Good info here. I gotta put that in my data bank.

 

While I know this isn't the case for the OP, for future reference and for the benefit of others, I'll mention a problem I've had a few times on a number of different vehicles with vacuum operated mix doors inside the heating and AC system that have failed and would send hot air into the cab with the fan depending on the amount of engine vacuum that was being created at the time. Typically they would perform properly at speed while coasting, which creates the most vacuum.

--Matt

 

I get 20" of vac at idle and between 20 and 15 at part throttle. Only under harder accel does it go really down to zero, then of course, boost.

 

On the subject, this is where I've had the most problems with my Ram (thankfully). I first had problemswhere the truck wouldn't switch vent locations when switched, then the blower sounded and vibrated as if it were out of balance or something, usually when set on high and the latest being the clutch assembly on the compressor. All were fixed promptly however by the dealership.

 

to undestand what is wrong you need to know what habbens in a refrigerator system.

freon in liquid is sprayed in to the evaporater, here it absorb the heat (inside the car) the freon evaporate (sprayed to easy evaporate) - (any evaporation needs heat, ore it cantevaporate.)
the heated freon goes to the compressor and gets compressed. even more heat is supplyed to the freon,
it goes in to the condensor-unit and gets condensed to liquid again and the process repeats.

if the heat cant get away(fan dont work), you have a mix of gas and liquid freon to spray in the evaporator, then the efficiency goes downand you can´t cool it down!

i know wise ***... but it´s my job for 10 years now
hope you understand my bad English/American
tommy

 

While this mode of failure isn't likely your issue, to clarify, how much vacuum your engine makes on a gage is irrelevant. The key to a failing heater mix gate is how much vacuum is supplied to the vacuum actuator that does the air mixing work. Most of the time the hoses slip off or crack etc. Actuator sees no or low vacuum, gets no actuation, you get heat or cold air when gravity decides to move the mix gate back and forth or when vacuum accumulates high enough to operate the actuator regardless of the cracked hose etc... Going around corners etc.

--Matt