I have a 96 F250 that is a great truck... I just stumped with a recent problem. When the truck is warm and your on the gas, the A/C switches from the dash vents to the heater. I'm thinking it's something vacuum controlled. Has anyone seen this type of problem before?
Thanks,
Dan
A common problem. That 'switching' is the default condition. You should be feeling cooled air from the defrost outlets as well, depending on where the controls are. There is a vacuum problem somewhere. Since it is showing up when the engine is under load, I'd start checking from the vacuum reserve tank in the engine compartment. Check all vacuum lines in the system for cracks/loose connections. The plenum system has two vacuum motors that control air distribution within. One motor opens/closes the door to the dash vents. With the function control in AC mode, that vacuum motor closes off the air flow in the plenum to the floor and defrost vents. The other vacuum motor is a two-stage motor. When the function control is in 'defrost'(no vacuum to that motor) all air is directed through the defrost outlets. With the function control in 'mix' the vacuum motor partially opens the door to the floor ducts so some air is directed to both the floor and defrost outlets. With the control in 'floor' full-vacuum is applied to the motor and the defrost door is held completely open, shutting off air to the defrost outlets, and all air is directed to the floor duct. With a no vacuum condition, the plenum door to the dash outlets is closed and air is directed to the defrost outlets. I hope I've described that so it makes sense. The best way to check anything in the vacuum system is with a vacuum pump/gauge.
Hope this helps
Dave S(Texas)
check the pcv valve
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