Dash AC

What make is the chassis?

F53

Based on the age of the coach my bet is you need to replace the vacuum lines. That is what fixed my 2003 Dolphin on a Workhorse W22. The Evcon Dash air uses vacuum lines and when there is a leak, the default is to defrost.

Not sure with the F53, but with some vehicles low Freon in the AC system will also cause a default to the def and floor. Is the air coming out as cold as you would expect for the AC? If not a recharge may cure the problem.

betting cracked or broken vacuum hose

The F53 chassis has no vacuum system. the chassis makers have to add one to operate the heating/ac dash vent system. This system consists of a small 12v vacuum pump and a small vacuum tank and associated vacuum hoses that go to the dash controls.
If there is no vacuum the default vent path is through the defrosters.
Check all these vacuum lines around the pump and tank, usually under the hood in front.

wanderingaimlessly wrote:
Not sure with the F53, but with some vehicles low Freon in the AC system will also cause a default to the def and floor. Is the air coming out as cold as you would expect for the AC? If not a recharge may cure the problem.

On Motorhomes, Low Freon will NEVER cause a default on the air discharge on the Dash Controls. Motorhome Dash Controls on older motorhomes(The OP 2002) are controlled by Vacumm either from the Gas engine or a Vacumm 12 volt pump on Diesel engines. Newer Motorhomes have Electric servo motor controlled Dash door activation. Doug

garym114 wrote:
The F53 chassis has no vacuum system. the chassis makers have to add one to operate the heating/ac dash vent system. This system consists of a small 12v vacuum pump and a small vacuum tank and associated vacuum hoses that go to the dash controls.
If there is no vacuum the default vent path is through the defrosters.
Check all these vacuum lines around the pump and tank, usually under the hood in front.

You are correct for newer model F53. But the OP is 18 years old and back then they did use engine Vacuum to operate the blend doors. They went to a 12 volt Pump to stop the lack of Vacuum when under heavy throttle causing the Vacuum to go to zero and having the air come out the defrost ducts until vacuum was restored. Doug

garym114 wrote:
The F53 chassis has no vacuum system. the chassis makers have to add one to operate the heating/ac dash vent system. This system consists of a small 12v vacuum pump and a small vacuum tank and associated vacuum hoses that go to the dash controls.
If there is no vacuum the default vent path is through the defrosters.
Check all these vacuum lines around the pump and tank, usually under the hood in front.

I beg to differ. The F53 absolutely uses manifold vacuum to operate the dampers for the air conditioner/heater. There's a small ball vacuum chamber to store the vacuum while the engine is under a heavy load and manifold vacuum isn't available. There's a check valve to prevent the chamber from loosing the vacuum. It could be a broken/cracked vacuum line or bad diaphragm in one of the damper motors. Keep looking for those vacuum lines.

Thanks, everyone. I’m still trying to find something, but so far the lines look intact. I haven’t give up.