I have a 2000 Ford Expedition with 4.6 litre Triton engine. The ac blower air quits at front vents and goes to defrost vents when acclerating or when I am going onto the freeway on the ramp. It seems the engine is losing vacuum. Is there a vacuum valve or hose cannister that prevents the loss of vacuum upon acceleration?

COULD BE BLOWER MOTOR OR RESISTER

there is most likely a vacuum leak or a hose off somewhere. The air will default to defrost if there is no vacuum

you have a vacuum loss to the ac control head, the motors that divert air are vacuum operated, if you loose vacuum the system goes to defrost for safety reasons, the vacuum comes from the engine, then goes o a plastic vacuum tank under the hook start there, it is a 3/16 id hose.

TRY JUMPING THE BLOWER MOTOR APPLY 12 VOLTS TO THE MOTOR IF IT STARTS THEN THE PROBLEM WOULD BE THE SWITCH IF IT DOES NOT START THEN THE MOTOR IS THE PROBLEM GOOD LUCK HOPE THIS HELPS

Listen and see if the vent flap is moving from the hot position, to the cold position. If that's working then the evaporator may be clogged. It sits inside the same box that the heater core is inside of and if it becomes clogged with hair or dust, then it wont allow the blower to blow through it. But it would then bring hot air through the vents because the heater core is there too.

Hope I was avle to help, glad you up and going.

the problem is a vacumm leak , the control panel works with vacumm from engine supplied by small hoses and also have an vacumm tank under right front fender , check for disconnected hoses or break, the main vacumm hose is on engine intake manifold, ahh , also check control panel

You either have a bad control, or motor running the air direction door in the front HVAC unit, or a door that is stuck.

The front and rear units are totally separate.

The door directs where the air, cold or hot, should come out, whether it is panel vents, floor, defrost, or any combination of those.

This door and motor are located inside the heater box which usually requires a lot of the vehicle dash assembly to be taken apart to get to these parts.

it is more than likely your blower motor, the control module only controls the position of the air flow, such as vent, floor defrost......

there is a vent opening linkage for the top vents if you had removed panel for sterio install this linkage might have come loose or the wiren to electric relay to open top vents came loose

also vent doors on front of a/c vents are turned to the closed position

also quite possible the passenger blower motor compartment door in side the blower mode has something blocking entrance

also your truck might be equiped with a blower motor filter i read that these need to be changed out during normal filter replacements the filters for ford i didnt know they started using filters for a/c on reading this since 2000