Character controller with "instant" acceleration?

Hi, I have what is hopefully a simple question.

How can I make a character controller have "instant" acceleration? While I'm holding the "W" key, I want my velocity to be 6... and when I'm not holding it, I want my velocity to be 0. Never anywhere inbetween.

I tried setting Max Ground Acceleration to 10000, but it seems to have no effect. Any suggestions?

Thanks!

EDIT: Sorry, I neglected to mention that I'm using the "First Person Controller" script that is included in the Unity standard assets.

Can you post a script? And have you checked your input options? http://unity3d.com/support/documentation/Manual/Input.html