kmb1966 wrote:
My dash a/c blows cold air, but at times when climbing hills/overpasses or traveling in a head wind, the air will decrease out of the vents, and blow out of the defrost vents. Upon slowing down, the air will return to the front vents. I've brought it to nearly every shop in town, with most scratching their heads after replacing lines and check valves and saying 'just live with it'. No. I don't want to live with it. I want it to blow out of the front vents. I've heard (on this forum) that a secondary vacuum pump could be added to solve this problem. My question is WHERE do I find this pump, and WHERE would it be installed? I'm clueless on where I would put this auxiliary pump for additional vacuum. :h
2000 Ford F53 Chassis
kmb1966,
What you're experiencing is age old for many Fords and even some Chevy equipped cars, trucks, RVs etc. There's some basic engine operating characteristics that you need to know, before you understand what EXACTLY is happening and why, with your problem. I'll give you the short version. I don't know if you've ever witnessed a vacuum gauge or not, when an engine is running. If not, the normal place for attaching the the vacuum gauge is someplace in the intake manifold.
When the engine is IDLING, there is HIGH VACUUM created. When you abrubtly step on the throttle, the vacuum decreases quickly. But, go back to idle and the vacuum returns. Now, engineers that create A/C systems in vehicles, know all this stuff and are prepared for it when operating conditions change. About 99.9999% of most dash A/C and heating controls in most of the last 30 or more years of vehicles and RVs etc. are VACUUM controlled.
That is, all the little doors, ducts, and more are all vacuum controlled with little vacuum biscuit type motors. When you select where you want your A/C to blow out, you in effect, are controlling vacuum to each of those little motors/doors.
Ok, enough on that. Now, as state when you're at idle or, very low throttle use, as in say, cruising on flat ground etc, you have fairly good vacuum being produced. So, all your controls for your A/C ducts and doors get all the vacuum they need for correct operation. But, as you start to climb a grade, guess what, you open the throttle more and that, as stated, decreases your vacuum production immensely.
For that reason, Ford and many other manufacturers created "STORAGE" containers for vacuum when those low vacuum production operations are encountered. And, through little check valves in all those little rubber lines, you now can draw vacuum from those storage containers, to keep your air conditioning flowing, to the ducts etc. that you've selected.
Once you level out and your throttle goes back to normal, you now begin to create more vacuum again.
So, with all that being said, what most likely has happened to you and your coach is, one or more of your vacuum storage containers, has developed a leak. I have seen this waaaaaay more than a few times. Now, sometimes, those vacuum storage containers look like something round, about the size of a soft ball. Sometimes, they look like a tomato Juice can, only painted black. Both of those have vacuum lines coming from them.
You can test those by simply removing the vacuum line and, put a short rubber line into the end and then blow into the little rubber line to create some pressure inside the can/container. Then, spray the can with some soapy water to see if there's bubbles emitting from any section of it. I've done just that test and revealed leaks many times. You can't see the leaks 'cause they are real sneaky. If you happen to find one or two leaks, fixing them is child's play. All you need to do is, get some Silicone from your local Home Depot, Lowes, ACE hardware or where ever and wipe things clean and paste some silicone over the leak area, DONE. Absolutely NO NEED TO PURCHASE A NEW ONE.
THE reason your A/C cold air reverts to the defrosters in a low or no vacuum situation, is due to mandated performance stats by the manufacturers. You see, if your system begins to fail, and you loose vacuum as you are, the system is designed to fail-safe to the defroster position. That they feel, is way more important to keep things clear, (as in the interior of a windshield) than it is to keep your little fingers cold from the regular dash ducts.
So, now that you have a bit of knowledge that what seems the "techs" you took it to DON'T HAVE, you can begin to check out your system for leaking vacuum storage containers and or hoses. I guarantee you'll find a leak.
Scott
P.S. With a correct fix like I explained above, there's no need for any "auxiliary pumps" to add to the confusion. Besides, I don't care how many pumps you add, you STILL HAVE A LEAK.
