Troubleshooting Trailer Light, Bulb Constantly On.

Updated 03/01/2021 | Published 02/25/2021

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Question:

My trailer passenger brake/signal bulb is staying on and getting really hot. What can be causing this?

asked by: Kenneth

Expert Reply:

My first thought is to verify the wiring connections, ensure your vehicle wiring harness is correct to the pins on trailer harness. Do you have a constant hot wire connected to the pin that this bulb is in circuit to?

If there has not been any recent changes to the wiring, if this issue is new but trailer has history of functioning properly, then check for cleanliness of contact points within the circuit. I use isopropyl alcohol and a wire brush if available, but WD40 works too. My preferred method of trouble shooting is to start at one end of circuit and work towards the back. For example, start at the bulb, clean contact points and see if that works first then replace bulb with another known to function. Swap that bulb with another on the trailer. If that doesn't fix it then work back in your circuit to the tow vehicle. Check all points for proper continuity and verify no shorts. Check integrity of wiring insulation to ensure that a constant hot wire is not feeding the circuit.

If you work all the way back to the tow vehicle and the issue is not found, then double check the vehicle wiring. I would do this first by connecting to another trailer, does everything function correctly? If it does then something on the first trailer was missed.

expert reply by: Michael R