99 3500 van..A/C doesnt come on for 10 minutes sometimes after turning it on.....and it switches from vent to floor...back to vent..to defrost..while your driving????

 

Something sure is wacky there. Have you tried the key on/off and leave on after three times? This will show any trouble codes in the odometer display. If it's electrical, a code will show up there for the AC.

Your HVAC AC system is the same in my 2001 B2500. Do you have the rear AC/Heater system as well?

Hayes makes a good repair manual for these vans dealing with the HVAC system. The book number is 30065. Use the trouble shooting section to find the root cause.

My suggestion is look for a bad vacuum line leak from the control unit on the dash to the HVAC system and trace each hose. Then I would look at the AC system electrical relays and under hood fuses.