ok guys my trucks heat/ac won't switch from defrost when i turn my switch... Is it my plenum.. i haven't really checked out the vacuum lines too much but it is still hooked up over by the dipstick where i've had it come loose before... thanks guys...

 

mine did the same and it was a vacuum line. so id check again

 

Mine did the same thing, it is a vaccum line in the engine compartment more than likely. Mine was on the fire wall, on the turbo side. But your leak could be anywere... Check all your little black vaccum lines and you'll find it.

 

As the others stated, you have a vacuum leak. With lack of vacuum, the HVAC defaults to defrost. A very common failure point is the plastic vac line on the cruise control servo. You'll find it under the driver-side battery. Battery acid drips down directly on the line and eats a hole in it. Very hard to see, even when you are looking for it.......

 

Alright thanks.. i'll keep looking... wonder if my chiltons would have a vacuum line diagram>???