chap91 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 91, the reader should
be able to:

•
•
•
•

Prepare for ASE Suspension and Steering (A4) certification
test content area “A” (Steering Systems Diagnosis and

Repairs).
Discuss the components and operation of power steering
pumps.
List the components of a typical power-recirculating-ballnut steering gear system.
Describe the operation of a power rack-and-pinion steering
system.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
BPP • cold climate fluid • EHPS • EPS • EVO
flow control valve • force • integral reservoir • magnasteer
pascal’s law • poppet valve steering gear • pressure • pressurerelief valve • PSCM • PSP

remote reservoir • rotary control valve
self-park • SPS • SSS • TFE • VES

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Power-assisted steering hydraulically boosts the mechanical
steering gear operation so the driver
can turn the steering wheel with less effort for the
same response.
Hydraulic power steering has been available since
the 1950s, and many late-model systems are
enhanced by electronic controls.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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POWER STEERING HYDRAULIC SYSTEMS
Hydraulics is the study of liquids and their use to transmit force and
motion through the use of fluid pressure.

Force is a push or pull acting on an object and is usually
measured in pounds or Newtons. Pressure is force applied to a
specific area. Pressure is usually measured in force per unit of
area, such as pounds per square inch (PSI), or kilopascals (kPa).
One PSI equals 6.895 kPa.
The Pascal is a unit of measure named after the French scientist
Blaise Pascal (1623–1662). One of his discoveries, known as

Pascal’s law, was that pressure on a confined fluid is transmitted
equally in all directions and acts with equal force on equal areas
See Figure 91–1.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Hydraulic systems transmit
force and motion through
liquids because a liquid
cannot be compressed.

Figure 91–1 Hydraulic fluid transmits the
same force whether it passes through a single

chamber or two chambers connected by a
narrow passage.

No matter how much pressure
is placed on a liquid, its
volume remains the same.
This allows a liquid to transmit
force much like a mechanical
lever.

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Figure 91–2 A fluid applies a force equal to the applied force on a surface that is equal in
size to the applying surface. If the surface is half the size, then the fluid exerts half the
force: if the surface is twice as large, the fluid exerts twice the force.

Because it assumes the shape
of its container, a liquid can
transfer force around obstacles or
through pipes and passages
of any shape.

As explained by Pascal’s Law,
a liquid can also decrease or
increase the force it transmits
depending on the area of the
output surface to which the
force is applied.
Continued
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POWER STEERING PUMP AND RESERVOIR
The power steering pump draws fluid from the reservoir,
pressurizes it, and delivers it to the power steering system. The
fluid reservoir may be either integral to (built into) the pump or
remotely mounted and connected to the pump by a hose.
The power steering fluid reservoir is usually made of either plastic
or stamped metal, and it includes the fluid filler neck, cap, and
dipstick. An integral reservoir is part of the pump, and the pump
itself operates submerged in power steering fluid.
See Figure 91–3.

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Once common, steering pumps with an integral reservoir have given
way to those with a remote reservoir on current-production vehicles.
The remote reservoir allows for a smaller, more compact pump
assembly better suited to a cramped engine compartment.
Figure 91–3 A typical
integral power stering
pump when the pump is
mounted in the reservoir.

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A remote reservoir is a separate assembly from the pump and
provides fluid to it through a suction hose. Most power steering
systems use an engine-driven hydraulic pump. Power steering
hydraulic pumps are usually belt driven from the front crankshaft
pulley of the engine.

Figure 91–4 Typical remote reservoir.

Figure 91–5 Typical integral power
steering pump assemblies.

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The power steering pump delivers a constant flow of hydraulic
fluid. A typical pump requires less than 1/2 horsepower, which is
less than 1% of engine power while driving straight ahead. Even
while parking at low speed, the power steering requires only about
3 horsepower while providing high hydraulic pressures. Typical
pressures generated by a power steering system include:
Straight ahead less than 150 PSI ( 1,400 kPa)
Cornering
about 450 PSI (3,100 kPa)
Parking
(maximum) 750–1400 PSI (5,200–10,000 kPa)
The power steering pump drive pulley is usually fitted to a chromeplated shaft with a press fit. The shaft is applied to a rotor with
vanes that rotate between a thrust plate and a pressure plate.
See Figure 91–6.
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Figure 91–6 General Motors vane-type pump.

