Hydraulic power steering is very dependable, but complex in operation. When problems occur a basic understanding prevents many expensive mistakes.
Virtually all vehicles built today employ power steering. Electric steering is becoming increasingly popular, but for the foreseeable future, hydraulic steering is likely to remain prevalent. Hydraulic steering uses a pump, normally driven by a belt. The pump supplies pressure, and driver control is supplied by either the rack and pinion or a steering gear box.
Much of the system is similar, whether a rack and pinion or a steering box is used. Rack and pinion offers lighter weight and more precise control. Steering gears are more durable and tolerate rough treatment far better. This is why many SUV and truck applications use steering gear boxes.
With either system, fluid pressure from the pump is used to push against a piston. When the wheel is turned, pressure flows to one side and the piston moves. The piston is attached to the steering gears. Hydraulic pressure does the work, and the driver controls the direction by turning the steering wheel.
The key to operation is a very sensitive valve system. Both systems use a similar, torsion bar driven valve to direct and relieve pressure in the cylinder. When the steering wheel is not being turned, the steering valve is at rest. In this position fluid flows around the valve and out to the reservoir. Slight pressure also enters both sides of the assist cylinder. Because pressure is equal on both sides of the piston nothing occurs.
As the steering wheel is rotated, a very sensitive torsion bar twist and rotates the steering control valve. The valve blocks the port to the reservoir and fluid now flows through an opening to one side of the steering gear. At the same time the other side of the cylinder is vented to the reservoir. With pressure on one side and none on the other, the piston moves and causes the wheels to turn.
When the steering wheel is released the valve returns to neutral, pressure equalizes and turning of the wheels stops. Turned in the opposite direction, the port that was previously pressurized is vented to the reservoir. Fluid pressure is now applied to the opposite side of the steering gear and the vehicle steers the other way.
The most common problems with steering gears and rack and pinions are leaks and slack in the steering. The pump, pressure and return lines also leak and are replaceable. Leaking from the gear box or rack and pinion, most often is repaired by installing a rebuilt part.
Steering gears are often damaged by out of round and out of balance tires
When a vehicle shakes or shimmies, the force is absorbed by the steering gear. Out of round tires or tires that were improperly mounted and balanced cause damaging shimmy. This force will damage bearings that support the gears and slack develops. Slack is free play in the steering wheel and is very annoying to the driver. It makes the vehicle far more difficult to control.
Many things other than the steering box can also cause slack in the steering. Properly checking tie rods and steering linkages helps prevent mis-diagnosis. Slack in the steering box is when the input shaft can be moved back and forth and the output gear does not immediately turn.
Many people have been mislead into thinking the adjusting nut on top of the steering gear is to remove slack. This adjustment is meant to set the initial lash, between the gears, when the box is built. This adjustment will NOT remove slack. Tightening this adjustment will force the gears together and cause them to bind. Ironically, binding gears make steering far more difficult and actually feel even more loose.
Steering valves can also be worn or damaged. Lack of power assist in one or both directions can result. Often the steering will start to turn normally and then become very difficult.
Steering death wobble
Steering death wobble can also be caused by a damaged steering gear. Death wobble is when the steering wheel turns violently back and forth, often after hitting a bump. Death wobble is more common on vehicles that have been modified, such as larger tires and offset wheels, but can occur on stock vehicles also.
Death wobble quickly destroys suspension components. It is not unusual to find tie rods and axle mountings badly worn, on vehicles with this problem. Many times worn parts are mistaken for the problem, rather than the symptom. All parts are replaced and the wobble continues.
A worn or damaged power steering control valve, no longer able to center, floats from side to side and creates the wobble. The affect is like turning the steering wheel back and forth, at highway speed. The steering box is causing the problem, but is usually also a symptom of other issues. Proper repair involves correcting the root cause as well as replacing the worn components and the steering gear.
Cleaning and removing air when repairing hydraulic steering
As with any hydraulic system, cleanliness is imperative. Routinely replacing power steering fluid may prevent many problems. When a steering component fails, it is necessary to remove any debris from the system. Filtering machines are often used by shops for this purpose. If no such machine is available, removing the return hose and continuously pouring fluid into the reservoir is better than nothing.
Hydraulic steering will not operate properly if air is caught in the system. Air most often enters when there is a leak, allowing the reservoir to empty. Air may also enter through weak seals on the input side or during part replacement. With air in the system, there is often a characteristic whine or moan when the steering wheel is turned.
Some systems will self-bleed when the fluid is replenished. It may be very difficult to remove the air from other systems. The following procedure gives good results in most instances, without special equipment.
Power steering bleeding procedure
Fill the power steering reservoir with the proper fluid and leave the cap off. Raise the front wheels off the ground and support them. Without starting the engine, grasp one wheel and slowly turn it in until it stops. Wait a few seconds, and turn out until the wheel stops. Repeat this procedure about ten times, waiting several seconds between cycles. Top off the reservoir, replace the cap and start the engine. If noise is still present, repeat the procedure.
Power steering is like most things, prevention is the best policy. Regularly replacing power steering fluid and repairing leaks immediately will prevent most major problems. Selecting quality tires also has a big affect on steering components. Low quality tires are simply too expensive when the cost of damage they cause is considered.