I believe your car is equipped with an A/C idle adjustment sensor...does what it says, adjusts the idle when it senses voltage at the compressor. Don't know much detail after that, location, cost, incidence of failure, etc.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car