Blower motor speed S500 1997
 

It seems to me that A/C blower motor works only for a half capacity. How can check it max speed and what control it.

Usually your problem reflects on plugged filters. Change all your A/C filters first, then see if the air volume improves.

While you are taking time to look at that filter you will see the blower motor & mounted to it is the electronic blower regulator..that is the most likely problem..

That filter should be replaced every 45K miles on that car..under hood, under the air intake plenum