Troubleshooting trailer lights
DOT regulations state that any and all lights on a tractor must be in working order, including lights on a connected trailer.
Below are tips from Phillips Industries on how to get lights working when they malfunction.
Total Lighting Failure: No part of the system appears to be powered.
Possible Causes: Burned 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.
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.
Check for a good ground.
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: Burned 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.
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.
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.
Visually check connections and wire jacketing.
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; bad switch.
Visually inspect all connections and wire jacketing.
Check the harness one piece at a time starting from the rear and working your way back up to the 7-way socket.