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A/C baffles shifts to floor when engine under strain

1997 Ford E150 Van, 190,000 miles.

When engine understain, as going up a hill, the a/c baffle shifts air from top vents to floor vents as if I had turned on the heater side; this occurs even if I have set the a/c on fresh air only.

This is pretty common. The baffles in this system are controlled by engine vacuum. You have a leak somewhere in the system. When the engine is under heavy load, the vacuum drops to near zero. This is normal for an engine. Normally the HVAC system holds vacuum and this doesn’t happen. With a leak, the system is always demanding vacuum from the engine, and when it drops to zero, the system loses vacuum ‘pressure’ and will slowly change to defrost, the default with no vacuum. The leak could be an old, crusty vacuum line, or an actuator in the system. There may be a check valve in the system to prevent this that has failed as well.

I agree with oblivion. It is a classic low vacuum thing.