GMC 4000 Series Service Manual page 504

From cavity (1) the oil flows into passage (2)
which is controlled in size to provide definitely
known oil velocities. From passage (2) a certain
quantity of oil passes through orifice (3) into p ass­
age (8) and then to the steering gear. Notice that
passage (8) is connected to cavity (10) by passage
(9). When the quantity of oil exceeds the predeter­
mined system requirements, the pressure drop
through orifice (3) exceeds force of spring (12),
flow control plunger (11) starts to move back,
thereby providing flow control through passages
(4), (6) and (7), with (7) being the suction or intake
part of the pump. Supercharging occurs as a result
of pressure oil in passage (4) discharging into
passage (6) at high velocity, picking up the make­
up oil from reservoir through passage (5) on the
jet-jump principle. Then by a reduction of velocity
in passages (6) and (7), velocity energy is convert­
ed into supercharge pressure (fig. 6, View B).
The pressure relief valve is contained inside
flow control plunger (11). If pump pressure ex­
ceeds a certain predetermined pressure, the p res­
sure relief ball will open, allowing a sm all amount
of oil to flow through passages (8) and (9), hole
(13), through flow control plunger (11) and into
passage (6). This flow of oil causes a pressure
drop across hole (13), thus creating a pressure
unbalance which moves the flow control plunger
back against spring pressure, allowing the major
portion of oil to bypass through passages (2), (4),
and (6) in the same manner as is accomplished by
flow control (View B, fig. 6).
Figure 6, View "A" is typical of pump oper­
ation when the vehicle is driven at low speed dur­
ing a partial turn. The oil pressure cannot build
up high enough to cause the pressure relief valve
to open as the external circuit still allows some
oil to flow through the system .
Figure 6, View "B" is typical of pump oper­
ation when vehicle is driven at high speed. In this
case, the flow control valve has opened to allow
oil flow in excess of system requirements to by­
pass into intake chamber of the pump.
PUMP REPLACEMENT
Removal
1. Disconnect hoses at pump and secure ends
in raised position to prevent drainage of oil.
2. Install caps at pump fittings to prevent
drainage of oil from pump.
3. Remove drive pulley attaching nut.
4. Loosen mounting bracket to pump bolts.
5. Remove pump drive belt.
6. Slide pulley from pump drive shaft. DO
NOT HAMMER PULLEY OFF SHAFT AS THIS
WILL DAMAGE THE PUMP.
7. Remove bracket to pump attaching bolts
and remove pump.
G M C S E R V I C E M A N U A L
D o w e l Pin
C ro s s o v e r
Installation
ket with holes aligned and install attaching bolts
loosely.
MER PULLEY ON SHAFT.
pulley.
ed in LUBRICATION (SEC. 0) of this manual.
(counterclockwise as viewed from the front) until
all air bubbles cease to appear.
then tighten attaching bolts securely.
ft.-lb s.
displacement type pump with a minimum output of
1.45 gallons per minute with pump idling at a speed
of 450 rpm against 665-735 psi pressure. The pump
has a maximum flow control range of 3.4 to 4.1
gallons per minute with a 1000 to 1100 psi p res­
sure relief valve. The pump is mounted at front
of engine and is belt-driven from the crankshaft.
PUMP OPERATION
loaded slippers, which contact the eccentric diam­
eter inside the pump housing, force oil from the
inlet side of the pump to the flow control valve
(fig. 7). Pressure inside the pump is controlled by
a valve which perm its fluid to be recirculated
within the pump. Maximum pressure in the system
is lim ited by a pressure relief valve which opens
into the reservoir when pressure exceeds specified
maximum of 1000-1100 psi.
POWER STEERING
P u m p Rotor
H
H o le
P u m p V a n e s
Figure 5 — Cross Section of Rotor and Vanes in Pump
1. Position pump assembly on mounting brac­
2. Slide pulley on drive shaft. DO NOT HAM­
3. Install pulley nut (finger-tight) against the
4. Connect and tighten hose fittings.
5. Fill pump reservoir with fluid recommend­
6. Bleed pump by turning pulley backward
7. Install pump drive belt over drive pulley.
8. Adjust belt tension as previously described,
9. Tighten drive pulley attaching nut to 25-35
SLIPPER TYPE HYDRAULIC PUMP
The slipper-type hydraulic pump is a constant
As the drive shaft rotates the rotor, spring
Sec. 9B
Page 493
P u m p R in g
C ro s s o v e r
H o le
D o w e l
Pin H o le