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AC stops blowing when van accelerates

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New User

Aug 25, 2016, 7:58 AM

Post #1 of 3 (1019 views)
AC stops blowing when van accelerates Sign In

I have a 2007 Ford E350 12-passenger fan with >200,000 miles. While driving, the a/c fan decreases it's velocity as the van reaches highway speeds and the rear vent blows warm air. As I stop accelerating the fan's velocity returns to normal and starts blowing cold air again. Might this be a vacuum problem?

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Aug 25, 2016, 11:11 AM

Post #2 of 3 (1008 views)
Re: AC stops blowing when van accelerates Sign In

It's probably not the fan velocity but the mode doors switching from loss of vacuum.

The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

Tom Greenleaf
Ultimate Carjunky / Moderator
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Aug 25, 2016, 11:17 AM

Post #3 of 3 (1007 views)
Re: AC stops blowing when van accelerates Sign In

At a glance seem like possibly evaporator is icing up, both blows warmer and less volume if so.

Other clue is let it and stop and shut all A/C off after looking for water dripping. When off for a minute if a flood of water starts dripping it's icing at evaporator for the one that's misbehaving.

A/C isn't very DIY friendly. I suggest you explain just what you did and have this checked out by a professional shop and tech. There are a few reasons it could do this so needs some time and pressures to figure out exactly why,


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