A/C blows through defroster when accelerating

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Sep 29, 2020 | 06:50 PM

Erik Sumen

Thread Starter

New User

Joined: Sep 2020

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A/C blows through defroster when accelerating

Hi Guys I have a 1990 Ford F350 Crew Cab 4x4. Ive tried everything I can think of without any luck.
When my truck is at idle or cruising the freeway at low RPM`s the a/c blows fine through the dash vents. As soon as I step on the gas to go faster or pass someone the a/c switches to blowing through the defroster. If I let off the gas is starts blowing through dash again.
Help!!!!!!!!!!!!!!!!!

Sep 29, 2020 | 07:04 PM

94ZR580

Tuned

Joined: Dec 2005

Posts: 400

Likes: 57

From: Saskatchewan

I'd think it has to be the dampers in the air box. Under heavy acceleration the g-forces cause the damper to move and direct the air to the defrost vents. Maybe you have to detune the engine to keep the A/C blowing out the dash vents.

Sep 29, 2020 | 07:15 PM

R&RFord

Fleet Mechanic

Joined: Aug 2015

Posts: 1,835

Likes: 364

From: Candy Mountain

You have a vacuum leak/s. Visually inspect and repair your under hood vacuum lines as needed. If you still have the factory lines in place, it would be a good idea to just replace them all with high quality vacuum lines. I am partial to Gates but use whatever floats your boat.

Sep 29, 2020 | 07:57 PM

kevinv10

More Turbo

Joined: May 2014

Posts: 587

Likes: 73

Quote:

Originally Posted by R&RFord

^^ what he said. As vacuum drops, fords default to defrost to keep the windshield clear for visibility.

Sep 29, 2020 | 08:02 PM

rla2005

Fleet Owner

Joined: Oct 2005

Posts: 20,789

Likes: 1,753

From: Kentucky

The most common cause of your issue, a quick search would answer your question, is the white vaccum line to the diverter valve on the blower box is leaking and/or broken.