The vehicle is a 1990 Dodge Caravan, 3.3L v6. The air conditioning system was converted four years ago by me to R-134a. At that time I did the required system flush, compressor replacement, expansion valve, and drier. It has performed very well for four years.

About a week ago the outlet air temperature began to cycle between warm and cold as I drove. I put the gages on today and here are the results:

Outside ambient air temp is 80 F, R/H 27%, static system pressure before engine start: 92 psi

After vehicle warmed up and idling high at 1500 rpm A/C was put to Max cool, hi fan: Hi side 225, Lo side 30
After 5 minutes run time: Hi side oscillates between 200-250. Lo side oscillates between 25-34. As hi is coming down, low is going up. Oscillation time is about 30-45 seconds.

My instinct tells me the expansion valve is fritzing, but I'd love to get a pro's advice.

Thanks very much!

 

Do you hear any change in sound of dash (duct work) when this happens?
Dodge used to install a check valve in the main vacuum supply line to your temperature control center, (it was up under the dash near the steering column If I remember correctly) that when it malfunctioned, would cause the airflow to be redirected, shifting back & forth etc, depending on acceleration etc. When you drive at a steady speed, or idle, vacuum is maintained enough to allow proper operation, but on acceleration, your vacuum drops off, and blend-air doors, etc, begin to move, back & forth. This small check valve , was the cure for that, which allows the vacuum at controls to be maintained.
About a 1" and $2.00 part that drove me crazy for a while. Just a thought.