How To Prevent Backfeed On Tail Light Wiring When Using Hopkins 7 Way Round Pin To 7 Way Flat Pin  

Question:

Expert Reply:

Hey Ron, what made you decide to install that converter?

The reason I ask is that a converter like the Hopkins 48845 is normally only needed when the tow vehicle has separate brake and turn signal wires, and the trailer uses combined brake/turn circuits. The converter takes those separate brake and turn signals and combines them so the trailer lights work correctly.

In this case, both the 7-way round pin and 7-way flat blade connectors should already be using combined stop/turn circuits - left brake/turn, right brake/turn, running lights, ground, etc. So that converter really should not be needed here.

I’d recommend using a standard 7-way round pin to 7-way flat blade adapter like part # PK12-724. As long as there aren’t any other wiring issues, that should get everything connected the way it needs to be.

Looking at your setup, it appears you may have had something similar installed originally, then it was cut and the converter was wired in. Were you having lighting issues before the converter was added? There could be another electrical issue going on, such as a bent pin in one of the connectors or a wire grounding out somewhere.

If I’m mistaken and that converter was needed for a specific reason on this truck, then you can add diodes to help prevent backfeed from traveling back into the light circuits. The Roadmaster Single Park Light Diode # RM-690 would be a good option for that.

expert reply by: Andrew N

0

Ron P.

6/9/2026

Hi Andrew. Thank you for your reply. The tractor does have separate brake and turn lights and the trailer has combined. Just plugging in the adaptor portion of the harness causes back feed to the turn indicators on the dash when you turn them on. ( The trailer is not connected at this point, just the 18” adaptor harness) You can attach the trailer and it works as it should with continued back-feed. Two adaptor harnesses cause the same effect. Is this normal? Do I have two defective modules? Is adding diodes a bandaid? If adding diodes is the solution, do they go in the green & yellow wires? If so, I assume they go on the input side of the module correct?

Andrew N.

6/15/2026

@RonP Hey Ron, thanks for the additional info. And sorry for the delay in getting back to you. So you will indeed use the converter in your situation, but I doubt you have two bad converters. It sounds like you just need some diodes to prevent backfeed. I'd add the diodes on the truck side of the harness so that way it stops voltage from feeding back into the truck turn signal wires (green and yellow as you said). Install them so the diode is allowing power to flow only from the truck to the output wires going to the trailer, so the arrow on the diodes will point away from the truck wiring if that makes sense.