Something is wrong with the pushbutton unit, or the switchover valves are screwed up. With no vac, the system defaults to defrost, when you have vac the airflow is controlled by the ACC. If it switches when you start, the vac servos are working. Either the electo-vac valves are bad or the PBCU is shot, or the electronics (behind the glove box) are bad.

You've exhausted my capabilities, I'm afraid.

Note that the solder joints can LOOK OK -- the usuall offenders are the ones where the switches are soldered to the board, so the switches don't make good contact.

With the unit off, the fan shouldn't run at all.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!