98XLT A/C Max

Vacuum leak. With no or low vacuum system defaults to defrost for safety. When you push down on pedel vacuum goes down. There is also a vacuum reservoir that "stores" vacuum. Maybe that has a crack or leak.

This could also be caused by a leaking check valve that holds the vacuum reservoir when the throttle is opened. Has happened to me on other ford things. Cheap fix.

Thanks for that - quick heads up - where is the vacuum reservoir located?

Doesn't sound like the blend door problem, Steve. Sounds like a vacuum leak somewhere. The direction of the air (dash, heat, defrost) is controlled by a vacuum operated system and it fails to the defrost mode. When you accelerate up a hill, the vacuum drops lower and that can be like losing vacuum as those of us old enough to remember vacuum operated windshield wipers know. Perhaps a split hose or cracked fitting somewhere is the cause for your symptoms.

=Vic= Bear Gap, PA

I "just" barely remember vacuum wipers....

The Exploder's going to the Ford Dealer on Monday for it's 90,000km service - I'll mention it to them and see if they can put a compound gauge on the system and see problem.

Interestingly if I have the A/C just on "normal" it comes out the dash vents and doesn't change to the defroster vents under load - I would have thought it would have acted the same?

All the best.

There is a damper that is pulled open by vacuum when you put it in max a/c. Maybe the connector to this or the vacuum solenoid is where the vacuum leak is.

When you are in Normal a/c no vacuum is sent to this system so the leak does not affect performance.

All fixed - Ford found a crack in the hose near where it enters the firewall - bad news was that they're of the opinion that it'll happen again due to the vacuum lines becoming brittle.....

Ah, well at least I'm happy on the drive home with the +30°C days;-)