I had similar symptoms on my '83 300D. The problem was the fan on the AC radiator. It was frozen. Now that the electric fan works so does the AC.

On the '83 removal of the monovalve put the heat on full. Not good except winter in Canada.

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JSR

1983 300D Turbo
1995 E320 Wagon
2001 MR2 Spyder
2001 Subura WRX Sedan