The servo arm rotates across a set of contacts, controlled by temperature and blower setting. When you shut the ignition off, the servo is programmed to move the arm to "park" where the power to the hvac system is completely off. If something is wrong and it can't get the arm to park, it will keep trying and drain the battery.

It is extremely unlikely that you will be able to fix this. What has happened is that coolant has leaked into the servo mechanism and corroded it. When they do this, they're gone. Go with a rebuild or the digital gizmo.

You should replace any coolant that you lost.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe