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.

i would pull vacuum on it first to see if there's a leak

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

Yeah don't worry about getting the compressor to run because it won't produce any cooling with zero pressure.

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ok i put two cans(12oz) of artic freeze in and the compressor ran for the first time low side pressure was 25lbs and high side was 190 lbs and my aux fan does not come on, can i jump it with 12volts form the battery to see of it works or what? i did notice the vent air was cool, not cold, but not hot outside air
kenny