Re: Air Conditioning not working in Texas
The mode selector defaults to defrost mode when it does not have any vacuum to operate the diaphragm that moves the paddle around inside the plenum. Better to see in the winter than not, more or less.
If the selector valve is leaky, the hoses have a leak or the vacuum reservoir springs a leak, there may not be enough engine vacuum to operate the selector. If it were to slowly swing to defrost when you accelerate, you may want to look at the vacuum reservoir check valve. Manifold vacuum drops when you step on the gas, and if the check valve is not functional, the vacuum in the reservoir will deteriorate also, allowing the paddle to move to defrost.