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 2500
Average mileage: 162,785 (84,000–275,000)
15 people reported this problem
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When accelerating with the vents on it switches to defrost until I quit accelerating.
switches to defrost under acceleration
Air flow switches to defrost when accelerating or pulling up hill. Have not found a fix as of now