Quote:
Originally Posted by MechEngVT
The cycling of the A/C compressor is based on the refrigerant pressure. When running fully cold with a low blower speed the evaporator will get very cold as there is little air blowing over it to warm it up. When this happens the refrigerant regains less pressure after passing through the evaporator, decreasing the pressure on the low side of the compressor. Some systems have a low-pressure cycle switch that shuts off the compressor to prevent freeze-up until the low-side pressure increases (which will happen as the evaporator warms). I'm not sure if many vehicles do this, but one could detect an increase in high side pressure that will correspond with the same events as the decrease in low-side pressure and control the compressor in that way.
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Modern cars cycle based on pressure of both high and low sides. They have a high side pressure switch, and a low side pressure switch. The low side pressure switch will not engage if the pressure is too low, and the high side pressure switch will not engage if the pressure is to high.
This also prevents the A/C from running if the coolant has leaked out!
-Steve