I too have had my car out of action whilst I sort out front end woes. Thankfully, nothing as bad as the engine - just niggles and the every worrying corrosion problems
My air con system should be the same as yours, and is not currently working - I am hoping to have it back this weekend as I believe it is now ready to pressure test, then fill.
The most common problem with these cars is that one of the air con rubber hoses connecting to the top of the compressor runs very close to the exhaust manifold. This obviously has the unwanted effect that placing any rubber in proximity to 300°C+ will have i.e. it weakens it.
Check for a split down these hoses - easy to remove this pipe with a large Allen key set (8mm required) and then some large spanners for the connection to the filter/drier. If it is the hose (Alfa 145 part60656137 £95.75 + V.A.T.) then you have no choice but to order from Alfa. I have purchased some aluminium heat reflecting insulating fabric to prevent this happening to the new hose - see picture below
It could be the compressor, but try the following:
- take the auxiliary belt off, and the hose mentioned above
- stick some tape e.g. black tape or masking over the two hose holes now exposed
- rotate the compressor by hand in the direction of aux belt travel
this will tell you two things
1. If the tapes shows blowing from one hole and suction from another, you know the compressor is pumping
2. when you rotate the compressor, if you feel any grinding at all, then it is possible that the vanes in the compressor have broken and there is debris in the compressor (new one needed)
I don't suspect there to be a problem
It could also be:
- the condenser i.e. the radiator in front of the engine radiator
- the evaporator i.e. the mini radiator inside the heater matrix behind the dash
- one of the other hoses (although these must be much less likely to fail than the one already mentioned)
- any of the dozen or so seals throughout the system
Any of the 4 above are unlikely to be detected by DIY. Better if you get to this stage to get the system leak tested with a UV dye at a specialist. This way, you know where all the leaks may now be.
If you replace the hose, I have some of the heat shielding left if you would like to buy it - exactly the right amount for the one hose closest to the exhaust manifold. I also have some hose clips than can be used to keep it really nice and tidy.
I also have about 5 different types of nitrile o-rings. I had to buy packs of 10 when I replaced the hose. You need some PAG oil as well to smear the o-rings with to ensure a good seal. Should be a few quid (under £5) from any air con supply/specialist.
Whilst you are at it you should replace your receiver/drier. As the system has been open, water vapour has got into your system. Once up and running again, this will move to the condenser, and freeze, causing the condenser to burst. A new drier will remove the water vapour before the condenser. An old drier will be saturated, and unable to remove any water vapour. The cheapest way to get these is to buy a Nissens OEM replacement. I have bought one for mine, but it is slightly the wrong size. If you go to nissens.com and log into the parts catalogue, you can search by Nissens part number. Either 95013 (which I bought) which is listed as the correct part, or 95192 which is listed as fitting an Alfa 155, but looks to me like the one I should have actually bought. I have hacksawed a bit of the mounting bracket to get the 95013 to fit, but 95192 would be better. If by some chance you are after the 95013 part, I have not used it yet, and would let you have it at a good price, and then fit the correct part myself.
Hope all this helps.