I recently experienced random M + S + Flashing D lights when getting up to around 55 mph. This led me to think my XYZ switch had some water in it and one of the contacts was corroded. I removed the switch, cleaned the contacts, and put it back in. Huzzah! The lights were gone! Until I started driving home. I got about 5 minutes from the shop I was working on the truck at, and the following scenario happened: https://photos.app.goo.gl/uYs6ER3D5xqEDzqGA. Flashing M + S + Flashing D, SES light, TC light, hill decent control (WTF?), and the self leveling system (that one is always on). When I stopped the car and turned it off, I turned it back on to find that the trouble lights were no longer flashing. Any ideas what might have caused this? I need to check and see if the adjustment bolt on the XYZ switch worked itself loose, but other than that I can't think of anything. This is possibly unrelated, but I had my alternator checked at O Reilly Auto Parts a couple of days ago and they said it was starting to go bad and I should replace it. I'm a bit stumped!

 

what’s your voltage ??
do you have real time monitoring?

there is a drain hose above the xyz switch. I would extend that somewhere else as it drips on top of xyz switch

 

The RAVE (factory workshop manual) lists 18 transmission faults that can trigger the flashing M&S lights. Only one has anything to do with the XYZ switch, and according to the RAVE that fault also triggers limp home mode. Unless you get the codes read, tearing into the XYZ switch is a shot in the dark. And, it's not likely the cause if the gear indicator in the instrument panel and the LEDs on the shifter bezel display correctly.

Low voltage is the most common cause of the flashing M&S lights, and that's not even among the 18 faults listed in the RAVE. I recommend you check and double-check your battery connections and grounds.