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Some power steering pumps are of the slipper or roller design instead of the vane type. When the engine
starts, the drive belt rotates the power steering pump pulley and the rotor assembly inside the power steering
pump.
With a vane-type pump, centrifugal force and hydraulic pressure push the vanes of the rotor outward into
contact with the pump ring. The shape of the pump ring causes a change in the volume of fluid between the
vanes.
As the volume increases, the pressure is decreased in the space between the vanes and draws in fluid from
the pump reservoir. When the volume between the vanes decreases, the pressure is increased and flows out
the pump discharge port.
See Figures 91–7, 91–8, and 91–9.

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Figure 91–7 Vane pump operation. In phase 1, the rotor moves past the opposed suction ports,
and the vanes move out to maintain contact with the ring. This creates a low-pressure area,
drawing fluid into the cavities formed by the vanes. As the rotor continues to move during
phase 2, the vanes follow the contour of the ring. The contour of the ring forms a larger cavity
between the vanes. This increases the suction and draws more fluid into the pump.

Continued
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Figure 91–8 Vane pump operation—continued. At phase 3, the vanes are at the end of the
intake port of the pump and the cavity has reached its maximum volume. In phase 4, the rotor
moves into alignment with the opposed discharge ports.

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Figure 91–9 Vane pump operation—continued. As the rotor continues to move during phase 5,
the volume of the cavity decreases, which increases the discharge pressure. At phase 6, the
last phase, the contour of the ring results in the minimum cavity volume, and the discharge of
fluid is completed.

Continued

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The pressure outlet hose connects to a fitting that threads into the
outlet port of the pump body. A modulator valve, commonly called
a flow control valve, installs in the same bore as the hose fitting.

Figure 91–10 Flow control valve.
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The modulator valve is a spring-loaded pressure-relief device
that bleeds off excess pressure to prevent system damage.
When pump output is more than the power steering system
requires, excess pressure overcomes spring force and moves the
valve down in its bore. This uncovers an orifice through which
the fluid can flow back into the inlet passages.
An orifice is a small opening that regulates fluid pressure and
flow. It can be a restriction in a fluid line or a hole between two
fluid chambers.
This particular opening is called a variable orifice because the
size of the opening varies with the amount of pressure applied
to the valve.
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Because the engine drives the pump, the power steering pump
places a load on the engine. When the steering wheel is turned to
or near full stop for more than a few seconds, pressure builds in
the system and the pump must work harder to keep up with the
demand.
The pump draws more power from the engine. If the engine is
running at idle, the extra load can cause it to stall. A pressure
switch, known as the power steering pressure (PSP) switch,
transmits a signal to the powertrain control module (PCM).
In response to the PSP switch signal, the PCM increases the engine
idle speed to prevent stalling.

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To handle excess pressure, a pressure-relief passage runs from a
point near the pressure hose fitting to the spring end of the
modulator valve, and contains a pressure-relief valve.
The parts of the pressure-relief
valve are the following:

Spring
Valve piston

Check ball
Seat

Normally, spring force holds the piston against the check ball to
keep it in its seat. When pressure becomes excessive in the
pressure-relief passage, the check ball raises off its seat to allow
fluid to flow through the relief passage and out through an orifice
in the modulator valve.
When power steering systems are pushed to the limit, most will
make a chattering or squealing noise until the pressure is reduced.
See Figure 91–11.
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Figure 91–11 The pressure-relief check ball unseats, allowing fluid to flow back into the pump
inlet if the pressure rises above a certain limit.

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POWER STEERING HOSES
The power steering pump and the steering gear are not part of the
same assembly. The system requires two hoses to connect the
power steering pump and gear assembly.
Figure 91–12 The power steering fluid cooler,
if used, is located in the return hose.

The pressure hose is connected
to the flow control/relief valve,

and supplies pressurized fluid to
the steering gear.
Some vehicles will use a cooler
in the return path to the pump,
to reduce the temperature of the
fluid before it enters the pump.
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INTEGRAL POWER STEERING
In an integral power steering system, the control valve and the
power piston are incorporated into the steering gear construction.
The control valve regulates the application of pressurized fluid
against the power piston, and the power piston helps move the

output member of the steering gear when pressure is applied to
one side of it.

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Integral Standard Steering Gear Most standard steering gears
with power assist are the recirculating-ball type, and the ball nut
functions as the power piston.
Hydraulic pressure from the steering gear control valve is applied
directly against the ball nut to help move it through the housing.
The ball nut in this type of power steering gear is called the power
piston because hydraulic pressure moves it as if it were a piston

traveling in a cylinder.
See Figure 91–13.

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Figure 91–13 Forces acting on the rack piston of an integral power steering gear.

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