How do I change the door closer arm in a commercial door?

Question

The closer unit is in the top frame and the arm is mounted to the top of the door. The closer has a square peg that the arm is clamped to. The door has to be open to access the arm hardware, but I know that if I loosen the hardware, the peg will rotate to the closed position. How do I prevent this from happening? Right now the door will not close all the way which is an indication that the arm is worn out or the peg is stripped.

Answer 1

turn the set screw that controls the closeing all the way in,