Assuming that your SLK is the original model 107.447 with the 111.973 engine...the supercharger engages upon cold start...acting as an air pump for up to 2 min.....then the engagement is dependent upon throttle position and engine RPM...it is engaged above 2200 RPM regardless of throttle position...and remains engaged also during deceleration to prevent boost lag. The latest version engine, the 111.983 has the supercharger running all the time...with no electric clutch.