Anything in from stuck pins which prevent the computer from charging to my cats deciding the adapters are made out of some rare form of addictive kitty-crack and shorting out the cord by chewing on it.
I used to buy new MagSafe adapters but that got expensive fairly quickly. Then I decided to splice out the shorted sections of cord; which did work but left me with relatively short cords for charging the computer. Next, I decided to figure out how to crack open the adapter and replace the entire cord and that's the process I'll demonstrate with this instructable.
Be careful and make sure the adapter is completely discharged. Capacitors can hold a charge for a while so it's best to leave the adapter unplugged for a bit prior to working on it.
Needle Nose Pliers
Clamp(s) or even a heavy book will do.
Replacement MagSafe cord- I found mine on eBay for like $16.
Step 1: Opening the adapter
Flip open the cord winding ears and insert the plier ends as the picture shows.
Open the pliers until you hear a satisfying pop and then do the same on the other winding ear.
This will typically separate the two halves almost completely but the case will still be held together near the wall plug and along that back seam. To get those to release you'll need to grab the two halves and pull them apart until those also pop apart.
Step 2: Ok so now it's open
Take the internals completely out and set the case components aside so we can use them later for reassembly. Then you'll want to peel the 3M tape up so we can move some of the metal shielding out of the way so that way we can access where we need to solder.
Step 3: Removing the current cord
Use desoldering braid to desolder the cord and then remove the old cord from the main board.
Step 4: Solder in new cord
Then once you complete that bend the shielding back into place and retape the other flap of shielding back down.
Step 5: Reassembly
Next replace the winding ears in their little holes in the outer casing.
Then place little dots of super glue at the indicated spots, slide on the other half of the case and clamp them together.