AC Doesn't Work At Elevation
 

My AC works fine at sea level, but as I go up in elevation the diverter closes and the air stops blowing. This has occurred repeatedly when driving from Seattle to Idaho & Montana, it's fine until I hit the mountains.

I can hear the fan running, but no air flow. I know that the diverter valve is vacuum operated, but where is it, which vacuum hose, how to test it, etc?

Year and engine please, however I think you have a vacuum leak.

'99 4wd w/5.9 L. I checked the vac lines in the engine compartment and all look good to me. But, I don"t know which line would be the one of interest for this problem as it looks like 3 or more go through the firewall.

Thanks for your help.

+1 Hydra on the Vacuum diagnosis.

Probably actually a bad check valve to the hvac system, when the engine is under load and looses vacuum pressure and/or there is a leak, then it would pull the vacuum away from the hvac system. I think the default position for the blend door is defrost... when vacuum is lost.

Where do I look for the check valve?

Not sure exactly what one he is talking about, but it could be the one on the brake booster.

Open the hood and look at the front of the Throttle Body on the bottom and you will see a vacuum hose about 3 inch long (about 5/16") and it plugs into a smaller harder black line that turns and follows under fuel rail to an access port on the cab fire wall under the drivers side front windshield. Anyway, this little 3" hose likes to crack right off the throttle body because of the heat.

Now from this port it goes to a "T". From one side goes to the brake booster, 1 to the inside of the cab through the left side heater hose grommet to the heater control on the dash, then the right side goes along the front cowl to the passenger side and under this cowl (under the front windshield) is a vacuum bladder. Now this bladder may have a crack in it, but I think the problem is more toward the engine side.

From what you are describing it sounds like a vacuum leak so you should look again and look close.