Namnp2007,

I'm not 100% sure, but I think that AC motors don't have brushes, so my fix isn't going to work for you. There are lot's of things that it could be; do a google search for "fanuc 401" and you will get lots of results. That's where I got the idea to clean out the brushes .

Fanuc is a good source for the manuals (http://www.fanuc.co.jp/en/contact/index.htm). Plus, their tech department is usually very good about debugging problems over the phone, even if you bought the machine used. I once had a tech on the phone for over an hour; I was checking things on the machine while he was feeding me instructions.

Chris Kirchen