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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.

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    The internal refrigerant pressures determines when it turns on and off. There are high pressure and low pressure switches located in the lines which read this and then lets the computer know when to turn the compressor on and off.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 13, 2016 at 0:19
  • @Pᴀᴜʟsᴛᴇʀ2 Well, at least it is functioning, and not destroying itself. I guess I will have it checked.
    – user15009
    Jul 13, 2016 at 0:34
  • What's the year and model?
    – Ben
    Jul 13, 2016 at 23:06

2 Answers 2

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There's probably too much system pressure. Ambient temp has an effect on A/C system pressure. If the high side goes too high than it trips a switch and cut's power to the A/C compressor clutch. Hence the short cycling.

I'm guessing you're on the mid to high side of 300psi on the high pressure line and when ambient temp goes up pressure goes above 400psi tripping the high pressure switch.

Have a shop check your pressures when it's > 100°F.

Also on a car that old there may not be an Evaporator temperature sensor. On some modern cars this sensor will signal a shutdown to ECM/PCM to turn off the compressor clutch when the evaporator temperature goes too low. To prevent freezing.

On cars with automatic A/C if the ambient temperature sensor isn't working (broken wires/short whatever) It'll prevent the ECM/PCM from operating the compressor clutch.

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  • I added refrigerant myself about a year ago, and it was low then. This year I checked it (with the nearly empty bottle I still had) and it registered normal pressure. But that was a cool day. Could I have overcharged it? There is a slow leak, but it held pressure enough to keep operating since last year. I didn't know it was possible to overfill it with the cheap bottle/hose kit you can buy at the auto parts shop.
    – user15009
    Jul 13, 2016 at 0:31
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    It's all too possible. The gauges that come with those bottles don't paint the whole picture in the A/C system. I ran into a car this morning that was doing the same thing. Low side pressure was just above 30psi and high side around 275psi @ roughly 65°F. Had the guy come back later in the day when it was 95°F the compressor was short cycling. Bled some pressure off and sent him on his way.
    – Ben
    Jul 13, 2016 at 0:36
  • @nocomprende there is such a thing as too much refrigerant. I believe Ben is spot on
    – Zaid
    Jul 13, 2016 at 1:11
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    @Ben is always spot on. He's an experienced and knowledgeable professional.
    – cdunn
    Jul 13, 2016 at 16:39
  • @ben Subarus of that era do have evaporator temp switches
    – Fred Wilson
    Jul 14, 2016 at 6:18
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Ambient temperature influences the pressure in refrigeration system. Cars a/c may have two sensors, one in the low pressure line and one in the high pressure line, that controls when to stop the compressor if some abnormality in the pressures are sensed. It is possible that your system is overfilled, since underfilled may also create ice in your evaporator (inside the car).

I don't think it would be the temperature control, since otherwise it works on less hot days.

Your car service manual may say something about this, but for instance, for a 4 doors "normal transportation" car a/c compressor, the low side pressure for 100F, 2000 rpm, should be between 31 to 42 PSI (20 to 90% humedity)

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