STEERING SYSTEM ON HAULTRUCK 789D

789D Off-Highway Truck 

STEERING SYSTEM

Identify steering system components

INTRODUCTION
The steering system on the 789D truck is similar to the 789C truck. The 789D truck now has three steering accumulators and some of the steering component locations have changed. As on other Cat Off-highway Trucks, the steering system uses hydraulic force to change the direction of the front wheels. The system has no mechanical connection between the steering wheel and the steering cylinders.

If the oil flow is interrupted while the truck is moving, the system incorporates a secondary steering system. Secondary steering is accomplished by accumulators, which supply oil flow to maintain steering.
The main components in the steering system are:
* Hand Metering Unit (HMU) (1)
* Case drain oil filter (2)
* Steering accumulators (3)
* Steering tank (4)
* Steering pump (5)
* Steering cylinders (6)
* Solenoid and relief valve manifold (7)
* Steering directional valve (8)

Explain steering system operation during no steer at maximum flow.

When the engine is started, oil for the steering system is drawn from the steering hydraulic tank (1) by the steering pump (2) and sent through a one-way check valve to the solenoid and relief valve manifold (3). Oil from the solenoid and relief valve manifold flows to the steering directional valve (4), the accumulator charging valve (5), and the accumulators (6). After the oil pressure increases to a predetermined pressure in both accumulators, the steering pump will destroke.

When a steering demand occurs, the accumulators supply the necessary oil flow for steering, and pressure in the accumulators decreases. When the oil pressure in the accumulators decreases to a predetermined level, the steering pump will automatically upstroke to maintain the oil pressure required for steering in the  accumulators. Oil from the accumulators flows through the steering directional valve to the Hand Metering Unit (HMU) (7). If the steering wheel is not turned, the oil flows through the HMU and the main steering oil filter to the tank. Allowing oil to circulate through the HMU while the steering wheel is stationary provides a “thermal bleed” condition, which maintains a temperature differential of less than 28°C (50°F) between the HMU and the tank. This “thermal bleed” prevents thermal seizure of the HMU (sticking steering wheel).

When the steering wheel is turned, the HMU directs oil back to the steering directional valve. The steering directional valve directs oil to the steering cylinders (8). Depending on which direction the steering wheel is turned, oil will flow to the head end of one steering cylinder and to the rod end of the other cylinder. The action of the oil on the pistons and rods in the steering cylinders causes the wheels to change direction. Displaced oil from the steering cylinders flows through the back pressure valve in the steering directional valve and returns through the main steering oil filter to the tank.

Identify steering tank components

STEERINGSYSTEMCOMPONENTS 
The steering tank is located on the right platform. Two sight gauges are on the side of the tank. When the engine is shut off and the oil is cold, the oil should be visible between the FULL and ADD OIL markings of the upper sight gauge (1). When the engine is running and the accumulators are fully charged, the oil level should not be below the ENGINE RUNNING marking of the lower sight gauge (2). If the ENGINE RUNNING level is not correct, check the nitrogen charge in each accumulator. A low nitrogen charge will allow excess oil to be stored in the accumulators and will reduce the secondary steering capacity.

A combination vacuum breaker/pressure relief valve is used to limit the tank pressure. Before removing the fill cap, be sure that the engine was shut off with the key start switch and the oil has returned to the tank from the accumulators. Depress the pressure release button (3) on the breather to vent any remaining pressure from the tank.
Supply oil for the steering system is provided by a piston-type pump. Case drain oil from the pump returns to the tank through the case drain oil filter (4). The remaining steering system oil returns to the tank through the main steering filter, which is located inside the tank and accessible by removing the filter cover (5). Both filters are equipped with bypass valves to protect the system if the filters are restricted or during cold oil start-up.

If the steering pump fails or if the engine cannot be started, the connector (6) is used to attach an Auxiliary Power Unit (APU). The APU will provide supply oil from the steering tank at the connector to charge the steering accumulators. Steering capability is then available to tow the truck. The steering oil temperature sensor (7) provides an input signal to the VIMS, which informs the operator of the steering system oil temperature. If the steering oil temperature exceeds 108°C (226°F), the operator will receive a warning on the VIMS display (STRG OIL TEMP HI).

Identify components inside right frame rail

The 789D is equipped with a load sensing, pressure compensated, piston-type pump. The steering pump (1) is mounted to the pump drive (2). The pump drive is located on the inside of the right frame rail near the torque converter. The Rear Axle Lubrication (RAXL) drive motor pump (3) is mounted to the front of the steering pump.

