Outside temp sensor can cause A/C to not come on for some BMW's; I can't recall for sure if it affects E46 but is your outside temp sensor working okay?
Also, check your coolant level. Another thing I'm not sure of but on some cars the A/C is disallowed unless the coolant is at an okay level.
That negative pressure is odd. Don't forget that it might be a wiring issue.
Last edited by fotsir; 10-22-2011 at 09:08 PM.