A high-pressure switch on the power rack and pinion or gear will be found on most fuel-injected vehicles. Its purpose is simple and straightforward. When the wheels are turned in a vehicle with power steering, the power steering pump requires additional power to increase the pressure to steer the wheels. This additional pressure has a tendency to decrease the engine rpm. At an idle or moving slowly, the rpm is low. Turning the wheel at this rpm would result in a rpm drop sufficient to stall the engine if not for the pressure sensor.
Computer Reaction To The Pressure Sensor
The computer senses the speed through the speed sensor. The power steering pressure sensor signals the computer when additional power is needed to limit the rpm drop associated with the demand placed on the power steering pump. The instantaneous signal alerts the computer of this impending rpm drop, and it increases the fuel injection on-time and the spark advance to head off the drop and maintain a smooth idle rpm regardless of the load. Without the sensor, the engine rpm would drop every time the wheels were turned, followed by a surge in rpm as the computer tries to bring the rpm back to within parameters. In essence, there will be a huge lag before the computer could correct the problem.
The sensor will also set a code if the computer has not seen the switch operate for over 40 seconds while the vehicle is moving at 50 plus mph. This code could mean the power steering reservoir is empty or the switch itself is bad or disconnected.
Power Steering Pressure Tester Location
The switch is located on the power steering gearbox on conventional steering vehicles. It is a small switch much like an oil pressure sensor. Either a deep well socket or a crowsfoot is needed to extract it from the gearbox.
On rack and pinion vehicles, the switch is located on the high-pressure line very close to the intake port on the rack and pinion unit.
Pressure Switch Operation
The power steering pressure switch has a spring-loaded diaphragm that is normally open when no pressure is applied to counteract the spring pressure. Every time the wheel is turned, high pressure builds in the line to the steering gear thereby counteracting the spring pressure and closing the switch. The computer provides power to the switch. When the switch closes it provides a ground for the computer, which is a signal of increased load.
Conclusion
The primary reason for the pressure switch is to allow a much quicker response by the engine computer to rpm fluctuations when turning at slow rpm and speed. A failure of the pressure switch will cause wide fluctuations in rpm and possible stalling when parking or turning while moving slowly with a low rpm. It will also cause dangerous surges as the computer is slow to react without the switch and the rpm will drop lower than normal before the computer recognizes the problem. It will overcompensate by raising the rpm to quickly to re-establish the pre-programmed idle setting.
