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How to Test a 4-Prong Trailer Connector Plug

by Richard Asmus

You've connected the trailer to the vehicle, but the turn signals and brake lights don't work. You don't know if the wiring is bad or you have bad bulbs. One way to troubleshoot the voltage on the connector plug is to use a circuit tester. On a four-prong plug, one prong connects to the right turn signal, one to the left and one to the tail lights. One is the ground. You will need a helper to operate the controls of the vehicle while you check the voltages with a circuit tester.

Tell your helper to turn on the ignition while you go to the connector plug. Connect the alligator clip on the circuit tester to a metal part of the vehicle.

Tell your helper to turn on the right turn signal. Place the probe of the circuit tester into the connection that has the green wire. The light on the tester should blink on and off.

Tell your helper to turn on the left turn signal. Place the probe of the circuit tester into the connection that has the yellow wire. The light on the tester should blink on and off.

Tell your helper to shut off the turn signal and step on the brake. Check the green and yellow connections again. The light on the tester should stay on for both.

Tell your helper to release the brake and turn on the lights. Place the probe of the circuit tester into the brown wire connection. The light should stay on.

Tell your helper to turn off the lights and shut off the ignition.

Tip

  • Green, yellow, white and brown are standard wire colors, but colors may differ depending on the manufacturer. If your connector has different colors, check the data sheet for your connector. You may also use the probe to identify the pins.

Items you will need

About the Author

Richard Asmus was a writer and producer of television commercials in Phoenix, Arizona, and now is retired in Peru. After founding a small telecommunications engineering corporation and visiting 37 countries, Asmus studied broadcasting at Arizona State University and earned his Master of Fine Arts at Brooklyn College in New York.

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Photo Credits

  • camping,trailer image by Greg Pickens from Fotolia.com