My 1981 240D a/c does not work. I have no clue on how to troubleshoot this problem. The person I bought it from said something about getting it charged every so often. I noticed that the compressor does not engage when I turn the dial to max. Is this because of the low charge and is a safety function so the compressor will not burn out? Looking for good input as the DC heat is about to be upon us.

TK