The A/C system is pressurized so you always have pressure even with the compressor not running. If you read 0 then you have no R134 in system. You do not need to charge to turn the compressor on. You could jumper the low pressure switch together at the rec/drier to simulate the system is charged. If your A/C works then the compressor would come on but do not run it for too long, under 1 minute, as there is no lubricant inside the compressor.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.