Hello. . . .
I’ve just changed the shell on my left JoyCon and it doesn’t seem to be recognised via Handheld mode.
There is no snapp annimation or sound. However it works fine wirelessly and charges fine.
I’ve replaced the rail on the Joycon as well as both ribbon cables. But hasn’t made a difference.
Which ribbon cable does what? and does anyone have an idea on how to fix this?
Thank you and regards
the top ribbon cable is for the SR and SL buttons and the Sync status. When you disconnect the joycon the green LED’s will flash and then go solid when connected to the switch.
The bottom ribbon cable is for power and handshakes along with button presses.
The joycon rails and the console rails could be dirty/corroded. You are getting the charge symbol when plugged in so that means at least some of the pins are connected.
If the ribbon cable for power/handshakes is damaged you will sometimes get this fault.
the way it works for reference is the joycon advertises it is available via the pins when connected and the switch plays the sound and animation however for the first time connecting it will show as grey instead of the actual colour of the joycon. After a second the console will show the joycon connected and its colour as the eprom the the joycon shares that information along with serial number/mac
if it was working before you swapped shells then chances are its a bad connection on the bottom ribbon cable or the ribbon cable is damaged. If these are good then check the pins on the console and the joycon for dirty/corrosion. An easy test is to use a different joycon on the console to see if it is seen.
Thanks for the reply.
Tried everything. My other left joycon works fine so there definetly nothing wrong with the rail on the console side.
I’ve replaced both ribbons and now I’m not even able to connect the joycon wirelessly anymore. Giving up on this I’m afraid. I’m guessing the problem is on the board where the ribbon connect into.
The videos on sheel replacment look real easy but i guess one wrong move and joycons become a brick. Gutted
Thank you for your help anyway I appreciate the reply. 
I fixed the problem.
Had to buy a new board off eBay for the JoyCon. The connector must have been too damaged for the ribbons to conect to. Anyway all working now.
Lesson learned. Be very careful taking apart the JoyCons. Those ribbons are very delicate! 