The car is an '83 300CD.

AC is not cold when:
1) Ambient temp. above 80 deg. F and parked at idle.
2) Stuck in a bumper to bumper traffic.

But blows cold air when:
1) Ambient temp. around low and mid 70's, even when parked and idling or stuck in traffic.
2) Ambient temps. above 80 deg. F, as long as driving at speeds above 30 mph.
3) Whenever driving in the rain or at night.

AC system condition:
1) Compressor is cycling and functioning.
2) Compressor belt tension is OK.
3) Auxillary fan is cycling and functioning.
4) Assuming that it's still charged with R-12, because there are no signs of conversions, such as 134A retrofit stickers and fittings.

What could be wrong?
1) Low refrigerant charge?
2) Bad expansion valve?
3) Bad receiver dryer/filter?
4) Bad compressor?

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'85 300D Turbo - CA Version