"MAX" A/C

[kok328]

Looking at my HVAC schematic for my 92 lesabre, I'm seeing that high speed is a direct 12V connection through the high speed relay.

My question is why does the fan run faster in MAX A/C mode than say VENT or HEAT mode on high setting?

Also, am I correct in assuming that MAX A/C locks the compressor in unless a high pressure situation occurs, versus normal cycling in NORM A/C mode?

[HotZ28]

The fan does not run faster in max, than in vent or heat when it is set on high speed, it just redirects the air using the blender doors. You are incorrect in assuming that max A/C locks the compressor verses normal mode. Cycling is a function of the low side pressure switch. If you run the normal mode, you are blending some outside air through the system; therefore your pressure fluctuates more than recirculating inside air, (constant & somewhat cooler temperature). Also, running the fan at a lower speed will cause more pressure fluctuations on the low side due to less air flowing through the evaporator. The high side pressure switch will only shut down the compressor in extreme cases, when the high side reaches near 300 psi. Normal operating temperature and operating pressure conditions, would never allow the use of the high pressure switch, much less use it to cycle.

Older cars (such as using the infamous GM R4 compressor), have a switch in the low pressure side of the system which opens at about 24 PSIG, cutting off the compressor. The evaporator warms up, and when the pressure reaches around 40 PSIG, the switch closes again, restarting the compressor. This method causes a lot of wear and tear on the compressor clutch. Newer cars, have variable displacement
compressors, which run continuously instead of cycling off and on. They have a swash plate, instead of a crank shaft, to anchor the pistons. The stroke of the pistons in this type of compressor may be varied from maximum to almost zero, by varying the angle of the
swash plate. A GM V5 is a common example of this type of compressor. These compressors usually have an internal mechanical arrangement to control the swash plate based on the suction (low side) pressure. They are usually set for about 28 PSIG. When the car is hot, the suction pressure will be much higher than 28 PSIG, so the compressor operates at full stroke. The car cools down, the suction pressure drops down through 28 PSIG, and the swash plate moves, reducing the stroke of the pistons, and lowering the capacity of the A/C, and the suction pressure will stop dropping, and stay at around 28 PSIG. The compressor will continue to reduce displacement by maintaining the suction line at 28 PSIG if the car cools down too
much. If the sun comes out, and the car begins to heat up, the suction pressure will rise above 28 PSIG, and the compressor will increase it's displacement to bring the pressure back down to 28 PSIG.