I think the compressor will engage even without gas. My last LS400 air con stopped working but the compressor still engaged, and it was holding gas OK. I bought a compressor from a breakers for £75 had it fitted and all was fine. I've just replaced the compressor on my 430, again from a breakers, this time only £50 + £10 postage, had it fitted and gassed and all was fine for a short time but then it stopped. Now it seems OK again, been blowing cold for a week, no idea why the sort term problem. If you do get one from a breakers, try to get one that's still on the car, if it's been laying around for ages it could dry the seals.