Originally Posted by Richard Wooldridge

Hi there,
You have a vacuum leak in the heater controls or air flap diaphragms. on the 76 - 80 ACII system there is a vacuum switch that controls the blower that requires a certain amount of vacuum to operate. The idea was that the heater wouldn't start until the engine water temp rose to a certain point so it wouldn't blow cold air in the winter. The defrost position bypasses the vacuum switch to allow defrost immediately. I am on a business trip, so don't have my factory manual at hand, but what you need to do is start checking the air flap diaphragms for a leak. The ACII system will not tolerate any leaks. You can get one of the CD manuals, look in the ACII system and find some troubleshooting procedures for the system. I have a '79 that had a similar problem, so I speak from experience - you will need a Mity-vac or equivalent to locate the leak, it only takes a very small leak to kill the system! You can also do a search on "ACII" for many threads on this troublesome system, originally designed by Chrysler Corp for their 1970 Imperials.