why does my ac compressor cycle on and off

Since the mid-1990s, most cars equipped with air conditioning have also been outfitted with electronic modules that tell your car s
to cycle on and off. Although it may be alarming the first time it happens at a stoplight or when you are idling at the store, it is very normal. The electronic module in your car s air conditioning system is informing your car s compressor that it can take a break because the car has reached the interior temperature you have chosen and the compressor is not needed at that moment to move the refrigerant around. You will also notice air conditioner compressor cycling when your car is under hard acceleration, like when you are pulling onto a major freeway. This is part of the programming that is built into the electronic system that takes all extra systems that might rob power from an engine under acceleration and redirects it to the. Again, this is normal operation.


If you do find that your car s air conditioner compressor is cycling on and off too quickly, then it likely indicates that your vehicle s electronic module is failing. Yes, it is easy to blame a mechanical system but your car s compressor system is engaged by the clutch and that, in turn, is driven by the ECU (electronic control unit) for the air conditioner/heating system. Why would this system fail? It could be temperature related. As systems age, they become more susceptible to failure, especially heat or cold-related failure. Although it isn t really advisable to look inside the module, if you could tear the cover off, you would see a bunch of tiny dark chips on the board with numbers on them. Some of these devices are really resistor packs that will change value over time depending on the car s operational environment (engines are not conducive to longevity for many devices because of the heat/cold cycle they run through).


Once a resistor pack (or thermistor it s temperature-controlled cousin) changes value, then the ECU signals that are supposed to happen at certain points may change, leading to an alteration in your car s compressor cycles. The solution to this problem is to take your car to a service area where they have the computerized equipment that will tell them the modules that have failed and need replacement. If you are a bit strapped for cash, you can find an aftermarket replacement module, purchase a service manual, and try to replace it yourself instead. This is only advised if you have at least some experience working with car computer systems or you could cause even more damage. Since it is better to let the professional handle this it is their responsibility to make it work let your service area take care of replacing the module after they ve identified the problem.


They will have the right part on hand and can get you on your way quickly. Best of all, you will not have to think about it as the work is finished and the cycling should have stopped. If not, bring it back and they will have to fix it free of charge. 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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