f 150 a/c vacuum leak

[jcougar88]

My a/c works fine, but when I accelerate, the air starts coming out of the defrost. I have traced down the best I can, and when the truck is at idle with the a/c off, you can hear vacuum leaking that is coming from behind the a/c switch(max ac, norm ac, vent, etc...). I pulled the panel out, and have 5 lines going to it(red,white,blue,black,yellow) I have traced them all, and all are connected on both ends. Don't understand why I hear vacuum leaking then. Noticed the blue line goes to the modulator that controls when you turn on vent, norm ac, or max ac. When you put it on vent or norm ac, the modulator goes in very fast. But when you put it on max ac, the modulator goes in very slow, and doesn't go in all the way. Please help. Thanks.

[jcougar88]

Also, changed vacuum selector valve, vacuum noise coming from here. Didn't help, though, still have vacuum noise coming from here. Any help would be appreciated.

[mstbone67a]

did anyone every find the resolution for this?

thanks

[unclebubbles]

If you`re having the same problem, start by checking the vacuum line at the selector valve that comes from the vacuum canister under the hood. Using a vacuum gauge you should have manifold vacuum there. Then check the lines going to the individual vacuum motors/actuators while operating the selector switch. If the vacuum is low and the selector valve/inside lines/vacuum motors/actuators are good and functioning ok, look for leaks under the hood, especially the reservoir. Sometimes heavy trucks/campers...ect that the engine runs loaded on most of the time have problems like this if the reservoir isnt big enuff to maintain the heater/ac controls.(manifold vacuum drops when the engine is in a strain) In this case i`ve had to add larger vacuum cannisters and that usually solves the problem. Some of the factory reservoirs are only about the size of a baseball.

[jyount]

Old thread I know, but mine is doing the same. Works fine until you accelerate then the blower goes to the defrost. Only thing it that I cannot find a leak anywhere. I am sure it probably has one, just can't find it. I took the vacuum motor off the side of the hvac case and it is fine, works fine, doesn't leak, etc. Pulled the controller out from the dash and there is no leaks there and it seems to work fine. It acts like it has NO reserve vaccuum, where is the reserve resevoir on these things? Maybe the line is busted to it.

[jyount]

Found my reservoir, I missed the vacuum line that runs right by the battery tray and to the reservoir which is mounted on the back side of the battery tray between it and the fender.

Anyway I didn't have any reserve because the check valve between it and the rest of the system was not working at all, so the reservoir was as good as uneffective. I got the valve lubed up and got it to work, it works fine as of now.

I suspect the valve I got unstuck will stick again, I will know where to go then though...

[Scrapper]

if you can find the leak or close to it try some carb cleaner on hose and when it idles up theres your leak but just spray a little on vaccuum hose or hoses some people don't believe in this but that's what i do if i hear a leak.

good luck......

[jyount]

I did, didn't have a leak though, a check valve was stuck, thanks though.