GMC 4000 Series Service Manual page 503

Sec. 9B
Page 492
POWER STEERING
Figure 4 — Power Steering Hydraulic Pum p Installed (Typical)
EXTERNAL LEAKS
To determine the source of external leaks,
thoroughly clean the steering gear and inspect.
A positive displacement vane-type or slipper-
type hydraulic oil pump is used on vehicles cover­
ed by this manual. Refer to "Hydraulic Pump Model
Application" in "Specifications" at end of this se c ­
tion for proper pump application on each truck
ser ie s.
VANE TYPE HYDRAULIC PUMP
The vane-type hydraulic pumps have a max­
imum output of 3.5 gallons per minute of Type "A"
Automatic Transmission Oil at 170°F., with pump
operating at 1500 rpm against 50 psi pressure.
Minimum output of the pumps is 2.35 gallons per
minute with pump idling at a speed of 590 rpm,
against 665 to 735 psi pressure.
The hydraulic pump is mounted at front of
engine and is belt driven from the engine crank­
shaft or water pump pulley (fig. 4).
NOTE: On "E" and "S" models, the hydraulic
pump is mounted on top of engine front cover and
is driven from the engine water pump pulley.
The oil reservoir encases the pump and on
some vehicles, an additional reservoir is remotely
mounted on the radiator front support or on the
transm ission control island support.
On all vane-type hydraulic pumps, the drive
shaft is supported by one ball bearing and one
needle bearing in the pump housing.
G M C S E R V I C E M A N U A L
POWER STEERING HYDRAULIC PUMP
External leakage may be due to loose hose con­
nections or damaged hose, adjuster plug seals or
torsion bar seals. Correct external leaks as
follows:
CAUSE
1. Loose hose connections . Tighten Connections
2. Damaged h o s e ................................ Replace Hose
3. Damaged adjuster plug seals . . Replace Seals
4. Damaged torsion bar seal . . . . Replace Seal
STEERING GEAR MALFUNCTIONS
Each of the malfunctions listed here have a
number of causes and each cause has a remedy.
These causes and rem edies are given in the
"Troubleshooting Quick Reference Chart" later in
this section.
1. Hard steering while driving.
2. Poor return of steering.
3. Leads to one side or the other.
4. Momentary increased effort on fast turn.
5. E xcessive wheel kickback.
6. Steering wheel jerks.
7. Hard steering when parking.
8. Effortless turn.
The pump rotary group is the heart of the
pump and consists of the drive shaft, rotor, vanes,
pump ring, thrust plate, and pressure plate. Flow
control valve is the nerve center of the pump and
includes flow control plunger, flow control spring,
and the pressure relief ball and spring. This a s­
sembly controls flow and pressure in the system .
PUMP OPERATION
NOTE: Refer to figures 5 and 6.
As the drive shaft rotates the rotor, the vanes
follow the cam surface in the pump ring. This cam
consists of two rising and falling areas which cause
a complete pumping cycle to occur every 180 de­
grees of rotation (fig. 5).
The spaces between rotor vanes pick up oil on
the rising portions of the cam from two openings
between thrust plate and pump ring and from open­
ing between the pressure plate and pump ring.
This oil is discharged on the falling portion of the
cam through two openings in the pressure plate
and two openings in the thrust plate which are con­
nected to openings in the pressure plate by cr o ss­
over holes in the pump ring (fig. 5). The oil passes
through the pressure plate into cavity (1) behind
it. A portion of this oil is directed back through
other passages in the pressure plate so that it may
enter behind the vanes forcing them to follow the
cam surface of the pump ring (fig. 6B).
REMEDY