I recently purchased a 92 400E and discovered last week that the A/C does not work. I checked the fuse in the box & the flat fuse in front. Both were ok and read 14.?? volts with a meter. The Compressor clutch does not engage and I cannot see or hear it attempt to. What should I check next? If the system has lost pressure, what is the most common place for a leak? Any recomendations?