What causes the abs light come on on a 2002 mountaineer?

First off, ABS means anti-locking brake system. And if there is something wrong with your brakes, then you better get to a repair shop or dealer. In most this case, it could almost be anything since your vehicle is over eight years old and the warranty is over. Best bet is that maybe the brakes are just really dirty. Unless you've done some serious driving with your Mountaineer, then it might be something a little more expensive.