A/C Vacuum direction??

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Putting this under engine, etc. as I think it is a vacuum situation. Driving today (HOT) I noticed that when I am cruising along ac is coming out of the vents as it should. Upon acceleration the ac is coming out of the deforst area up into the windshield. Is this some sort of vacuum problem that is causing the re-direction upon acceleration? If so how do I correct it?
 
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Start the engine, remove the output line from the cannisiter, put your finger over the output port and then turn off the engine. Remove your finger and you should hear/feel the air being drawn in. Or use a vacuum gauge.

It's also possible that your have a line disconnected under the dash or the selector is leaking.
 
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Here is a trouble shooting guide to test your system. As mentioned above it sounds like you are loosing vacuum at wot so most likely your canister. Hope this helps.
 

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If the vac tank is good but check valve is leaking, a small check valve like this one will fix it. Only a couple of $$.

View attachment 64727
well I put in a new (repro) cannister, still have the problem but not as much I think. Will try an in line check valve. I assume if the cannister is not up to snuff then under acceleration the vacuum is either cut off from the cannister or drawn back into the engine? Thus the check valve in line to keep the vacuum from leaving the cannister?

m
 

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Sounds like you may have another issue. Selector valve leaking or a leaking vac motor. A check valve will not help if your new tank is ok. Test the new tank first. Crimp the small vac line going into the firewall or put another piece on and block the end. Start the engine for a short time and turn off. Wait a couple of minutes and then pull the small hose off the tank. You should hear the vacuum release if the tank is ok. (A vac gauge is obviously the best) If ok, reconnect the small hose and go to 2 TESTING in the docs above. Hopefully that will indicate where the leak is.
 

Aus73Mach1

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Took me a while to sort my issues, bad check valve in the tank, one leaking vac motor and one hose connected to the wrong vac motor. Now even after a couple of weeks without an engine start, vacuum holds and controls work until the vac tank "runs out".
 
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If I put a vacuum gage onto the small output portion of the air tank and run the engine I will show vacuum on the gauge. When I turn the engine off should the gauge still show holding vacuum or go to zero when engine is shut off?
 
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If I put a vacuum gage onto the small output portion of the air tank and run the engine I will show vacuum on the gauge. When I turn the engine off should the gauge still show holding vacuum or go to zero when engine is shut off?
That is not as easy to answer as one might expect. If you are speaking of the Air Conditioning Reserve Vacuum Tank/Canister, there should be a check valve on the tank that is used to manage vacuum used for the A/C control head unit. The Check Valve should have 2 ports. One leads to the Intake Manifold Tee, the other leads to the A/C control head unit. The vacuum hose leading from the vacuum canister to the A/C control head unit should hold vacuum even when the engine is off, although it may bleed off after a while - especially in an older A/C system. But, the hose leading to the Intake Manifold Tee will lose vacuum the moment the engine stops running. I hope this, and the attached image, helps.

1657260343194.png
 
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What can confuse the issue is the presence of a Fuel Evaporative Control Storage Tank/Canister that looks a little like the Air Conditioning Reserve Vacuum Tank/Canister. It has one vacuum hose leading to it, but it is not meant to be to manage engine vacuum. It is meant to help direct evaporated fuel vapors from the fuel tank into the charcoal impregnated Fuel Evaporative Storage Tank/Canister. Take a look on PDF file pages 19-21 in the attached file for details. The Fuel Evaporative Storage Tank/Canister is not connected to Intake Manifold Vacuum. Rather, there is a foil/paper low pressure hose that leads from it to the air cleaner housing. The low pressure vacuum from the Air Cleaner's interior draws the light fuel vapors out of the Storage Tank/Canister so the vapors get burned in routine engine combustion.

Here are some YouTube videos showing what the Fuel Evaporative system (including canister) looks like:



 

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certainly talking about the ac cannister. I put a vacuum gauge on the output nipple, the one that goes to the ac control panel (in car). With engine running it shows vacuum, when engine shut off vacuum drops immediately to zero.
 
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certainly talking about the ac cannister. I put a vacuum gauge on the output nipple, the one that goes to the ac control panel (in car). With engine running it shows vacuum, when engine shut off vacuum drops immediately to zero.
It sounds like you have the correct canister identified for the A/C control system. If the vacuum from the canister into the cabin is immediately dropping to 0 when the engine is off there is a vacuum leak "somewhere" (I know, duh...). That vacuum in the line leading into the cabin should hold steady for at least a "a while" But, over the years vacuum leaks will pop up. Sometimes the leak is found within the vacuum control head itself, other times in the vacuum lines and connectors to/from the control head, sometimes the Vacuum Check Valve in the canister itself is bad, sometimes it is the canister itself that is leaking, and of course (you guessed it) i is a combination of any of those potential problem areas.

I know that Classic Air Conditioning offers kits to provide A/C into Mustangs (and other vehicles also) that were not originally A/D equipped vehicles. Their kits use electric motors as opposed to vacuum motors to control air direction doors and the hot water (coolant) flow valve for the heater core. It is "possible" their Original Air division (for oem systems) just might have new control heads that are electrically managed as opposed to being vacuum managed that will work on oem A/C vehicles. If I were to find I was having a vacuum problem in our 69 Shelby's oem A/C system I would be very tempted to find a replacement solution that used electric motors and an electrical control head unit. Here is their web site (Original Air). In prior phone calls I have found their folks to be very experienced and useful.

 
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I took the mustang for a ride tonight, wish I had not.... Did discover the ac works fine even under acceleration with it set on outside air but when switched to max air the problem pops up, vents cut out and ac comes out defrost. BUT i hit the worst pothole ever and now have another problem to deal with. Something broke on the passenger side, rides fine but making metal noises when I hit a bump or not smooth part of the road. Hoping for just a shock but who knows. Ac problem on back burner now.
 
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now that I am back to thinking about the ac while I am getting ready to tackle my damaged control arm....

During the drive that I damaged the control arm, while I was playing with the ac controls I noticed 2 things: In regular fresh air mode the a/c did not change to the defrost ducts when under heavy acceleration, but stayed steady on the vents. When switched to max under heavy acceleration it goes to the defrost vents as noted in original post. With the knob on max i put a little more pressure on it to the left and the ac cut out completely, so wondering if the vacuum control in the cabin switch is the culprit? I had purchased a freshly rebuilt one when I was doing the resto but wonder if I should pick up a used one, swap it out and see if that is the culprit.

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If it is changing while you are driving it sounds like you have a vacuum leak. Inspect the lines for cracks/breaks.
 
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