I had an issue where my EC button didn't light up, and the compressor was running all of the time. Turned out to be a dirty switch, anfter a few minuted of firmly pushing it over, and over, and over, it flashed then lit up. At idle you should hear the engine stutter as you turn the EC on and off, as the clutch disengages.

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2000 C230 Kompressor "Frosty"

2007 PT Cruiser
2001 VW TDI Beetle
1999 Landrover Discovery II
1971 Corvette

Gone: 97 Cabrio, 83 240D, 93 VW Eurovan, 95 Volvo 850, 93 Chevy Astro Van, 77 VW Bus, 99 VW Jetta, 74 VW Bus