Auto Air Conditioning & Heater Repair: Plymouth Voyager Airconditioning Problem, plymouth voyager, twin blower

Question
Hi David,
Greetings from Canada and trust you and yours are doing well and keeping safe.
Need help with an airconditioning problem and hope you could pl help.
I have a 1991 3.3 Litre Plymouth Voyager LE minivan. The stock airconditioning (R-12 system) works great with excellent cooling. Its not a twin blower system but only blows from the dashbord. There are buttons to activate air flow directions, eg. to face level, to foot level, to both foot and face area, and to windscreen for defrosting.
The problem I have is that with air flow switched to face level, which is the most often used position, it works okay for a time and then it shuts down and air flow is diverted to the feet. I hear a soft but solid thud like a vent closing when that happens. When I switch the blower off I hear another similar thud. Then when I switch the blower on again I get the air flow back to face level again but only for a short while before it goes back to the feet area like before. And the cycle repeats. I notice this diverting happens most with heavy accelleration like when going uphill.
Pl let me know what could be the problem and whether it would be an expensive job to fix.
Thanks very much and take care.
Warm rgds.
Gibson

Answer
HELLO MR.G.,THE PROBLEM YOU HAVE IS VERY COMMON WITH VACCUM OPERATED CONTROLS.SOUNDS LIKE A LOST OF VACCUM WHICH IS SUPPLIED FROM THE ENGINE INTAKE.AT THE CENTER OF THE FIREWALL FROM THE ENGINE COMPARTMENT, IS THE MAIN LINE THAT FEEDS VACCUM TO THE DASH. CHECK THAT POINT AND SEE IF THE PLASTIC TUBING IS BROKEN. IF YOU HAVE VACCUM THERE,FOLLOW IT TOWARDS THE SELECTOR CONTROL.SOMEWHERE ON THE LINE IS A BREAK.

GOOD LUCK,DAVIDT