My A/C comes out the panel vents ice cold at idle or cruising speed but then suddenly starts come out of only the defroster vents up at the windshield every time I accelerate. When I let off of the gas a little, the air flow returns to normal out of the panel vents and my wife stops griping. It seems to be dependent on engine RPM. Can't see or hear anything unusual under the hood, but I did have the heater core replaced a while back during the winter and I may not have noticed the problem till this summer. Should I be looking for a vacuum leak/crossed-up connection or something else? Everything else works perfectly.

 

Sounds like a vacuum leak is your problem.

 

Yep, vacuum leak. Check these common failure locations. There's a small vacuum tube that goes around the back side of your battery, these have been known to develop a leak. Also a small tube that goes through the firewall by the battery. Also check all the vacuum connections at the PCV valve.

 

Default position (no vaccum) is defrost, that's why it is blowing out of that. If you have checked everything and don't find the leak, I have seen posts on here regarding the a/c head unit itself where others have replaced the o-rings and solved the problem. You might want to search for that.

 

I just had this problem and fixed it this morning. There is a black valve, about 2"x3" with 3 vacuum hoses connected to it. One of these hoses had become disconnected. As soon as I connected it back the problem with my A/C went away. The valve and hoses are located directly on the left side of the plastic engine cover. Hope this helps.

 

Glad you got it fixed, thanks for letting us know the outcome.

 

Hey guys, I got a '95 doin the same thing. As long as its idling, it does fine. While it's sittin runnin, I checked the vac (the blue hose) goin to the vac diaphram that runs the vents. I got 20" of vacuum at idle and even as I reved the engine up to 3,000 RPM's. I know for a fact that this is the correct vac diaphram, cause I disconnected the hose and manually worked the door and made the air come out of the vents. Are these diaphrams bad about gettin weak over time and not holding enough vacuum to keep the door open? The truck has 217,000 miles on it and is a 2WD truck with a 351W in it.

I also have an '05 Taurus doin the exact same thing I checked it the same way and it also has 20" of vacuum at the hose during the vent selection.

Thanks for ANY help guys.

 

Connect the vacuum gauge to this hose and take a drive. (Simply reving the engine doesn't put enough of a load on it to drop the vacuum.) Does the vacuum drop off when accelerating?

 

Yep, it does switch during uphill acceleration. I didnt know if it would be an accurate test by simply reving the engine? I may just do that. If it drops, then its probably the switch itself? I do have 20" of Vacuum coming into the switch. Thanks for the idea. I dont have an adaptor to hook the vacuum pump/gauge to the diaphram, but will make one soon so I can pump vacuum to the diaphram and see if it "bleeds" off.

Thanks for the suggestion dude

 

If the vacuum reading drops off during acceleration, then you either have a leak, or the check valve is no good. If however the vacuum reading stays fairly steady then you probably have a problem with the actuator. Sight unseen I would first suspect a leak, then the check valve, and the actuator last.

 

I had the same issue on my 2004 E-150, the blower would switch to defrost with vacuum drop (acceleration). It would only come back through the vents once I let off the gas. Eventually, it wouldn't come back at all until I came to a stop sign. My issue was caused by a defective and worn out A/c vacuum check valve, I assume it was letting air leak in while under a vacuum drop.
I fixed it with the following steps:
1) Purchase a A/C vacuum check valve at Napa p/n 207449, its a 3 way valve.
2) Purchase 6' of vacuum hose (or a new canister) to serve as a vacuum reservoir.
3) Pull the front dash bezel off so that the A/C control panel can be removed. The light switch needs to come off, it's tricky.
4) Cut the black hose leading into the A/C switch. Leave enough room to attach a new vacuum hose. If you want to check first, pop the access panel on the dash off and disconnect the vacuum hose connector. You can put a vacuum gauge on the black line, this is your incoming vacuum line. Compare vacuum readings between vacuum off manifold and vacuum at the A/C switch. I had a large discrepancy between the 2 readings.
5) Reroute the vacuum line off the manifold through the firewall and into the area under the dash.
6) Connect the vacuum line to the black nipple of the 3 way check valve.
7) Connect the 6' of vacuum hose or canister to one of the 2 nipples, plug the other end of the 6' hose to create a canister.
8) Connector the other nipple to the black A/C switch. You can search a vacuum diagram online for pics of how the valve connects to the lines
9) Test the setup under hard acceleration. If your check valve was bad, this should be the fix.
10) Stuff the new vacuum canister or 6' of hose up under the dash.
11) Reassemble A/C controls, radio and bezel.


Another method is to replace the defective A/C vacuum check valve. This can be done by cutting a 4" x 6" hole in the fender well on the passenger side of the van. It will allow you access to get to the old check valve. I opted not to follow this method but l have read it works too.

 

Thank you for your suggestion. My vacuum line to the vacuum box behind the battery was broke. I didnt want to pay the $15. for a new line so used a vacuum hose off of a 2011 International Pro Star Semi!. worked great.
Now i can have a/c come thru the vents when i hit the gas!

 

I have a problem kinda like this but my a/c blows hard sitting still and when I accelerate it just starts blowing like it’s on low but only out the dashboard vents it doesn’t go to defrost or anything it sounds like it’s on high but it’s barely any air coming out

 

just an FYI. I have an 02 f150 king ranch. It has this issue and this forum is the first place I've looked that had a real solution . THANK YOU SO MUCH !!!! when it's 110° in the shade , this a/c problem is a MAJOR problem. THANKS AGAIN !!!!

 

I have some similar going on. A/c works great cruising down the road but when Im accelerating harder it’s switches to defrost and blows out heat in the back of the van but still cool air on the dash. I have a 2009 ford e350 econoline passenger van