Service Training
MALAGA
777D Off Highway Truck
STEERING SYSTEM HAND CROSSOVER
METERING UNIT (HMU) RELIEF VALVES
HOLD
ELECTRONIC
CONTROL
MODULE
STEERING
HYDRAULIC
TANK
OIL
FILTER MAIN
RELIEF
MAGNETIC
SWITCH CHECK
VALVE
SECONDARY
STEERING
PUMP CHECK
VALVE
FLOW
SWITCH
ACCUMULATOR
STEERING
PUMP
STEERING
FLOW CYLINDERS
CONTROL VALVE
VII STEERING.
1. Components Layout
2. Operation
3. Maintenance
4. Testing & Adjusting
Adolfo
August 1997
777D Off Highway Truck
CHAPTER : STEERING
TYPICAL OFF-HIGHWAY TRUCK STEERING
COMPONENT LOCATIONS
FLOW
SECONDARY CONTROL
STEERING VALVE
PUMP
STEERING
ELECTRIC CYLINDERS
MOTOR
STEERING
PUMP
HYDRAULIC
TANK
HMU
STEERING
ECM HOIST PUMP
OIL FILTER ACUMULATOR
HMU
CROSSOVER
RELIEF
VALVES STEERING
PUMP
Page: VI-1
Page:
VI-2
STEERING SYSTEM HAND CROSSOVER
METERING UNIT (HMU) RELIEF VALVES
HOLD
ELECTRONIC
CONTROL
MODULE
STEERING
HYDRAULIC
TANK
OIL
CHAPTER : STEERING
FILTER MAIN
RELIEF
MAGNETIC
SWITCH CHECK
777D Off Highway Truck
VALVE
SECONDARY
STEERING
PUMP CHECK
VALVE
FLOW
SWITCH
ACCUMULATOR
STEERING
PUMP
STEERING
FLOW CYLINDERS
CONTROL VALVE
Page:
VI-3
STEERING SYSTEM HAND CROSSOVER
RIGHT TURN METERING UNIT (HMU) RELIEF VALVES
ELECTRONIC
CONTROL
MODULE
STEERING
HYDRAULIC
TANK
OIL
FILTER
CHAPTER : STEERING
MAGNETIC
SWITCH
777D Off Highway Truck
SECONDARY
STEERING
PUMP
ACCUMULATOR
FLOW
STEERING CONTROL VALVE
PUMP
STEERING
CYLINDERS
777D Off Highway Truck
CHAPTER : STEERING
STEERING FLOW
CONTROL VALVE
TO
HMU
MAIN
RELIEF
VALVE CHECK
VALVE
FROM
HMU
FLOW
CONTROL
ORIFICE
TO CHECK
TANK VALVE
CHECK VALVE
FROM
SIGNAL TO SECONDARY
SECONDARY STEERING PUMP
STEERING
FLOW SWITCH FROM PUMP
CLOSED
Page: VI-4
Page:
HAND METERING UNIT
VI-5
HOLD
STATIONARY COMMUTATOR
BALL IN GEAR
HELICAL
GROOVE
INPUT SHAFT
CHAPTER : STEERING
777D Off Highway Truck
TORSION BAR
VALVE SPOOL TO TO ROTATING
TO
TANK CYLINDER CYLINDER GEAR
FROM
PUMP DRIVE
LINK
Page:
HAND METERING UNIT
VI-6
LEFT TURN
COMMUTATOR
STATIONARY
GEAR
BALL
INPUT SHAFT
CHAPTER : STEERING
777D Off Highway Truck
TORSION BAR
TO FROM
VALVE SPOOL TANK
TO PUMP
CYLINDER
FROM
CYLINDER ROTATING DRIVE
GEAR LINK
Page:
VI-7
HAND METERING UNIT
RIGHT TURN
STATIONARY
GEAR
COMMUTATOR
BALL
INPUT SHAFT
CHAPTER : STEERING
777D Off Highway Truck
TORSION BAR
FROM
VALVE SPOOL CYLINDER DRIVE
ROTATING LINK
TO GEAR
TANK FROM
TO PUMP
CYLINDER
777D Off Highway Truck
CHAPTER : STEERING
CROSSOVER RELIEF SYSTEM
EXTERNAL IMPACT
CROSSOVER
RELIEF VALVES
HAND
METERING UNIT
Page: VI-8
777D Off Highway Truck
CHAPTER : STEERING
LAB EXERCISES
LAB A: STEERING SYSTEM COMPONENT IDENTIFICATION
Directions: Place the correct letter or number next to the component and briefly describe the
function of each.
Steering System Components
Flow Control Valve and Flow Switch
Hand Metering Unit
Oil Filter
Steering Pump
Secondary Steering Electronic Control Module
Secondary Steering Magnetic Switch
Secondary Steering Pump
Crossover Relief Valves
Accumulator (777D truck only)
Steering Hydraulic Tank
Page: VI-9
777D Off Highway Truck
CHAPTER : STEERING
LAB EXERCISES
LAB B: STEERING SYSTEM TESTING
Machine Model __________________________ Date_______________________________
Serial Number ___________________________ Technician___________________________
VISUAL CHECKS:
Oil level _______________ Oil Viscosity _______________ Oil leaks ? ______________
Air/Water in Oil ?_______ Suction Screen Debris ? _______ Oil Filter Examination ______
PRESSURE TESTS
TEST SPECIFICATIONS ACTUAL
Full Cycle Time at High Idle
Main Relief Valve Pressure (Left Turn) at High Idle
Main Relief Valve Pressure (Right Turn) at High Idle
Crossover Relief Valve (Left Turn) at Low Idle
Crossover Relief Valve (Right Turn) at Low Idle
Accumulator Charge Pressure
SECONDARY STEERING SYSTEM TEST
The truck should be on flat level ground with the parking brake engaged. Turn the ignition
switch ON (engine off) and actuate the secondary pump automatic/manual switch to the
MANUAL position. Then, attempt to turn the steering wheel. You should be able to hear the
secondary pump motor run and the front wheels of the truck should turn.
