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Trailer Wiring Guide for Trailers and Trucks


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Trailers generally come with a 4 or 5 wire flat connector or a seven wire round connector. If your trailer has an electrical wiring connector this does NOT mean your trailer has electric brakes. Electric brakes are a separate connection.

The purpose of the wiring connection is to match the action of the lights on the tow vehicle to the trailer lights. This includes the running (night) lights, emergency flashers, turn signals and brake lights. A four wire connector contains a ground wire (connected to the trailer frame), running light wire (goes to both right and left trailer lights) and turn signal/brake light wire for right and left lights.

If you have a five wire connector, the 5th wire is used for an electrical solenoid on a hydraulic actuator used to stop the brake when backing up. This are commonly called a “back up solenoid", “reverse flow solenoid" or “stop flow solenoid". The fifth wire from the tow vehicle is connected to the reverse lights of the tow vehicle. This activates the solenoid when the vehicle is placed in reverse.

Most trailer light problems stem from a bad ground connection between the tow vehicle and the trailer. Since there is not a ground wire going back to each light (very few trailer manufacturers add additional grounding wires), the proper ground for each light is dependent on the wiring ground from the vehicle to the trailer and the ground connection at every light.

Indicators that you have a bad ground connection are:

  1. Weak or dim trailer lights
  2. Opposite light dimming when turn signal is on
  3. Running lights go dark when turn signal is on
  4. All lights dim and lightly blink when turn signal is on

Check the simple things first if all lights act strangely.

  1. Check the wiring connector between tow vehicle and trailer. Clean the exposed connectors with a fine sand paper or steel wool. Make sure wires have not been cut or scrapped on the pavement.
  2. Check the ground wire from the connector to the trailer. This wire cannot just be wedged under the coupler lip. The wire has to be against clean bare metal, with a screw or bolt attaching it to the frame. Preferably a separate drilled hole in the frame for this connection.
  3. If only one light is acting up, check the bulb first for proper connection, Carefully remove the bulb and clean the connectors on the back of the bulb and inside the light housing. Check the ground to the frame. This can be a short wire of just the bolts holding the light to the trailer frame.
  4. If you have a long bell wire or similar, you can check the ground at each light. Attach an alligator clip at each end of a wire that is long enough to go from the tow vehicle to the end of the trailer. Connect one end of the wire to the vehicle frame, making sure to get a good connection to bare metal. Connect the other end to the ground on the light. Compare results. If the light works better, that is the most likely spot where the ground connection is bad.
  5. If your lights flash rapidly and not a full brightness, you may need a new flasher in the tow vehicle. Consult with a local auto parts store to replace the existing flasher with a HD model.

Remember, your trailer lights tell the traffic around you what you are doing. Improperly working lights are a hazard to you and everyone driving around you.

Article by Chris Turner
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