The steering pump operates only when the engine is running and provides the necessary oil flow to the accumulators for steering system operation. The steering pump contains a load sensing controller (4) that works with an accumulator charging valve to monitor and control steering pump output.
The steering pump will produce flow at high pressure until the steering accumulators are charged with oil and the pressure increases to approximately 18300 ± 350 kPa (2655 ± 50 psi) at LOW IDLE. This pressure is referred to as the CUT-OUT pressure. When the CUT-OUT pressure is reached, the accumulator charging valve reduces the load sensing signal pressure to the pump load sensing controller, and the pump destrokes to the LOW PRESSURE STANDBY condition. During LOW PRESSURE STANDBY, the pressure should be between approximately 2070 and 3600 kPa (300 and 525 psi).

The pump operates at minimum swashplate angle to supply oil for lubrication and
leakage. Because of the normal leakage in the steering system and Hand Metering Unit (HMU) “thermal bleed,” the pressure in the accumulators will gradually decrease to approximately 16470 ± 350 kPa (2390 ± 50 psi). This pressure is referred to as the CUT-IN pressure.
When the pressure in the accumulators decreases to the CUT-IN pressure, the accumulator charging valve blocks the load sensing signal line to the load sensing controller from returning to the tank, and the pump upstrokes to maximum displacement (full flow).

Identify components inside right frame rail

A pressure tap (1) is located on the pump pressure switch manifold (2). The manifold is located inside the right frame rail. If steering pump supply pressure is measured at this tap during LOW PRESSURE STANDBY, a gauge acceptable for testing maximum steering system pressure must be used to avoid damaging the gauge when the steering pump upstrokes to provide maximum oil flow.
Two pressure switches monitor the condition of the steering system on the 789D. The low steering pressure switch (3) monitors the output of the steering pump during LOW PRESSURE STANDBY.
The high steering pressure switch (not shown) is mounted next to the steering directional valve. The high pressure switch monitors the steering system accumulator pressure.

The low steering pressure switch provides input to the Transmission/Chassis, which sends a signal to the VIMS. The high steering pressure switch provides input to
the VIMS. The VIMS informs the operator of the condition of the steering system. A steering system warning is only displayed if the ground speed is above 8 km/h (5 mph) or the actual gear switch is not in NEUTRAL. Also visible in this image is the solenoid and relief valve manifold (4).

Explain operation of steering pump during maximum flow

After the engine is started, pressure increases in the steering accumulators (2). The pump load sensing controller (1) is spring biased to vent the actuator piston (3) pressure to drain. Venting pressure from the load sensing controller and the actuator piston positions the spring biased swashplate piston (4) to maximum displacement (full flow).

As pressure increases in the accumulators, pump supply pressure is sensed in the accumulator charging valve (5) and on both ends of the flow compensator (6). With
pressure on both ends of the flow compensator, the swashplate is kept at maximum angle by the force of the spring in the pump housing and pump discharge pressure on the swashplate piston. The pistons travel in and out of the barrel and maximum flow is provided through the outlet port. Since the pump is driven by the engine, engine rpm also affects pump output (7).

NOTE: Because the signal lines are sensing pump supply pressure and not a “load” pressure, the steering system does not operate the same as other load sensing systems with a margin pressure.

Explain steering pump operation during low pressure standby

Pump supply pressure will increase until the accumulator pressure acting on the accumulator charging valve (5) shifts the spool, and the load sensing signal pressure (8) is vented to the tank. The accumulator charging valve spool shifts (cut-out) when the pump outlet oil pressure is approximately 18300 ± 350 kPa (2655 ± 50 psi).

An orifice prevents supply pressure from filling the drained load sensing passage above the flow compensator (6). Pump oil (at low pressure standby pressure) flows past the lower end of the displaced flow compensator spool to the actuator piston (3). The actuator piston has a larger surface area than the swashplate piston (4). The oil pressure at the actuator piston overcomes the spring force of the swashplate piston and moves the swashplate to destroke the pump. The pump is then at a low flow, LOW PRESSURE STANDBY condition. Pump output pressure
is equal to the setting of the flow compensator. The LOW PRESSURE STANDBY setting must be between 2070 and 3600 kPa (300 and 525 psi). In the NEUTRAL or NO STEER position, demand for oil from the accumulators (2) is low. The pump operates at minimum swashplate angle to supply oil for lubrication and leakage.

Because of the normal leakage in the steering system and HMU “thermal bleed,” the pressure in the accumulators will gradually decrease to approximately 16470 ± 350 kPa (2390 ± 50 psi) (90% of the accumulator charging valve cut-out pressure).
When the pressure in the accumulators decreases to approximately 16470 ± 350 kPa
(2390 ± 50 psi), the accumulator charging valve shifts (cut-in) and blocks the load sensing signal line pressure from the tank. Pump supply oil flows through the orifice and pressurizes the load sensing signal line. The load sensing signal shifts the flow compensator spool and drains the actuator piston oil to the tank. Venting pressure from the actuator piston positions the spring biased swashplate to maximum displacement (full flow). At LOW IDLE in the NEUTRAL or NO STEER position, the pump will cycle between the cut-out and cut-in conditions in 25 seconds or more.

