Is the A/C vent door vacuum controlled?

[SilverHammer]

I have noticed that when I am driving with the A/C on, the air will redirect out the defrost vents when the truck is accelerating or under load. Is this normal?

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[SilverHammer]

BTW, It's a 1998 Ram 1500 Laramie SLT 4x4 Quad Cab Short Bed Pick up

1998 Ram 1500 Laramie SLT 4x4 Quad Cab Short Bed Pickup

 

[Fast69Mopar]

What you are experiencing is a vacuum loss to the HVAC system. If you open the hood and from the passenger side of the truck, look against the firewall near the evaporator you will see a small black hard vacuum line. In the line will be a 1-way check valve that is installed with short rubber hose to connect it to the hard vacuum line.

This check valve holds vacuum one way and allows vacuum to pass the other. This check valve is worn out an allowing the vacuum in the system to bleed down when engine vacuum drops while under heavy acceleration.

I have a pic of what it will possibly look like and the part number I believe is
PN# 4549209

 

[Mop]

Sorry, dupe post.

 

[Mop]

I had this problem with my '99 Sport, but it wasn't the check valve. It was a crack in the hard plastic line leading to it. I bought 3mm diameter rubber hose, attached it to the existing hard line, and it's fine.
Check for cracks on that hard line around the check valve, just in case.