RACK & PINION STEERING
Rack & pinion steering is used on almost all front-wheel drive cars and minivans, as well as many rear-wheel drive cars. Most
have "power-assisted" steering.
A power rack is nothing more than a manual rack with hydraulic muscle. Around the rack bar is a cylinder which is divided into
two chambers by a piston seal. Pressure is routed into one chamber or the other via a spool valve on the pinion input shaft.
Turning the steering wheel opens ports in the valve that routes pressurized fluid into one side of the rack. The fluid pushes
against the rack piston and helps steer the wheels in the desired direction.
Operating pressures within a power rack generally do not exceed 100 psi when the wheels are straight ahead. In an easy turn,
pressure can increase to 300 psi -- and up to 700 psi in a tight turn. The highest pressures are usually encountered when
parking. If the wheels are up against a curb or if the steering wheel is turned hard against the stop, it can climb to 700 to 1,400
VARIABLE ASSIST STEERING
A growing number of import and domestic racks have a new feature called "variable assist." The effort required to steer the
wheels changes with the vehicle's speed and/or steering input. Maximum assist is provided at low speed when extra muscle is
needed for parking. At higher speeds, the amount of assist is reduced up to 50% or more with a solenoid or bypass valve to
improve road feel and steering stability.
Great as they are, no rack lasts forever. Two common problems that afflict power racks are "center wear" and "morning
sickness." Center wear can allow steering wander when driving straight ahead. It's caused by internal fluid leakage around the
rack piston due to wear in the rack piston housing. Morning sickness is a condition where the steering feels stiff when the car is
first started on a cold morning. Normal power assist doesn't return until the car is driven awhile and warms up. It's caused by
wear in the spool valve housing.
Another problem is fluid leakage. The presence of fluid in either bellows is bad news because it indicates an internal seal leak.
Fluid can also leak around the pinion input shaft.
The appearance and condition of the power steering fluid in the pump reservoir can reveal a great deal about the condition of a
rack. Dark fluid would indicate oxidation or contamination from dirt or metallic debris. A low fluid level would indicate a leak that
needs fixing (possibly a hose or the rack itself).
A rack problem doesn't always mean the rack has to be replaced. Minor problems such as loose inner tie rod sockets, split or
torn bellows, or deteriorated or loose rack mounts are obviously items that can be repaired and do not require replacing the
rack. On some vehicles where the rack is easy to reach, repairs can often be made without having to remove the rack from the
Replacement is recommended, however, when a rack has leaks or internal wear.
Seal repair kits are available for overhauling racks, but they are not very popular because overhauling a power rack is not a job
for a novice. Special tools (which are expensive) are needed to remove and install the internal seals. A mistake here can result
in a leaker -- which is why most remanufactured racks are pressure tested before they leave the factory. Some racks also can't
be rebuilt because of internal wear. So most installers buy new or remanufactured racks rather than waste a lot of time trying to
overhaul racks themselves. Most new and remanufactured racks are guaranteed.
Two types of replacement racks are available: long and short. Long racks come completely assembled and ready to install with
new inner sockets, outer tie rod ends, bellows and mounts. Short racks, which don't include the tie rods and bellows, may be a
more affordable alternative for applications where the inner sockets and tie rod ends are in good condition and can be reused.
When a rack is replaced, the system should be flushed to remove all traces of the old power steering fluid. This will prevent
fluid contamination that could shorten the life of the replacement rack. The system also needs to be purged of air by cycling the
steering slowly back and forth until there are no air bubbles in the fluid.