Dash A/C blows out of defrost when climbing or head wind

Probably leak in a vacuum line. A good Ford truck mechanic can find it.

Yes, I fixed one of those a few years ago. In that case someone had done some work and covered the line going to the reservoir with something that wore into it.

If it has the 460 in it, the white hard vacuum line to the HVAC selector is typically cracked under the hood on the passenger side. Pretty obvious if it is the culprit.

Loss of engine vacuum which is normal when the engine is under heavy load. I had the same problem on a class c 2004 chassis. When I had a leak the in the heating system it would only blow air out defroster. I had a broken vac line in the engine compartment that a pack rat chewed on.

This one should get the job done.

Ford Vacuum Pump

Fix the leak in the vacuum line. The leak could be in the controller or an actuator, it doesn't have to be in a line.

The vacuum reservoir/check valve could be leaking also.

If you have no leaks you can also install an additional canister in the same line as the original.

ALL Class A motorhomes have aftermarket Dash AC systems. They have inherent Vacuum leaks. Always have and impossible to fix. The actuators are the culprits as they are not 100% leak free. When you go to zero Vacuum on full throttle, the vacuum reservoir runs out of vacuum. The default is then to go to defrost. As the other poster showed, get the Ford 12 volt vacuum pump. It will mount on the firewall up front by the rest of the controls from the outside. You will see a round black ball with 2 small hoses and that is your vacuum reservoir. The larger hose is connected to the engine manifold vacuum. THAT hose you remove from the reservoir and plug. The vacuum hose from your new 12 volt pump connects to that vacuum reservoir ball. Connect ground and 12 volts to your ignition side of the chassis and your problem is fixed. This is the same system Diesel Motorhomes use. Doug

at the very bottom of this link when you click will be a pic of the most common Pump and reservoir.

http://www.evanstempcon.com/service_parts.php

dougrainer wrote:
ALL Class A motorhomes have aftermarket Dash AC systems. They have inherent Vacuum leaks. Always have and impossible to fix. The actuators are the culprits as they are not 100% leak free. When you go to zero Vacuum on full throttle, the vacuum reservoir runs out of vacuum. The default is then to go to defrost. As the other poster showed, get the Ford 12 volt vacuum pump. It will mount on the firewall up front by the rest of the controls from the outside. You will see a round black ball with 2 small hoses and that is your vacuum reservoir. The larger hose is connected to the engine manifold vacuum. THAT hose you remove from the reservoir and plug. The vacuum hose from your new 12 volt pump connects to that vacuum reservoir ball. Connect ground and 12 volts to your ignition side of the chassis and your problem is fixed. This is the same system Diesel Motorhomes use. Doug




at the very bottom of this link when you click will be a pic of the most common Pump and reservoir.

http://www.evanstempcon.com/service_parts.php

****AWESOME ****
THANK YOU!!!!
THIS MIGHT BE A PROJECT I CAN DO MYSELF! THANKS AGAIN!