Assuming we are talking about a late model van, I would bet that you
have a bad AC fan resistor.  You will find it behind the grill, near
the driver's side headlite.  Pull it out of there and test it with an
ohmmeter.  When they burn out the low speed doesn't run as it should
when the AC is on and then the temp goes up and up until the high
speed kicks on.
  I have had three late model vanagons with these burned out the same
way.  With some care and determination they can be repaired.

Larry A.

On Tue, Jul 17, 2012 at 11:17 PM, JRodgers <jrodgers113@gmail.com> wrote:
> AC was putting out the barest of cool air on full at night - could not
> feel but hot air during the heat of the day. Added R134a and now it
> works great. BUT - it seems the radiator/condenser fan up front runs as
> long as the AC is turned on - and I think that is as it should be -or is
> it? But this evening - while ambient temp was still quite warm, not far
> from home I turned the AC off when stopping for a light (idle stabilizer
> doesn't work so engine would stall otherwise) and when I turned it back
> on - I noted that radiator/condensor fan didn't work - didn't turn on.
> The AC continued to put out cold air - but now fan doesn't run - which
> incidentally up to now sounds like a jet engine running.
>
> Can anyone tell me about how this is supposed to work - and do I have
> something wrong? Or am I fretting over nothing?
>
> Thanks,
>
> John