Jeep Repair: 1994 Jeep GC Air Conditioning, jeep grand cherokee, vacuum line

Question
I have a 1994 Jeep Grand Cherokee with 150,000 mile on it that I bought new.  It has been a good vehicle and my son is now driving it.  I had an air condition compressor clutch installed today.  When I picked it up, the shop said that during the test drive when he accelerated the A/C fan only blew air out of the defroster vents only and shut down the dash vents.  I told him that has been doing that for a long time and maybe even since it was new.  It even does it while climbing a hill.  He thought it might be a vacuum check valve that was bad but did not have time to check it.  Could that be it or is there something else I should check too?  Where is this check valve?  Thank You, Dennis

Answer
I would have to agree with the mechanic that fixed the other part of the a/c.

Basically, the vent selection (mode door) is held in place and moved by engine vacuum.  Vacuum is only present at idle, cruise and some at low throttle positions.  Heavy acceleration allows more air in to the engine thus taking away the vacuum source.  Without a check or with a check valve the door is able to move where ever it wants.  With a check valve in place the vacuum in the lines can not bleed off when you accelerate.  

The check valve is located in a vacuum line leading from the intake manifold to the vacuum line that goes through the firewall.  This line goes through the firewall in one of two places.  Either where the heater hoses go through or where the a/c lines go through (I can't remember which right now).  Once you find the vacuum line at the fire wall start tracing it back towards the engine.  There are at least two of these (the other is for cruise control I beleive) so replacement of both may help but if you only replace one make sure it's the right one.
Doug