Sandy,
Thanks for your reply.
Is the 405 system different from the 505? I'm asking this because
on my '86 505 turbo the fan kicks in without shutting off the
AC compressor; furthermore, the air the AC blows is much colder
when the fan is on.
Anyway, here's more: I just did the refrigerant flow and humidity
checks, as specified in the Haynes manual pertaining to these 405's,
and it looks like the freon has higher than normal humidity (continuous
flow of milky air bubbles seen through the glass). Also, I didn't mention
that outside temperature might play a role here: AC works fine
while driving at night, even at highway speeds.
Thanks again for your advice,
Radu