On my 2000 F-150 5.4 it just started doing something weird, when I go up this one long hill the AC stops blowing from the panel where I have it set and blows from the defrost vents. When I top the hill it switches back to the dash panel. I am assuming this is some kind of vacuum issue so I checked all the vacuum lines and can't find any loose or worn/cracked.

I don't think it could be related in any way but I just had a new battery installed (the original one lasted 7yrs and 8 mos, not bad!) <!-- google_ad_section_end --><!-- / message -->

 

Keep looking, it's a vacuum issue .......

 

if you loose vacuum, the default is to switch to the defrost instead of dash vents. the vacuum comes off the passenger side of motor, toward the firewall in front of the glove box. there is a TEE that you need to check. other side of the TEE goes to an accumulator BEHIND THE BATTERY in the wheel well area. Maybe it came loose when moving the battery.

 

First of all, thanks for the quick and informative response. I have posted this same question at another ford site and in 3 weeks it never got a response.


Today I found the vacuum lines and tee and inspected them, they all look fine. I removed the battery and tray and looked at what I assume was the accumulator and it looked fine also. It was a square box that clips onto the back side of the battery tray with a single line going to it. I checked all the lines between the intake manifold and the accumulator. Only ones I did not check is on the other side of the firewall under the dash.

I noticed the other day that it does not seem to switch to the defrost vents unless I have the AC on, with just the vent and fan blowing it didn't switch from the dash vents. Don't know if this means anything or not.

 

I'll have to look but I think the 2000 has the same vacuum reservoir under the right front fender as my 98 has. It's not likely that it's damaged but mine was, broken in half.
So as to not over post graphics here since I've posted these on another thread, here are the links to the vacuum schematics.
http://s11.photobucket.com/albums/a1...uumSchemat.gif
http://s11.photobucket.com/albums/a1...ematic4.2L.gif

 

BTW, this IS ! !
Where in NC are you located?

 

I didn't see anything that big. The box I saw clipped to the battery tray was about 1/3 the size of the battery and square. Had a single vacuum line going into it. Maybe I missed the rest of the system.

I am in Advance, NC. About 10 miles west of Winston Salem.

 

Pictured here is the behind battery reservoir and the line routing through the inner fender to the outside under fender reservoir.Just FYI as I do doubt the under fender reservoir is the problem.




Yep, I know where Advance is, pass it weekly on my way to Winston-Salem.

 

Well after checking all of this stuff I am at a loss as to what to do next. It seems like it only does it with the AC on. Could it be in the control unit inside the cab? Or maybe there is a small leak in one of the lines that's too small to see?

 

Vacuum inversely follows the gas pedal, more gas means less vacuum. So if this is a vacuum problem then you should be able to make it happen with the gas pedal, not just a hill. With the airconditioner on, can you make it swing the air by just giving it more gas? Check out the defrosters to see if does the same thing, since the airconditioner runs with the defroster. Look on the bright side, very soon you won't be using the airconditioner, so you better check out the heater to see if that works ok under the same test conditions.

 

It sounds like the vacuum check valve might be bad. I don't remember off the top of my head where it's at, but when it's messed up all those reservoirs don't do you any good.

 

You can see them in the right side of my second photo.

 

the second photo of the 9/22 post or the 9/23 post?

 

9/23 They are the blue things on the right.

 

It won't do it with just the pedal or without much load on the engine. The only time it does it so far is on a long hill, I get about 3/4 of the way up, still in OD and it's beginning to lose rpm's, even a little more gas will make it downshift but I just let it slow down until I top the hill.