Smart Cover not working.
Hi, I know this is an old thread but for people that end up having this same issue here's how it worked out for me. I too had accidentally tore my power flex cable & had to replace it with the iPad turned on. I got the same clicking noise as though the unit was turning on & off when connecting the silver connector on the end of the new flex cable to the power control (PCB) board. After this my sleep function did not work any longer with the Smart Cover. I replaced the ribbon again as my first action, thinking I may have shorted it out & when it still did not fix the issue I even replaced the PCB board with a new one & replaced the power flex cable 2 more times. I think the cables were poor quality aftermarket cables because this is how I ended up solving the problem.
I bought a more expensive ($10) OEM flex cable on
eBay
Before doing the replacement of the cable I turned off the Smart Cover function in settings->general->turn switch to off next to Smart Cover function. (I don't know if turning off this function was necessary but I'm just saying what I did).
Turned off the iPad & replaced the flex cable with the higher quality one.
When reassembled & turned back on....also turning back on the Smart Cover function....it worked perfectly!
So it is possible that I received 4 bad flex cables in a row or that turning off the Smart Cover function was the key to this repair. Either way this worked for me & I hope this helps someone else having this same issue.