I have a 20 year old Subaru wagon, and the A/C seems to work fairly well except when it is extremely hot outside (> 100 F / 40 C - which happens a lot in the summer here). What I wonder about is that the compressor cycles on and off regularly even when it is quite hot inside the car. I would think it would stay on until the interior of the car cools down?
The blower has 4 settings: on 1, the air from the vents is very cold, but there is not enough circulation to be useful, so I would expect the compressor to cut off sometimes then. On 2 it has a good balance of cold air and movement, but still cycles on and off. On 3 the air is not as cold, but it still cycles on and off. It is loud and I usually do not run the blower on 3. On 4 it is a howling tepid wind and I cannot tell if the compressor cycles then, but I do not use it on that setting unless it is outrageously hot in the car. I keep the temperature slider all the way to the cold side at all times except perhaps in January.
So, what controls the compressor cycling, since it does not seem to be very dependent on the air temperature inside the car? Is it measuring the temperature directly at the cooling core?
Addition: Is there any actual temperature sensing in a car this old? If the A/C system pressures were correct, would the compressor simply run continuously? My understanding is that the temperature slider is not a thermostatic device, it just allows air to blow through the heater core.