Just to add a little bit more to what Steve posted:

If you have any button on the HVAC panel pressed other than the EC button, the a/c is always "on". It functions this way by design

On W124 cars, it is best to not set the wheel in the fully hot or fully cold position, and also to not set the fan blower speed to continuously on "high" or "low", but rather use the "Auto" button.

Drivers have the feeling that full heat or full A/C will get the passenger compartment to the correct temperature fastest.

This doesn't allow the system to work as designed.

To get the longest life from the automatic climate control system, allow the system to exercise all components.

If the temperature inside the car is more than 4 degrees away from the setting on the temperature wheel, the system will use full heat or full A/C regardless.

As the interior temperature approaches the setting at the climate control panel, the automatic functions will slowly reduce blower speed.

Allowing the system to work in its automatic mode, with all components functioning, will prolong the life of the system.

When flap controllers and valves (vacum operated) don't move for long periods, they tend to stick in position and ruin the component.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".