Page: VI-10
777D Off Highway Truck
CHAPTER : STEERING
LAB EXERCISES
LAB C: TRACING STEERING SYSTEM OIL FLOW
I. Directions: Have each student trace the flow of oil during the conditions listed below
using different colored pencils to show each path:
• Steering system when the steering wheel is stationary
• Steering system when the steering wheel is turned to the right or left
HAND CROSSOVER
METERING UNIT (HMU) RELIEF VALVES
STEERING
HYDRAULIC
TANK
OIL
FILTER
STEERING
CYLINDERS
SECONDARY
STEERING
PUMP
ACCUMULATOR
FLOW
CONTROL VALVE
STEERING PUMP
HAND CROSSOVER
METERING UNIT (HMU) RELIEF VALVES
STEERING
HYDRAULIC
TANK
OIL
FILTER
STEERING
CYLINDERS
SECONDARY
STEERING
PUMP
ACCUMULATOR
FLOW
CONTROL VALVE
STEERING PUMP
Page: VI-11
777D Off Highway Truck
CHAPTER : STEERING
LAB EXERCISES
LAB C: TRACING STEERING SYSTEM OIL FLOW (CONT.)
II. Directions: Have each student trace the flow of oil during the conditions listed below
using different colored pencils to show each path:
• Flow control valve under normal conditions
• Flow control valve when the secondary steering system is in operation
• Flow control valve when system pressure exceeds the setting of the main relief valve.
Page: VI-12
777D Off Highway Truck
CHAPTER : STEERING
LAB EXERCISES
LAB D: STEERING SYSTEM TROUBLESHOOTING
Machine Model __________________________ Date_______________________________
Serial Number ___________________________ Technician___________________________
VISUAL CHECKS:
Oil level _______________ Oil Viscosity _______________ Oil leaks ? ______________
Air/Water in Oil ?_______ Suction Screen Debris ? _______ Oil Filter Examination ______
PRESSURE TESTS
TEST SPECIFICATIONS ACTUAL
Full Cycle Time at High Idle
Main Relief Valve Pressure (Left Turn) at High Idle
Main Relief Valve Pressure (Right Turn) at High Idle
Crossover Relief Valve (Left Turn) at Low Idle
Crossover Relief Valve (Right Turn) at Low Idle
Accumulator Charge Pressure
Page: VI-13
777D Off Highway Truck
CHAPTER : STEERING
POST TEST
Off-Highway Trucks Steering System Posttest
I. Directions: Match the component to the correct statement below.
1. Crossover Relief Valves 5. Steering Hydraulic Tank
2. Oil Filter 6. Secondary Steering Pump
3. Flow Control Valve 7. Flow Switch
4. Hand Metering Unit 8. Accumulator
Dampens pressure surges in the steering system.
Contains an oil filter.
Contains a check valve to prevent pressure surges from the steering cylinders from
returning to the HMU when a turn is in progress.
Meters and redirects oil to the steering cylinders as the steering wheel is turned.
Equalizes the pressure in both steering cylinders if the pressure in one cylinder exceeds
the relief valve setting.
Utilizes a bypass valve to protect the steering system during cold oil start-up.
Is energized by a magnetic switch.
Indicates to the electronic control module a loss of oil flow.
II. Directions: Indicate whether the statement is True or False by circling the corresponding
letter.
1. The secondary steering motor is tested every time the ignition switch is turned to the ON
position. (T) or F
2. The crossover relief valve pressure setting is higher than the setting of the main steering
relief valve (in the flow control valve). (T) or F
3. The crossover relief valves should be checked at low idle and are adjustable. T or (F)
4. When the steering wheel is stationary, steering oil circulates through the Hand Metering
Unit (HMU), back to the steering hydraulic tank, and oil flow is blocked to and from the
steering cylinders. (T) or F
5. All truck models are equipped with an accumulator that uses a nitrogen gas charge to
cushion the steering system from pressure surges. T or (F)
Page: VI-14
777D Off Highway Truck
CHAPTER : STEERING
POST TEST
III. Directions: Match the components with the associated letter.
STEERING SYSTEM
A
I
E F
H
K
G
C
Flow Control Valve Hand Metering Unit
Oil Filter Steering Pump
Crossover Relief Valves Steering Cylinders
Secondary Steering Pump Accumulator
Steering Hydraulic Tank Magnetic Switch
Automatic/Manual Panel Switch Electronic Control Module
Page: VI-15
777D Off Highway Truck
CHAPTER : STEERING
POST TEST
IV. Directions: Match the components and ports with the associated letter.
FLOW CONTROL VALVE
Flow Switch From Hand Metering Unit
To Hand Metering Unit Steering Pump Inlet
Relief Valve Flow Control Orifice
Secondary Steering Pump Inlet To Steering Hydraulic Tank
G
F
Page: VI-16
