Quote Originally Posted by Cate&Rob View Post
Gene - It would make sense that the truck would be checking the trailer brake apply circuit . . . but, Jim was able to turn the warning message on/off by changing light bulbs from incandescent to LED. These would be different circuits than the brake apply.

Rob
Rob,
Yes, they're on different circuits, only thing is the common -12V connection between them that might couple noise, like common-mode coupling?

From Ford's owners manuals, they say only checking the brake control line.

2016 F-150:
http://www.fordservicecontent.com/Fo...G1684894&div=f
Explanation of Information Display Warning Messages
The F-350 (2018) pdf owners manual says the same thing.

GM's bulletin says it's monitoring trailer control circuit 47, so saying the same as Ford.
https://gm.oemdtc.com/TSB/SB-10081241-2280.pdf

edit: Of course they may be checking for trailer light burned out bulbs, but not putting it in writing.

I did a double take when reading thru the GM Service Bulletin, reminded me of Jim's junction box picture.

Gene