Car Power Steering Pressure Switch
On vehicles with small capacity engines and power steering, the power steering system has little effect on the performance of the engine under normal driving conditions. But during parking maneuvers, the power steering system imposes its heaviest loads on the engine. When parking, the engine speed and power output are usually low, so an air conditioning compressor is likely to impose even further demands on the engine, and can even cause rough idling and stalling.
To prevent these problems, some vehicles are equipped with a power steering pressure switch, which disengages the compressor whenever the power steering hydraulic pressure exceeds a predetermined level.
Some vehicles are equipped with a more sophisticated version of this system in which the switch sends a signal to the ECU or PCM, which adjusts the engine idle speed to compensate for the higher steering load without disengaging the compressor.