2004 Volvo V70 General A/C Question

  • 2004 VOLVO V70
  • 89,000 MILES
I understand an auto a/c compressor runs at it's designed constant maximum ouput regardless of the outside temperature or the temperature control setting inside the car. I also understand the inside temperature is controlled by the temperature setting controls. If the temperature is set to maximum cool there is no mixing of outside air with the inside air. If set to less than maximum coolling only then will some outside air be mixed in. Please confirm if my logic is correct.
Do you
have the same problem?
Friday, August 30th, 2013 AT 10:23 AM

1 Reply

Not exactly sure what you're asking but you have kind of the right idea. Most AC systems have a "Max AC" that blocks fresh outside air from coming in. It may also be called "Recirculate". That is meant to cool the same air over and over to get the cabin temperature down quickly right after you enter the car. That only takes a minute or two, then you switch to the regular mode which brings in outside fresh air. Cooling that air condenses the humidity on the cold evaporator in the dash. That water drips outside. The goal of all AC systems is to lower the air temperature by only 20 degrees, although in actual practice they do a better job than that. The main comfort comes from lowering the humidity.

The controls in the system are going to keep the evaporator temperature near 40 degrees Fahrenheit. Any colder than that and that condensing humidity will freeze into a block of ice and block air flow. The customer controls allow you to choose anywhere between that 40 degrees and whatever the outside temperature is. That's your half of the system. If you're too cold, you move the adjustment to allow a larger percentage of uncooled air in.

The compressor side of the system is concerned with "heat load". That has to do with how hard it has to work to maintain the evaporator at 40 degrees. On hotter days that hot air will warm up the evaporator quickly and more refrigerant will have to circulate through it and evaporate in it to keep it cold. That refers to the volume of refrigerant pumped. On cooler days a lower volume must be pumped to maintain that 40 degrees. There's a number of ways to control that volume which leads to part of what you were asking. On older systems they monitored the refrigerant pressure which very closely matched its temperature, to cycle the compressor on and off. The refrigerant had to squeeze through a tiny orifice where it expanded and became very cold in the evaporator. Temperature was sensed there and the size of the orifice was adjusted to control the flow rate to maintain the 40 degrees. There was a temperature switch on the pipe leaving the evaporator that turned the compressor off when the temperature got too cold. Once the temperature rose enough to prevent that condensation from freezing, the switch and compressor would turn back on. That was how the system regulated the evaporator temperature.

In that story you were right about the compressor always pumping to its maximum ability, but regulation was done by turning it on and off. Today people are so concerned with a tiny wind noise over a wiper arm, or not enough cup holders, and all the other silly things to complain about, and the cycling on and off of the compressor was another one. Years ago we were thrilled to have cool air and that cycling was a welcome "so what". Today, to address the whiners, the engineers came up with a compressor design that is much less reliable but it does not cycle off. It runs continuously, and to adjust the volume, they use a variable "wobble plate". That's the plate that pushes the pistons back and forth. On older compressors that plate was welded to the shaft at a specific angle. Each piston took the same length stroke with every revolution of that plate. On the newer ones the angle of that plate varies and the stroke of the pistons can vary from almost no volume to maximum volume. The angle of the wobble plate is adjusted by the pressure in the line leading out of the evaporator. When the pressure goes up, indicating less cooling is taking place in the evaporator, that higher pressure pushes on the wobble plate to increase the length of the stroke the pistons take, and the compressor starts to pump more volume. You don't feel the change in load on the engine as the compressor adjusts.

So you're actually still right that compressors will pump at their full volume, but it's that full volume that changes as necessary to maintain 40 degrees in the evaporator.
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Friday, August 30th, 2013 AT 1:38 PM

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