190 Auto Suspension and Steering
are turned to lock (their maximum turning position in
either direction), the power steering pump produces maxi-
mum pressure. This pump pressure increases the load in
the engine. When maximum pressure is reached, the pres-
sure switch diaphragm deflects enough to close the con-
tacts and the switch sends an electrical signal to the ECM.
The ECM then compensates for the extra load by raising
engine idle. On some vehicles, the ECM may also deacti-
vate the air conditioning compressor when signaled by the
pressure switch.
Power Steering Coolers
Pressurizing the power steering fluid creates heat. If
this heat is not removed, it will overheat the fluid, causing
oxidation and sludge formation. Excessive heat will also
cause hardening of the hoses and other rubber parts. Power
steering systems used on smaller vehicles generate less
heat than those used on larger vehicles, and the fluid
temperature does not become dangerously high. Many
larger vehicles have enough airflow through the engine
compartment to remove excess power steering system
heat.
However, some vehicles develop high power steering
system temperatures and have cramped engine compart-
ments with low airflow. On these vehicles, an oil cooler is
used to prevent fluid overheating. Some oil coolers are
simply extra lengths of aluminum tubing installed on the
bottom of the frame where air can pass over them, as
shown in Figure 9-51. Other coolers are finned heat
exchangers installed ahead of the radiator, Figure 9-52.
Rack-and-Pinion Power Steering
A rack-and-pinion power steering gear is similar in
design to a conventional system. A self-contained
power piston is added to the rack assembly. The power
piston creates two pressure chambers in the rack-and-
pinion housing. A control valve is installed in the pinion
section. The operation of each of these units is explained
in the following sections.
Pressure passages may be built into the rack assembly,
or the unit may have external piping to deliver the pressure
where it is needed. Seals are more complex and stronger to
seal under power steering pressures and to hold in the thin-
ner power steering fluid.
Power Piston
The rack-and-pinion power piston is installed inside
the rack housing and is directly connected to the rack
assembly, Figure 9-53A. Seals prevent pressure from
crossing between the two sides of the piston. When the
vehicle is not being turned, there is no pressure on either
side of the power piston.
When the driver begins a turn, pump pressure is
directed to one side of the power piston. This causes a
pressure differential, meaning that there are unequal
amounts of pressure on each side of the piston. The piston
will move toward the side with the least pressure, taking
the rack with it. This provides the power assist. Fluid on the
side of the power piston that is not pressurized is exhausted
into the pump reservoir.
During a left turn, pressure enters the left turn side of
the power piston, Figure 9-53B. No fluid pressure is
delivered to the right side. This causes the power piston to
move the rack, assisting with steering effort in that
direction. During a right turn, Figure 9-53C, fluid pressure
is delivered to the right turn side of the power piston,
causing it to move.
Control Valve
The power steering control valve is called a rotary
valve. It is composed of a spool valve inside a valve body.
Since the valve body turns in the rack-and-pinion gear
housing, it is equipped with sealing rings. The turning effort
from the steering wheel passes through the valve assembly
Fan shroud
Oil cooler
Steering
gear
Return
hose
Pump
Figure 9-51. A power steering fluid cooler pipe assembly
is commonly mounted to the vehicle frame. Styles vary.
(General Motors)
Reservoir
hose
Pump
pulley
Power
steering
pump
Inlet
hose
Outlet
hose
Bumper
support
plate
Finned cooler
assembly
FRONT
Cooler outlet
hose
Figure 9-52. A finned heat-exchanger unit. Study the construc-
tion and layout of the parts. (General Motors)