What Does the Power Steering Pressure Switch Do?by Derek Powlison
The power steering pressure switch lurks quietly under the hood until a problem develops with a vehicle, such as a power steering leak requiring line replacement. If the vehicle idles poorly and the check engine light turns on, a check of the diagnostic codes may reveal an error with the power steering pressure switch. The power steering pressure switch feeds information about the demand on power steering to the engine control module.
The power steering system's fluid is split into two sides. The high-pressure side feeds fluid from the power steering pump to the power steering gearbox or the power steering rack. Most heavy-duty trucks and many older cars were designed with a power steering gearbox, but front wheel drive vehicles and most new light duty trucks utilize a power steering rack. The fluid returns through a low-pressure line back to the pump reservoir, where the cycle repeats. The power steering pressure switch is always installed on the high-pressure side of the system.
When the steering wheel and front tire positions do not match, the power steering fluid pressure pushes the tires in the desired direction, and the pressure in the power steering system spikes higher. The power steering pressure switch contains a diaphragm that actuates a contact switch. A spring pushes outward on the diaphragm keeping the switch normally open. The switch closes as the pressure of the power steering fluid on one side of the diaphragm rises high enough to overcome spring pressure. Once the front tires match the steering wheel position, the steering gear redirects the fluid back to the power steering pump and the tires no longer move. The system pressure drops, the pressure no longer overcomes the spring pressure on the diaphragm, and the switch opens.
The power steering pressure switch feeds information about demand on the power steering system to the vehicle's computer. At low speed, such as during parking lot maneuvers, the engine produces little power. Instead of waiting for the engine speed to decrease from the increased demand on power steering, the vehicle computer can compensate immediately for the increased load demand to keep the engine running smoothly.
When the power steering pressure switch indicates the system is always on or always off, the engine control computer illuminates the check engine light to the problem. During low speed parking lot maneuvers or when the engine is in idle, turning the steering wheel may cause the engine to bog down, and then surge as the computer overcompensates for the load. In some cases, the vehicle may stall at low speed when the wheel is turned, as the engine computer did not know there was a sudden demand for power and could not compensate fast enough.
- Alabama Cooperative Extension System: "Functional Features of the 1988 GM P-4 2.0 Turbo PFI System"
- "Bosch Fuel Injection and Engine Management: How to Understand, Service and Modify"; Charles Probst; 2003
Derek Powlison has over 13 years experience in the automotive aftermarket industry and has written automotive and business articles since 2007. He is an ASE-certified Master Automotive Technician and service writer.