Trailer Wiring Guide For Trailers And Trucks

Trailers frequently come with a four or five wire flat connector or a seven wire round connector. If your trailer has an electrical wiring connector this does NOT imply your trailer has electric brakes. Electric brakes are a separate connection.

The objective of the wiring connection is to match the action of the lights on the tow car to the trailer lights. This contains the operating (evening) lights, emergency flashers, turn signals and brake lights. A 4 wire connector includes a ground wire (connected to the trailer frame), operating light wire (goes to each appropriate and left trailer lights) and turn signal/brake light wire for appropriate and left lights.


If you have a 5 wire connector, the 5th wire is employed for an electrical solenoid on a hydraulic actuator employed to cease the brake when backing up. This are frequently named a “back up solenoid”, “reverse flow solenoid” or “cease flow solenoid”. The fifth wire from the tow car is connected to the reverse lights of the tow car. This activates the solenoid when the car is placed in reverse.

Most trailer light complications stem from a poor ground connection involving the tow car and the trailer. Considering that there is not a ground wire going back to each and every light (incredibly handful of trailer makers add further grounding wires), the right ground for each and every light is dependent on the wiring ground from the car to the trailer and the ground connection at each and every light.

Indicators that you have a poor ground connection are:

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

Verify the uncomplicated factors very first if all lights act strangely.

  1. Verify the wiring connector involving tow car and trailer. Clean the exposed connectors with a fine sand paper or steel wool. Make positive wires have not been reduce or scrapped on the pavement.
  2. Verify the ground wire from the connector to the trailer. This wire can not just be wedged beneath 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 a single light is acting up, verify the bulb very first for right connection, Cautiously eliminate the bulb and clean the connectors on the back of the bulb and inside the light housing. Verify the ground to the frame. This can be a quick wire of just the bolts holding the light to the trailer frame.
  4. If you have a extended bell wire or equivalent, you can verify the ground at each and every light. Attach an alligator clip at each and every finish of a wire that is extended sufficient to go from the tow car to the finish of the trailer. Connect a single finish of the wire to the car frame, creating positive to get a very good connection to bare metal. Connect the other finish to the ground on the light. Examine outcomes. If the light functions far better, that is the most most likely spot exactly where the ground connection is poor.
  5. If your lights flash quickly and not a complete brightness, you could need to have a new flasher in the tow car. Seek the advice of with a nearby auto components retailer to replace the current flasher with a HD model.

Keep in mind, your trailer lights inform the website traffic about you what you are performing. Improperly functioning lights are a hazard to you and absolutely everyone driving about you.


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