A/C vents stop working under engine load
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New User
Joined: Jul 2003
Posts: 12
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From: Charlotte, NC
A/C vents stop working under engine load
I have a 99 ford explorer 4dr. 4wd, automatic tranx. When the A/C is turned on and the dashboard vents are blowing. Whne you step on the gas the dash vents shut off and the A/c air comes out of the defrost vents. It sounds like a vacuum problem. But I need all your experience in what to check ,and possbily fix. Also the blower motor does not blow at maximum level when you put it on the high setting.
thank you
thank you
Postmaster
Joined: Dec 2001
Posts: 3,742
Likes: 2
From: UT
A/C vents stop working under engine load
I do believe that the vents are controlled using engine vacuum. Assuming that's right, there's supposed to be a check valve in the vacuum tubing between the intake manifold and the vent controls. What can happen is this check valve gets stuck open. When it does and you stomp on the gas, engine vacuum goes to zero and the vent controls revert to the "default" defrost setting. I'm not familiar with the layout on the '99, but on my '92, there's a check valve and a vacuum reservoir for these things sitting on top of the A/C evaporator housing.
Elder User
Joined: Sep 2000
Posts: 580
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From: Kelseyville, CA
A/C vents stop working under engine load
In addition to what MrShorty said (which is also correct for your '99), there are several hoses and fittings behind the A/C panel inside the dash. I've seen a hose pop off inside their that will cause this problem. Finally, your vacuum reservoir is under the hood, kind of below your air intake box. Look for a black plastic bottle looking thing with hoses coming from it. Check the conditions of these. This is another very likely source for A/C vacuum leaks. If both of these areas check out OK, then you'll have to look at the entire system to find out where it is leaking. It is most likely a vacuum leak somewhere.
