A/C switches to defrost upon acceleration

[psu x]

(1997 EB with automatic climate control)

Recently, my A/C has decided to switch to 'defrost' whenever I accelerate and/or climb hills. It switches back by itself as soon as I let off the gas. I am guessing that this is related to a vacuum issue, but I do not know where to begin looking.

 

[Turdle]

http://www.explorerforum.com/forums/showthread.php?t=192842

This may help--although your controls are different--

 

[BrooklynBay]

I've had this happen several times. The plastic vacuum lines develop a crack near the vacuum servo piston.

 

[Ritsui]

When that happened to me, it was the vacuum reservoir gone bad. A $10 replacement reservoir fixed it.

The thread jtsmith pointed to has the info you need.

 

[DC Larson]

I have a 2000 V8 Merc Mounty. Auto climate control. My air gets cold and hot but when turning fom vents (AC(max or outside) or Fresh Air) to heater it won't come out of the bottem heater vents...it goes to defrost. When using Panel and Heater mode together, it only comes out the defrost. Defrost and heater setting I get defrost only, no heat by my feet. Any idea what may be up? When on vent setting it comes out the vents...cold or hot whatever I want.