It is an electrical problem if the compressor does not come on. It has nothing to do with the expansion valve. This is what I would do.

1) short the low pressure switch connector at the recv/drier. Call for A/C at the CCU, does it come on?
2) Run it for a short while as there is no freon in the system. This will confirm the CCU is working.
3) Confirm system is evacuated by pressing the low pressure schrader valve momentarily.
4) Freon/R134a should go in if system is in vacuum. If it does not then some valve in the manifold gauge may have issue.

Good luck.

__________________
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.