I have a 1998 Caravan and recently had the evaporator replaced. Occasionally when the blower is on the lowest setting, you can hear the HVAC damper door constantly cycling so the air goes to the dash registers and then to the floor and then back again. I am wondering if there is a vacum line that could be either loose or a hole in the line that is causing the cycling. This doesn't seem to happen on the higher blower settings nor does it happen all the time.

Any suggestions would be appreciated.

 

your system is motors not vac servos. Possible door issue or accuator issue.