Isuzu N-Series. Manual - part 121

POWER STEERING 3B1-3

Power Steering Pump

Tandem Hydraulic Pump

A tandem hydraulic pump has two independent hydrau-
lic pumps inside one casing. Its main parts can be clas-
sified into the following three.

• hydraulic pump (vane type)
• flow control valve
• pressure relief valve

The following is a description of the functions and oper-
ation of the main parts.

Hydraulic pump

Functions

The heart of the system, namely the part that supplies
hydraulic pressure, which operates the hydro boost and

the P/S. The hydraulic pump is installed directly to the
engine and is driven by gears. It smoothly adapts itself
to severe conditions of use, including a wide range of ro-
tation speeds, changes in the temperature at which it is
used, and frequent changes in load pressure, and it is
able to fully perform its functions.

Operation

Its drive is transmitted by gears from the engine to the
drive shaft of the oil pump, and it turns a rotor that is fit-
ted into the spline part of the drive shaft. When the rotor
rotates, the ten vanes built into the rotor groove fly out
by centrifugal force, are pressed against the inside sur-
face of the cam ring, and rotate along the inside surface
so that the pump operates.

Legend

A. 4J series engine

B. 4H series engine

N3A0117E

3B1-4 POWER STEERING

Suction and discharge of the hydraulic oil is done by a
pressure plate and side plate attached to both ends of
the cartridge.
Suction openings and discharge openings are provided
in two locations each in symmetrical positions with re-
spect to the axis of rotation, and because the load on the
bearings due to the discharge pressure is balanced, a
tandem oil pump is also called a balanced hydraulic
pump.
Because it has a double pump effect per rotation, this
pump provides large discharge flow in proportion to the
space it occupies. Also, some of the hydraulic oil that is
discharged is led to the base of the vanes and ensures
the quality of the seal between the vane tips and the
cam, and even if the vanes and cam rings become a lit-
tle worn, the seal is hardly affected at all (with respect to
discharge, oil tightness).

POWER STEERING 3B1-5

ON-VEHICLE SERVICE

Power Steering System

Preparation

• Inspect the fluid reservoir for proper fluid level.
• Inspect the pump belt for proper tension.

System Test Procedure

1. Place a container under the pump to catch the fluid

when disconnecting or connecting the hoses.

2. With the engine NOT running, install the power

steering fluid pressure gauge unit between the
power steering unit and discharge side of oil pump.
The gauge must be between the stop valve and oil
pump.
Gauge kit: 5-8840-0162-1

3. Check the fluid level. Fill the reservoir with

Dexron

®

-III to “Full” mark. Check the connections

at gauge kit for leakage.

4. Fully open the stop valve, and bleed the system.

Refer to “Power Steering System Bleeding” of this
section.

5. Start the engine and being idled, and raise oil tem-

perature to 50 — 60

°C (122 — 144°F). Check the

fluid level again. Fill the reservoir with Dexron

®

-III

to “Full” mark if required.

6. Hold the steering wheel in the forward position.

Fully close the stop valve, and record the highest
value of fluid pressure.

Caution:
Do not continue this test. It should be done within 10
seconds, because otherwise oil temperature would rise
to excess to cause trouble.

Standard Operating Pressure

• NQR without HBB: 11,278 kPa (115 kg/cm

2

/1,635

psi)

• NPR/NPS/NQR, with HBB: 10,787 kPa (110 kg/

cm

2

/1,563 psi)

• except Above Models: 10,297 kPa (105 kg/cm

2

/

1,493 psi)

HBB: Hydraulic Booster Brake

• If the pressure recorded is within specified range,

the steering system is functioning within its specifi-
cations.

• If the pressure recorded is higher than specified

range, oil pump relief valve is defective.

• If the pressure recorded is lower than specified

range, oil pump or relief valve is defective.

7. Fully open the stop valve and keep engine idled. 

Turn and hold steering wheel to full left lock, and
record the highest value of fluid pressure. 
Turn and hold steering wheel to full right lock, and
record the highest value of fluid pressure.

Caution:
Do not continue this test. It should be done within 5 sec-
onds.

Standard Operating Pressure

• NQR without HBB: 11,278 kPa (115 kg/cm

2

/1,635

psi)

• NPR/NPS/NQR, with HBB: 10,787 kPa (110 kg/

cm

2

/1,563 psi)

Legend

1. From oil pump

2. To steering unit

N3A0118E

3B1-6 POWER STEERING

• except Above Models: 10,297 kPa (105 kg/cm

2

/

1,493 psi)

HBB: Hydraulic Booster Brake

• If the pressure recorded is within specified range,

the steering system is functioning within its specifi-
cations.

• If the pressure recorded is lower than specified

range, steering unit is leaking internally and must
be disassembled and repaired.

Power Steering System Bleeding

1. Raise the front wheels with a jack. (It is recom-

mended that a turn-table should be placed under
right hand and left hand side front wheels at this
time.)

2. Replenish the fluid tank with ATF Dexron

®

-III, and

repeat the full steering (turning to the extreme ends
of the steering) in both right hand side and left hand
side directions quietly with the engine stopped.

Caution:
Under this condition, the air trapped in the fluid piping
will be released from the fluid tank to the atmosphere in
the form of large air bubbles.

3. Whenever the fluid level in tank is lowered, replen-

ish the tank with ATF Dexron

®

-III in the amount

equivalent to the consumption.

Caution:
Pay extra care over the fluid level so that fluid will not run
out.

4. Start the engine and repeat the full steering several

times quietly with the engine kept idling.

Caution:
The time needed for full steering should be less than five
minutes. (Prolonged full steering of more than five sec-
onds will cause the fluid temperature to raise significant-
ly.)

5. Lower the front wheels and turn the steering wheel

in both left hand and right hand sides several times
with the engine run. If no abnormal noise can be
heard at this time, air might be completely re-
leased.

Caution:
If the abnormal noise (creaky, “zi-zi” sound etc.) can be
heard in the piping after the step No. 5 has been com-
pleted, the air is still trapped in the piping and air release
has not been completed.
In this case, continue repetition of steering wheel turn in
the both directions of left hand and right hand sides until
the fluid temperature is increased up to around 50 —
80

°C (122 — 176°F). When the temperature reaches

this degree, stop the engine and leave the engine for
about five minutes. (Within five minutes, air bubbles can
be collected.) Start the engine again and repeat the full
steering several times.

6. After air release has been completed, check the

fluid level in the tank and also confirm that the
joints have no fluid leak.