Okay, I posted this question a few months ago, but have done some more research. I have 99 F350 SRW 4x4. A few months ago my vents stopped working, will only go on defrost. Also, my 4x4 won't work unless I get out and lock the hubs in. I assumed a vacuum problem. When I take the hose off that goes from the pump to the canister and plug the end, the pump shuts off. That tells me the pump is functioning as it should. So I pressurized the canister with my compressor, and no leaks. Now I am thinking it is the selenoid mounted on the inner fenderwell next to the pump, Ford part # 6C3Z9H465 BA. I am planning on buying one after work and replacing it, but the part is $51.00 and non-returnable. If that doesn't work I was going to replace the pump, at $135, also non-returnable. Does this sound like what the problem could be? Has anyone else had this problem and replaced these? Did it fix it? I forgot that I also replaced the vacuum lines down to the front hubs as I read somewhere on here that they tend to go bad, and in fact mine were all cracked. I would really appreciate any advice. I hate to blow $$$ that I don't need to, especially at Christmas time with so many gifts to buy. Thanks!!

 

Did you carefully follow all the vacuum lines under the dash? Seems like some of the problems originate there. They can unplug, etc.

 

I think your assumption is correct, you have a vacuum leak. The vacuum controlled air doors are not working because you have low vacuum. The leak may be coming from the hub since when you lock the hubs the truck runs ok. I would disconnect and plug the lines to the hubs then see if the pump stops, you will have to wait untill the canister empties before the pump will shut off. If that does not work follow the vacuum lines to their control point, disconnecting and blocking off as you go untill you find the leak.

Jeff

 

I did follow lines under dash, all are connected as far as I can tell. When I manually lock my hubs in, my 4x4 will work but the vents still will not. Thanks!

 

Try putting a vacuum gauge on the lines to the vacuum doors and see if their is enough present on those lines.

Jeff

 

what should the pressure be at??

 

I don,t know. when I go home tonight I will read mine and let you know

 

Just wanted to let you know, I replaced the selenoid or whatever it is called and the problem is solved. Everything is working again! It is really nice to be able to have heat on my feet again when I am out plowing! Thanks for your help!

 

Great news Grunt0311. Thanks for letting us know.

 

Glad you fixed it. Just for your info I measured my vacuum and it read 17" when the pump shuts off.