Connecting a pressure gauge to the pressure tap on the bottom of the steering directional valve will indicate these steering system pressures. If pump pressure cycles in less than 25 seconds, leakage is in the system and must be corrected. Typical sources of leakage can be the accumulator bleed down solenoid or the back-up relief valve located on the return manifold.

Explain operation of load sensing controller

Pump pressure limiting (high pressure cutoff) is adjustable. To adjust the pump high pressure cutoff valve, turn the accumulator charging valve (1) adjustment screw all the way in, or disconnect the load sensing (LS) line (2) at the pump as shown in this illustration. Plug the line to the accumulator charging valve and cap the fitting on the pump. Operate the engine at LOW IDLE, and check the pump (accumulator) pressure at the pressure tap below the steering directional valve.

Turn the compensator (high pressure cutoff) adjusting screw (3) while watching the pressure gauge. One turn is equal to approximately 2800 kPa (405 psi). Adjust the pressure to approximately 20000 ± 350 kPa (2900 ± 50 psi). When the adjustment is complete, reconnect the LS line to the pump.
The high pressure cutoff setting must be a minimum of 1720 kPa (250 psi) higher than the accumulator charging valve setting. If the high pressure cutoff setting of the compensator valve (in the load sensing controller) is lower than the accumulator charging valve setting, the pump will stay at MINIMUM FLOW, and the steering system will take too long to recharge.

The high pressure cutoff adjustment provides a back-up if the accumulator charging valve malfunctions. Pump LOW PRESSURE STANDBY is also adjustable. Connect a gauge to the low pressure standby pressure tap located on the pump pressure switch manifold. With the signal line connected, operate the engine at LOW IDLE and check the pump pressure. The pump will cycle to low pressure standby every 25 seconds or more. Low pressure standby must be between approximately 2070 and 3600 kPa (300 and 525 psi). If adjustment is required, stop the engine.

Turn the low pressure standby adjustment screw (4) clockwise to increase the pressure and counterclockwise to decrease the pressure until the pressure is between approximately 2070 and 3600 kPa (300 and 525 psi). Each 1/4 turn changes the pressure setting approximately 345 kPa (50 psi).
NOTE: If the steering pump is adjusted on a hydraulic test stand, set the margin pressure to 2070 ± 100 kPa (300 ± 15 psi) with a flow of approximately 115 ± 12 L/min (30 ± 3 gpm), 1838 rpm, and approximately 15000 kPa (2180 psi) discharge pressure. The low pressure standby reading measured on a truck is higher than the test stand margin pressure due to parasitic loads in the truck steering system.

Identify components on frame rail near front of truck.

The solenoid and relief valve manifold (1) and the accumulator charging valve (2) are located on the front right side of the front crossmember.
Steering pump supply oil flows through a check valve (3) to the solenoid and relief valve manifold. The check valve prevents accumulator oil from flowing back to the steering pump when the pump destrokes to LOW PRESSURE STANDBY. The solenoid and relief valve manifold connects the steering pump to the accumulator charging valve, the accumulators, and the steering directional valve. The solenoid and relief valve manifold also provides a path to drain for the steering oil.
The accumulator bleed down solenoid (4) drains pressure oil from the accumulators when the truck is not in operation.

The back-up relief valve (5) protects the system from pressure spikes if the pump cannot destroke fast enough or limits the maximum pressure if the steering pump high pressure cutoff valve does not open.
Steering system oil samples can be taken at the steering system S•O•S port (6).
To operate the steering system on a disabled truck, an Auxiliary Power Unit (APU) can be connected to the secondary steering connector (7) and to a suction port on the steering tank. The APU will provide supply oil to charge the accumulators. Steering capability is then available to tow the truck.

The pressure setting of the accumulator charging valve can be changed by adjusting
the spring force that keeps the valve seated (closed). Change the setting by removing the protective cap (8) and turning the adjustment screw clockwise to increase or counterclockwise to decrease the pressure setting.
One turn of the adjustment screw changes the pressure approximately 4000 kPa (580 psi). If the accumulator charging pressure cannot be adjusted within specifications, an adjustment of the high pressure cutoff valve is required. The high pressure cutoff setting must be a minimum of 1720 kPa (250 psi) higher than the accumulator charging valve setting.
 NOTE: When testing or adjusting any steering system pressure settings, always allow the accumulator charge cycle to occur at least three times before testing the pressures. Failure to allow the charging cycle to occur three times will result in inaccurate readings.

Explain operation of accumulator bleed down solenoid.

