The power-steering pump is beltdriven off the engine and is what provides fluid pressure to assist the mechanical effort of the steering box. This is what allows you to turn the tires with a single finger on the steering wheel.
Such pumps are a rotary-vane design. That is, a set of sliding metal vanes spins on a rotor shaft, and the vanes move inside an elliptical chamber. This action pressurizes the fluid being pumped from the steering box.
The pump is designed to output a fixed fluid volume for each revolution of the pump. As such, the faster the engine speed, the greater the output volume. Pumps are designed to provide adequate pressure (approximately 1,000 psi) and volume for assisted steering at low engine speeds, and this means they provide excessive flow at high engine speeds. A relief valve is used to reduce the output pressure at higher engine speeds.
Whenever a power-steering system fails to operate properly, check the easy things first.
Check the fluid level in the reservoir and check the drivebelt tension. Fluid leaks at the pump may signify loss of pump pressure, resulting in poor steering boost. Obviously, a leaky high-pressure hose can cause the same problems.
Should you find the pump leaky or otherwise at fault it can be replaced, or it is possible in many cases to rebuild it if kits are available. These usually consist of seals and gaskets, and may also include a shaft bearing. For best results, take care in cleaning all parts and assemble in a clean work space.
Anytime a steering box, pump, or hose is removed from the system, air will enter it. You'll need to make sure to bleed these pockets of air from the fluid path after the component replacement. We have seen several recommended methods for bleeding air from the power-steering system, so we suggest you follow the directions included with the replacement part.
Some manufacturers suggest you fill the reservoir with fluid and then run the engine for short periods of time and, working the steering wheel back and forth, refilling in between runs. Others suggest you jack the front tires off the ground and manually steer the tires back and forth without the engine running to work fluid into and air out of the system to prevent pump damage on engine start-up.
Check your vehicle manual or the instructions that come with your new pump. Many pump manufacturers specify that you use only certain types of power-steering fluid and not use auto-transmission fluid, as it may not contain needed additives and friction inhibitors.
If you're running large tires or need more power assist, there are aftermarket pumps available that provide greater fluid flow and pressure. These can also be combined with specially modified steering boxes for even greater help turning big meats.
From the factory, many vehicles have a small steel-tube loop in the fluid path that serves as a mini-cooler. If you're working your steering aggressively, you may want to consider a supplemental inline cooler to increase fluid capacity and cooling.