installed in the vehicle to avoid having any splices on the underside of the vehicle.

There are two types of lamp assembly installed on boat trailers; the old bulb and socket assembly and the newer sealed units which usually consist of a conventional lamp and socket assembly, housed in a sealed plastic housing. This newer type eliminates two trailer light problems; bulb failure due to shock when the trailer is submerged in the water with the lights on, and corrosion between the lamp base and the socket. However, when a bulb in this unit fails, the whole sealed unit must be replaced.

Trouble Shooting

1. If no trailer lamps are working, but the vehicle lamps are okay, check ground circuit at vehicle-to-trailer connector.
2. If only one lamp in a circuit is not working, check the bulb, then the contact between the bulb and the socket and spring contact. Clean and replace bulb. If it still does not work, check the lamp ground to the trailer chassis and the continuity of the positive supply to the lamp.
3. If the turn signals operate erratically or either of them is dim, check the ground of both the lamp assemblies.
4. If there are no lights on both the vehicle and trailer, check the vehicle fuses. Refer to the vehicle handbook for the location and correct amperage of the fuses.

Courtesy of the CanBoat / NautiSavoir Boat and Engine Maintenance Course. Breaks, tires, wheel bearings, hull and engine maintenance and more. Sign up for a maintenance course at: www.boatingcourses.ca. 

Next tip, learn how to safely Tow and Trailer your boat.