I have just picked up an 87 420 SEL and I am having a similar problem with the AC also I don't know the history of the AC but it does not come on, is there a safety catch that will not allow the compressor to kick in when there is no freon in the system I jumped the compressor and it does turn on could it be that there is no freon in the system and it will not turn in order to protect it self? It is still using freon I am assuming. Could I convert the system over to R134A? Is it something I can do on my own? any help would be appreciated.