Would suggest that if you are operating a business as a bike mechanic, and you can't diagnose and fix simple things like gears, it isn't the business for you.

You need to provide more info about what you are working on, what Shimano model number, have you looked it up on the Tech Docs, or on the Park Tools how to's.

If you want to be a mechanic, go do a course, so you know what you are doing, http://www.bbinstitute.com/index.php otherwise you are a liability to your customers and yourself.