Troubleshooting Low Output Voltage on Hopkins Trailer Taillight Converter Box

Updated 05/22/2025 | Published 08/02/2017

Question:

Hello Experts, Application: Trailer hookup from International school bus. Qty 4 ea model: 310-0041-560 Rev. c09-10 All four converters outputs are 5vDC or less to trailer. Voltage in to converter is 12vDC. What could cause the low output?

asked by: Mike M

Expert Reply:

It's possible the converter has an internal short and is damaged. Or it could be possible that you have a weak ground for the converter. Most likely though the box has shorted out. They are designed to fail as a way to prevent a short in trailer wiring from making it back up to the vehicle wiring so that is what is happening.

You'd want to track down the short on the trailer wiring and fix it before replacing the converter.

expert reply by: Jameson C

Dave

5/22/2025

I am having the exact same issue and am pulling my hair out. All of my outputs are 5 - 6V. I just got a new box and it is wired up correctly. Using a keychain tester everything works fine, but plugged into the trailer I get no lights at all. I tested the trailer using a separate 12V battery and everything works fine. The lights are new a year or two ago, and the car is new. All fuses are fine and the cars tail lights work fine, The trailer has a dedicated ground from the harness to the 2 lights.

Jameson C.

5/22/2025

@Dave If the outputs of the converter box are only 5 to 6 volts that is a red flag as they need to be closer to 12 volts for the lights to work. What vehicle do you have? Some newer vehicles are designed to put out a much lower voltage and if you aren't using a converter box that was designed for that it won't put out the correct voltage to the trailer lights.