Pontiac FIREBIRD 1972 Service Manual page 1169

9-16 1972 PONTIAC SERVICE MANUAL
The higher the resistance to turning between the roadbed
and the front wheels, the more the valve spool is displaced,
and the higher the oil pressure on the upper end of the
rack-piston nut. Since the amount of valve displacement
and, consequently, the amount of hydraulic pressure built
in the cylinder is dependent upon the resistance to turning,
the driver is assured of the proper amount of smooth
hydraulic assistance at all times.
As the driver stops applying steering effort to the steering
wheel and then relaxes the wheel, the spool valve is forced
back into its neutral position by the untwisting of the
torsion bar. The spool valve lands and grooves align them-
selves with the grooves and lands in the valve body, prov-
iding a balanced clearance between the land-groove
alignment. When this happens, the oil pressure is again
equal on both sides of the rack-piston nut and the steering
geometry of the car causes the wheels to return to the
straight-ahead position.
CHECK VALVE POPPET
The check valve is located in the pressure port of the
housing under the connector. The valve consists of a pop-
pet and a spring and its purpose is to reduce the possibility
of steering wheel "kick-back". If when making a turn, the
front tire hits a bump which forces it in a direction oppo-
site the turn, the impact will be carried up to the rack-
piston nut by the pitman shaft. If the force is great enough,
the rack-piston nut will tend to move against the applied
oil pressure and force oil back through the valve assembly
and out through the pressure port where the poppet valve
is now located. If the rack-piston moved in the opposite
direction, the steering wheel would resist momentarily or
would "kick-back". The poppet valve is designed to pre-
vent the above action from occurring by trapping the oil
inside the gear.
POWER STEERING PUMP
DESCRIPTION
Hydraulic pressure for the steering system is provided by
a constant displacement vane type pump. It is attached to
the left front of the engine by brackets and is belt driven
from an engine crank shaft pulley.
The housing and internal parts of the pump are inside the
reservoir (Fig. 9-14) so that the pump parts operate sub-
merged in oil. The reservoir has a filler neck fitted with
a vented cap. The reservoir is sealed against the pump
housing, leaving only the housing face and shaft hub ex-
posed. A shaft bushing (bronze on steel) and seal are
pressed into the housing from the front. The drive shaft
is inserted through this seal and bushing.
A large hole in the rear of the housing contains the func-
tional parts; namely pump ring, rotor, vanes and plates
(Fig. 9- 15). A smaller hole contains the control valve as-
sembly and spring.
THRUST PLATE
The thrust plate (Figs. 9-14 and 9-15) is located on the
inner face of the housing by two dowel pins. This plate has
four central blind cavities for undervane oil pressure. The
two outer blind cavities direct discharge oil through the
two cross-over holes in the pump ring (Fig. 9-16) through
the pressure plate, and into cavity 1 (Figs. 9- 17, 9- 18 and
9- 19). The two outside indentations in the thrust plate are
for intake of the oil from the suction part of the pump.
PUMP RING
The pump ring (Fig. 9-16) is a plate having the mating
surfaces ground flat and parallel. The center hole is two
lobed cam in which the rotor and vanes operate. The ring
is placed next to the thrust plate, and located with the
same dowel pins.
PRESSUREPLATE
The pressure plate is fitted against the ring and located
with the same-two dowel pins. This plate has six through
ports. The four central through ports connect from cavity
1 (Figs. 9-17, 9-18 and 9-19) to supply undervane oil
pressure. The two outer ports pass oil under discharge
pressure to cavity 7. The two indentations are for oil in-
take from the suction part of the pump, cavity 7 (Figs.
9-17, 9-18 and 9-19) into the rotor.
RESERVOIR AND OIL LEVEL INDICATOR
The reservoir is for oil storage. It receives and directs the
return oil back to the make-up passage of the pump.
"Hot" and "cold" marks appear on the reservoir filler cap
oil level indicator. Fluid level should be at the "cold"
mark when the pump and fluid are at normal room tem-
perature, or approximately 70°E If fluid level is checked
with fluid at room temperature, the pump is underfilled
when the fluid level is below the "cold" mark.
The proper fluid level must be maintained to prevent
foaming conditions in the power steering hydraulic pump.
This is especially important in the colder regions of the
country when diagnosing noise due to oil foaming. In most
cases, this condition is caused by low fluid level in the
pump reservoir.