A/C moves from panel to defrost and back under acceleration

MDB2210 · #1 (**permalink**) 09-07-2009, 08:22 AM

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.

cbradford · #2 (**permalink**) 09-07-2009, 09:04 AM

Sounds like a vacuum leak is your problem.

madsonp · #3 (**permalink**) 09-07-2009, 11:01 AM

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.

99ExpyProblems · #4 (**permalink**) 09-07-2009, 08:57 PM

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.

Rush985 · #5 (**permalink**) 09-08-2009, 11:47 AM

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.

madsonp · #6 (**permalink**) 09-08-2009, 06:44 PM

Glad you got it fixed,

thanks for letting us know the outcome.

Truxx1956 · #7 (**permalink**) 11-05-2009, 06:20 PM

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.

alloro · #8 (**permalink**) 11-05-2009, 06:40 PM

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?

Truxx1956 · #9 (**permalink**) 11-05-2009, 06:48 PM

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

alloro · #10 (**permalink**) 11-05-2009, 07:26 PM

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.