Article

Troubleshooting Trailer Lights

January 2013, TruckingInfo.com - Feature

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One of the most common reasons for an officer to pull a driver over is for a faulty light. Below are some basic guidelines on how to troubleshoot some of the problems that may arise with a trailer lighting system.


Total lighting failure: No part of the system appears to be powered.

Possible causes: Burnt out light bulb causing a break in the system; corrosion in harness, light plug or pigtail; ground problem; hydraulic lock from too much dielectric grease; bad spot in wiring harness; issues in the 7-way plug; issues from the tractor.

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1. Check to see if you have power at the 7-way of the trailer.

- If there is no power there, check the back of the 7-way cable from the tractor.
- If there is no power there, check the 7-way socket on the tractor.

2. Check for a good ground.

3. Check the condition of the 7-way connector.

- Check for a secure connection.
- Check to see if corrosion is present.

Single lamp failure: Problem appears to be isolated to one lamp.

Possible causes: Burnt out light bulb; corrosion in harness, light plug or pigtail; ground problem; hydraulic lock from too much dielectric grease; bad spot in wiring harness; issues in the 7-way plug; issues from the tractor.

1. Check the lamp with a lamp tester.

- If the lamp functions, try re-installing it. If the lamp is not functional, move on to step two.

2. With the power on, check the pigtail for continuity.

- If the pigtail does not function, check the jumper that connects the pigtail to the harness.
- If the jumper does not function, check the main harness.
- Keep working upstream in the system until you get a connection that functions. From there you know the problem is downstream in the system.

Lamps working intermittently and/or flickering: a single lamp or entire circuit

Possible causes: Small amount of corrosion build up; ground problem; short in wiring harness; loose pins in 7-way; loose wire in 7-way; problem in the tractor.

1. Visually check connections and wire jacketing.
2. Start testing from the front of the trailer beginning with the 7-way socket. Disconnect one piece of the harness at a time and check the circuit.


Lit lamps that should be off: This is the most difficult failure to find.

Possible Causes: Electrical short, crossed wires or a bad switch.

1. Visually inspect all connections and wire jacketing.

2. Check the harness one piece at a time starting from the rear and working your way back up to the 7-way socket.


From the December 2012 issue of the Phillips' Qwik Tech Tips newsletter. To subscribe to Phillips' Qwik Tech Tips newsletter, visit www.phillipsqwiktechtips.com.

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