jimbo1111

A/c works great. than all of a sudden it would switch to heat than back to a/c. I changed the valve by the fire wall and checked the clips nothing broken there. I did notice vaccum going from 19 to 15 in.hg when the fan is on. the servo motor that swings the flapper door between the two coils seams ok but why would it swing to heat when the system is on a/c. Any help is appreciated.

James86-951

Where is the a/c temp dial set? It sounds like the climate control is trying to keep the inside air a certain temp. If the set temp is higher than the temp the system is detecting, it will begin heating.

Alternately, if the system is trying to keep the heater bypass valve closed and there isn't enough vacuum or the diaphram is damaged; then it may open. (A good check for that is to disconnect the actuator from the valve itself.) If the problem goes away then it is either the vacuum or the diaphram in the actuator.

jimbo1111

I replaced the valve as noted above. The dial is set to full cold.

From my understanding even if the valve is open it dosn't matter because there is a damper between the heater core and the evaporator coil that directs the air from one or the other.

What would make the door swing back and forth between hot and cold other that the **** on the dash? Is there any other sensor that controls the damper motor?

I noticed a sensor with a small blower in it behind the glove box that looks like it's pulling air in to sample it. If this device was faulty would I be getting these symptoms?

I also check the tstat above the gas peddle and it was in the closed position. Looks to be some kind of return air sensor.

Robert D

I do believe that sensor behind the glove box is for climate control. When it goes bad I am pretty sure it will flip to full heat...but...it hasn't happened to me yet, thats something I remember reading a few years ago.

James86-951

That very well could be the problem. I don't know where you might find technical info for the sensor. (Resistance/Voltage specs) Is the sampling fan running? If not, then it may be detecting the temp dropping as a result of it's proximity to the evaporator core. (Speculation)

RussRPJ

I have developed the exact same problem in the last week. Compressor kicks in and nice cold air for a minute or so. Then suddently, the compressor goes off (you can hear it) and really hot air blows in my face. Not nice when the outside air temp is over 100F like it is now. I fixed a broken heater clip, and the vents look OK. There is another sensor behind the glovebox, with its own fan? How do you get to it?

Dick Ruddell

I have the same problem. Just talked with my P-car mechanic and he says it sounds like it is the small vacuum check valve on the outside of the firewall.

He will be looking at it on Monday Morning, and I will post the results