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 Van 3500
Average mileage: 168,400 (133,000–210,000)
15 people reported this problem
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Same symptom on 2001 ram van 2500.
1995 dodge ram 1500 5.9 vent changes to defrost during accelleration