I can't believe I missed the most obvious option, Mike is right, first check the servo. My 79 went south as well. The servo fails in a normally open position which equals full heat.

This AC system is a pain, I don't miss it one bit. Like I said, if you do a search, you'll find plenty of info on where to test with a multi meter to see if various things like the temp sensors are reading right. It doesn't take long at all to check this. If they are working, then the servo or amp are likely bad.

Frank.

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'82 300SD