This is likely a vacuum supply problem to the mode control switch. A vacuum line from the engine’s intake manifold goes to a vacuum reservoir ball under the hood; from there a vacuum line leaves this reservoir, passes through the firewall and connects to the mode control switch.
The mode switch directs vacuum to actuators that position the doors to the desired vents. When there is no vacuum the doors are pulled by springs to a rest position; defaulting back to defrost.
The vacuum reservoir is used to store vacuum when the engine is under load and not producing large amounts of vacuum. The inlet vacuum line may have a check valve; stopping vacuum from going back to the engine. Make sure that this valve (if there) will only allow vacuum to pass through it in one direction.
The rest of this repair is like search and rescue; look for any cracks in the hard plastic lines, which may allow vacuum to escape; also checking that the reservoir ball itself isn’t leaking.
A question that I forgot to ask; does the van run ok, producing its normal amount of power? If the engine isn’t producing vacuum there won’t be any for the heater system.
A vacuum gauge is handy to have for this but not necessary.
Dan.
Canadian "EH"
MAIN
SEARCH
WHO'S 
LOG 