The W124 ACC's temperature selection thumbwheel is bypassed in the "Defrost" mode. When using the defroster, forget it exists; if functioning properly, the system recognizes no input from the thumbwheel nor the fan speed buttons in "Defrost" mode, as it is hardwired to simultaneously activate the A/C compressor, the heating system, blower motor and vacuum flap elements at certain settings. No user imput is recognized. If the defroster works only with the thumbwheel in "Max" heat position, you have a concurrent second problem, in addition to the A/C, probably the ACC pushbutton control unit.

In the defrost mode, power is sent to the A/C compressor clutch, so as to bring the compressor on-line, and the blower motor is sent to maximum speed; at this point the A/C system's primary job is not to cool the vehicle's interior nor maintain a pre-selected interior temperature but to provide cold air in the vicinity of the windshield so as to assist in dehumidification.

Where to start? Visually check the compressor's clutch when "Defrost" is selected; you should hear a subtle click and see the clutch start turning. If the clutch is not operating you either have a low freon situation or an electrical problem. If the clutch is functioning and the compressor operating, inspect the receiver/drier's sight glass; a continuous stream of bubbles is indicative of a low freon charge. The balance of the diagnostics rapidly diverges after this, depending on these initial findings.

M-B's W124 ACC service manual details pushbutton control unit and temperature thumbwheel diagnostics; rebuilt pushbutton control units are available from companies such as CCI/Uptime for a fraction of the price of a new unit.