How can i diagnose a gear box system of a car and an automatic operate gear box? what procedures can i use during the dismantling a gear box?

How can i diagnose a gear box system of a car and an automatic operate gear box?

what procedures can i use during the dismantling a gear box? 

This question i have asked because i tried to disassemble the parts of the gear box of my car since i myself i specialize in that field, it failed me during the reassembling because of the wrong procedures in used at the disassembling. Thank you

Requesting @Nitin Pathak for response. 

Clutch is used in manual transmission to change gears and in automatic gear box gears changes automatically depending on speed 

You need to disconnect drive shaft prior removing gear box,it’s easy to overhaul manual then automatic 

fine thank you. I really appreciate all your positive responses