Wells braces for toughest merger yet: Its computers.

The success or failure of Wells Fargo and Norwest's 23-month union may hinge on an unreadable ATM card.

That's a seemingly tiny matter in the grand scheme of the banking industry and the nation's sixth-largest bank. But it's the sort of detail that Wells Fargo & Co. is focusing on this month as it comes to the critical point in blending the two companies' computer systems.

If Wells is successful in rolling out the new system with a minimum of problems or inconvenience to customers, starting Oct. 20, it will go a long way to setting a new standard for integration. That's a task that has frustrated and complicated many bank mergers.

It also will rid Wells of a ghost that has haunted it for four years, since computer integration and other …