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ac stops blowing at acceleration.

gladesrunner on Wed October 21, 2009 2:46 PM User is offlineView users profile

Year: 1995
Make: Ford
Model: F-150
Engine Size: 4.9
Country of Origin: United States

Hi. My ac blows cold at lower speeds on the road. When I get to around 60mph and want to go to 65 or 70mph it will just stop blowing. It seem I get some air out of the defrost vents and I already replaced blower. Also removed shims from compressor because the gap was too much, which seemed to help but it has consistently stopped blowing air through ac vents and noticed it only happens at higher speeds. What can I do? Is it vacuum or a sensor. I am not sure where to start. Any advice would be greatly appreciated. It is a 4.9 inline 6 automatic and runs ok but does kind of hesitate on acceleration. Thanks John

1995 F-150 XL 4.9 inline 6, 157k

JACK ADAMS on Wed October 21, 2009 7:02 PM User is offline

I would start by checking all of your vacuum lines under the hood. Look close by the battery; I think Ford has a one-way valve the spits as a 3way as well.

Please post back and let us know what you found. Hope this helps and good luck..

Cussboy on Wed October 21, 2009 10:51 PM User is offline

Sounds like your issue is that the volume of air is substantially less under your stated conditions, likely it's exiting at the windshield or under the dash instead. Some vehicles use vacuum as Jack stated to control the flap operation, some use electrical motors, some use cables. Investigate those.

Probedude on Thu October 22, 2009 11:40 PM User is offline

Are your heater/ a/c door vacuum operated? If so, does your truck have a check valve and vacuum reservoir to run the doors?
My 1993 Ranger doesn't have any of these things and when I accelerate, the air doors change state until I let off on the gas and the manifold vacuum goes up again.
You may not have an a/c problem at all.

abossram on Fri October 23, 2009 5:11 PM User is offline

Yeah-the default position of the vacuum assisted doors is to the defrost position. Some Fords of this era have an access panel on top of passenger side dash-my 92 e350 does. After a year of dealing with this, I found that the inboard vacuum operated control pot won't hold vacuum and keep the flow thru the dash-not defrost. Annnnd------part is NLA from Ford. So I'm gonna wire it open and live with it. Good luck.

Edited: Fri October 23, 2009 at 5:20 PM by abossram

bohica2xo on Fri October 23, 2009 5:36 PM User is offline

Your 4.9 likely has vacuum leaks all over the place if it hesitates.

There should be at leat two 5/32 vac lines right at the throttle body - those crack on the underside all the time.

Like Jack said, down near the battery there is a fuel vapor line that does a "Y" merge & a size change - these crack from fuel vapor regularly. This line leads to a solenoid valve that dangles near the intake manifold, check the vac lines there as well.

Another sneaky leak on those trucks is the rubber connectors that mate the nulon tubes to the various nipples. The most common one is the actuator near the passenger side hood hinge on the airbox. When that coupler cracks, my F150 will switch from dash to defrost on a long grade...

Go leak hunting. Carefully inspect each hose, all the way around at every connection. A small mirror & a flashlight can be a big help.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

JACK ADAMS on Fri October 23, 2009 5:48 PM User is offline

Focus on the white in color line that comes out of a harness on top of the evaporator case next to it will be a black vacuum line as well. But check the white one; it will connect to a fresh air recirculation door motor on top of the case. This is a common problem with those older Fords. I thought you had the newer body style F-150.

gladesrunner on Thu October 29, 2009 7:26 PM User is offlineView users profile

Thanks for your help everyone who posted. I am going to check the vacuum lines for leaks of course. I have a friend with the same model truck and the same year and his just started doing this also. The information posted by you guys gives me a much more definite idea at what I need to fix. Thanks John

1995 F-150 XL 4.9 inline 6, 157k

gladesrunner on Thu October 29, 2009 7:27 PM User is offlineView users profile

1995 F-150 XL 4.9 inline 6, 157k

gladesrunner on Thu October 29, 2009 7:28 PM User is offlineView users profile

And I will be back with the results in a few days.

1995 F-150 XL 4.9 inline 6, 157k

JACK ADAMS on Mon November 09, 2009 5:35 PM User is offline

Just wondering if the problem(s) has been resolved.

gladesrunner on Sun December 13, 2009 1:09 AM User is offlineView users profile

The problem has not been resolved because I have not completed all the suggestions yet. Did a thorough vacuum hose check. Ok there. I am back to work full time and have not checked all the tubes, couplers and the inboard vacuum control pot. thank god we are enjoying cooler weather now and am able to get by at higher speeds with the window cracked. thanks

1995 F-150 XL 4.9 inline 6, 157k

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