The Max A/C stops blowing out the vents when i get to 60 mph. Does anyone have this problem, its a first for me.

That's a new one for me...

does it start coming from the defrost vents? Thats the default position. Or just stop blowing all together as in the fan shuts off?

When Max A/C is on its coming out of the dash vents, then when I reach around 60 mph it stops blowing, out the dash vents. Not sure if it defaults to the defrost vents or not. Will check! But i have notice this several times now. I have tried it several times and I am able to replicate the problem at will. It baffles me! Just wondering if anyone has any ideas.

This would be a first for all Ford techs too.

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I havent talked to any techs about this. Thought i would ask you guys. But i dont think transmission fluid is the fix on this one.

The only thing that even remotely comes to mind is that for some reason you loose vacuum pressure randonly and the vent actuator returns to defrost, or you have a wiring problem and the fan is just randomly stopping.... theres no correlation between how fast you're going and how the hvac system works lol.

I bet you have a vacuum leak, or a weak engine. The 'vent' selection is done by vacuum power, and if your check valve leaks, and the engine is under load, the spring will move things back to the default of 'defrost'. You could have leaking vacuum lines, or the check valve that contains the vacuum when manifold vacuum drops off(floor it, and it may move to defrost also). If it is wonky, or your system has slight leaks, vacuum will drop, and there you go.
You can add a second check valve in the line from the manifold to the reservoir, or leading to the in-cab vacuum system.
tom

Thanks Tom i will check this out! Is there a schematic showing the check valves and reservoir? After about 60 MPH it does go to defrost. So maybe check valve or reservoir.

I don't understand how hitting 60 would cause less vacuum to the blend door than going under 60. If there is a vacuum leak, it would happen at all speeds.

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Engine under load has less vacuum. It could be "load" related but impossible to be strictly "speed" related. Faster speeds increase load but simply revving in first gear would also increase the load and drop the vacuum....

DV get out your vacuum gauge, and connect it up. Cruise. Watch. If the engine is geared one way, it will have very low vacuum at cruise. That would be the case for a long-legged final drive ratio. Less vacuum == less pumping loss == better mpg. To a point.
tom

The vacume controlled gate thats infront of my fan under the dash tries to open in Max AC, but doesnt. I dont have enough vacume pressure. I havent got a Vac. Guage or a Vac. Hand pump yet as i work seven days a week and have litte free time.But i will get those and try to find the source. Of the loss. Thanks for all the info guys.

Old video I made but should help you find your issue:

https://youtu.be/7tSFPSY19MI


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Automotive Electronics Engineer
2000 XLT 2.5L A/T
Mods:
Explorer OHC
Explorer Autodim Mirrors w/ Autolamps
Explorer Sport White Face Gauges

The vacume canister was bad but still have the problem of a/c going to the defrost vents. Motor makes 20 hg of vaccume on the guage and at the vacume canister? Any ideas?