The AC/heater (HVAC) system may unexpectedly switch to the defrost mode when accelerating. This system is operated by engine vacuum and should be inspected for any vacuum leaks if this problem develops. There is also a revised vacuum check valve available to address this concern.
HVAC Air Flow Switching to Defrost When Accelerating on Dodge Ram Wagon 3500
Average mileage: 163,000 (120,000–206,000)
10 people reported this problem
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Based on how fast the van is going the air from the vents switches to defrost and heat vents.