a/c stops blowing
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Senior User
Joined: Jun 2004
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a/c stops blowing
OK, I haven't been driving it lately but when I do I notice that when im hitting close to 70 mpg The ac stops working, if i slow down I hear a thump and it kicks back in.... Do i have a vacume loose?? Any help would be great appreiciated. !!
Junior User
Joined: Dec 2007
Posts: 61
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From: Broken Arrow, OK
Could be the reduction in manifold vacuum that occurs when the throttle is opened sufficiently to achieve 70MPH. The blend door of the HVAC system is operated by vacuum. When the hammer is down on a tired motor, they sometimes don't have enough vacuum to keep the blend door in the position that directs air towards the panel vents. If when the AC "stops blowing" you can feel it coming out of the floor and defrost vents and when you slow down the air comes back out the panel you'll know... The blend doors default to defrost/floor sans vacuum.
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Senior User
Joined: Jun 2004
Posts: 238
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Could be the reduction in manifold vacuum that occurs when the throttle is opened sufficiently to achieve 70MPH. The blend door of the HVAC system is operated by vacuum. When the hammer is down on a tired motor, they sometimes don't have enough vacuum to keep the blend door in the position that directs air towards the panel vents. If when the AC "stops blowing" you can feel it coming out of the floor and defrost vents and when you slow down the air comes back out the panel you'll know... The blend doors default to defrost/floor sans vacuum.
Logistics Pro
Joined: Mar 2005
Posts: 4,698
Likes: 47
From: Mexico, Missouri
Also, just a thought...This maybe the white vaccum line that is on the passenger side, up near the cowl and the vaccum plunger that works the blend door switching.
Pop the hood and look into the wire loom just between the A/C Compressor and the Fender well. There should be a white vac line. Look/feel/inspect for crumbling hose or other deteriating spots in it. My A/C would do the same thing, and after searching for the problem, FTE members directed to the problem.
This is also covered in the 87-96 F-150 Sub-Forum here:
https://www.ford-trucks.com/forums/8...cuum-line.html
If this the problem, it is an easy fix.
Let us know what you com eup with.
Matthew
Pop the hood and look into the wire loom just between the A/C Compressor and the Fender well. There should be a white vac line. Look/feel/inspect for crumbling hose or other deteriating spots in it. My A/C would do the same thing, and after searching for the problem, FTE members directed to the problem.
This is also covered in the 87-96 F-150 Sub-Forum here:
https://www.ford-trucks.com/forums/8...cuum-line.html
If this the problem, it is an easy fix.
Let us know what you com eup with.
Matthew
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Senior User
Joined: Aug 2004
Posts: 394
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From: Nashville
There is usually a big metal can under the hood on these which is the vacuum resevoir, my old lincoln did the same thing and it turned out the vacuum can had a hole rusted in it. if your engine isn't building enough manifold vacuum at 70 to keep the plungers in their position than thats another problem, but i'd check this can just to cover all your bases
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Logistics Pro
Joined: Jan 2002
Posts: 4,907
Likes: 39
From: suburban atlanta
Most Fords had a check valve in the system that kept vacuum high when intake manifold vacuum fell off. Examine the underhood vacuum line diagram, or trace the vacuum line from the intake to a manifold near the power brake booster. This check valve will prevent vacuum from leaking down, and allow for one or two boosted stops if the engine were to fail. It also keeps the heater from defaulting to 'defrost' when you accelerate hard.
tom
tom
Senior User
Joined: May 2009
Posts: 294
Likes: 0
From: Ponchatoula, LA
Also, just a thought...This maybe the white vaccum line that is on the passenger side, up near the cowl and the vaccum plunger that works the blend door switching.
Pop the hood and look into the wire loom just between the A/C Compressor and the Fender well. There should be a white vac line. Look/feel/inspect for crumbling hose or other deteriating spots in it. My A/C would do the same thing, and after searching for the problem, FTE members directed to the problem.
This is also covered in the 87-96 F-150 Sub-Forum here:
https://www.ford-trucks.com/forums/8...cuum-line.html
If this the problem, it is an easy fix.
Let us know what you com eup with.
Matthew
Pop the hood and look into the wire loom just between the A/C Compressor and the Fender well. There should be a white vac line. Look/feel/inspect for crumbling hose or other deteriating spots in it. My A/C would do the same thing, and after searching for the problem, FTE members directed to the problem.
This is also covered in the 87-96 F-150 Sub-Forum here:
https://www.ford-trucks.com/forums/8...cuum-line.html
If this the problem, it is an easy fix.
Let us know what you com eup with.
Matthew
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