Shown is a sectional view of the solenoid and relief valve manifold. The accumulator bleed down solenoid (1) is energized by the bleed down solenoid shutdown control (located in the compartment behind the cab) when the key start switch is moved to the OFF position. The bleed down solenoid shutdown control holds the solenoid open for approximately 70 seconds.
Pressure oil from the accumulators (2) is sensed by the bleed down solenoid. When the solenoid is ENERGIZED, the plunger moves and connects the pressure oil to the tank (3). Pressure oil flows through an orifice, past the plunger, to the tank. The orifice limits the return oil flow from the accumulators to a rate which is lower than the  flow limit (restriction) of the steering oil filter in the hydraulic tank. When the solenoid is DE-ENERGIZED, spring force moves the plunger and pressure oil cannot go to drain. The back-up relief valve (4) protects the steering system if the steering pump malfunctions (fails to destroke). Pressure oil from the steering pump works against the end of the back-up relief valve and the spring. The relief valve unseats (opens) if the pressure reaches approximately 20670 ± 400 kPa (3000 ± 60 psi) at 8 ± 2 L/min (2 ± .5 gpm). Oil then flows past the relief valve and drains to the tank.

The back-up relief valve must be adjusted only on a test bench. The pressure setting of the back-up relief valve can be changed by adjusting the spring force that keeps the relief valve seated (closed). To change the relief valve setting, remove the protective cap and turn the adjustment screw clockwise to increase or counterclockwise to decrease the pressure setting. One revolution of the setscrew will change the pressure setting approximately 3800 kPa (550 psi). A functional test of the back-up relief valve can be performed on the machine by installing a manual hydraulic pump at the location of the Auxiliary Power Unit (APU) connector and installing blocker plates to prevent oil from flowing to the accumulators. See the service manual for more detailed information.
NOTE: Using the functional test procedure to adjust the back-up relief valve will provide only an approximate setting. Accurate setting of the back-up relief valve can only be performed on a hydraulic test bench.

Identify accumulator bleed down solenoid shutdown control.

Shown is the shutdown control (arrow) for the steering accumulator bleed down solenoid. The control is located in the compartment behind the cab. The steering accumulator bleed down solenoid is activated by the control when the key start switch is moved to the OFF position. The bleed down solenoid shutdown control holds the solenoid open for 70 seconds. The charge pressure for the steering accumulators is approximately 5512 ± 345 kPa (800 ± 50 psi).

Identify steering directional valve and high steering pressure switch

The steering directional valve (1), located on the inside of the left frame rail, is pilot operated from the HMU in the operator’s station. Five pilot lines connect these two components. The pilot lines send pilot oil from the HMU to shift the spools in the steering directional valve. The spools control the amount and direction of pressure oil sent to the steering cylinders. Four pilot lines are used for pump supply, tank return, left turn, and right turn.

The fifth pilot line is for the load sensing signal. When checking the steering system cut-out and cut-in pressures, a gauge can be connected at the pressure tap (not visible, mounted on the rear of the valve). The high steering pressure switch (2) is mounted next to the steering directional valve. The high pressure switch monitors the steering system accumulator pressure and informs the VIMS when the steering pressure is out of range.

Explain operation of steering directional valve during no turn.

Shown is a sectional view of the steering directional valve. The main components of the steering directional valve are: the priority spool (1), the amplifier spool (2) with internal combiner/check (3) spool, the directional spool (4), the relief/makeup valves (5), and the back pressure valve (6).
Pressure oil from the accumulators (7) flows past the spring biased priority spool and is blocked by the amplifier spool. The same pressure oil flows through an orifice to the right end of the priority spool. The orifice stabilizes the flow to the priority spool and must be present to open and close the priority spool as the flow demand changes. The same pressure oil flows to the HMU (8). After all the passages fill with pressure oil, the priority spool shifts to the left, but remains partially open. In this position, the priority spool allows a small amount of oil flow (thermal bleed) to the HMU and decreases the pressure to the HMU supply port. The “thermal bleed” prevents the HMU from sticking.

With the truck in the NEUTRAL or NO TURN position, all four working ports are vented to the tank through the HMU. The directional spool is held in the center position by the centering springs.
The four work ports are: Accumulator •
* Tank (9)
* Right turn cylinder (10)
* Left turn cylinder (11)
While the truck is traveling straight (no steer), any rolling resistance (opposition) acting on the steering cylinders creates a pressure increase. The increased pressure acts on the relief/makeup valve in that port. If the pressure increase exceeds approximately 28000 ± 1000 kPa (4065 ± 150 psi), the relief poppet will open. A pressure drop occurs across the orifice. The pressure drop causes the dump valve to move and allows oil to flow to the tank passage.
The relief action causes the makeup portion of the other relief/makeup valve to open and replenish oil to the low pressure ends of the cylinders.

The excess (dumped) oil flows across the back pressure valve and enters the outer end of the other relief/makeup valve. A pressure difference of approximately 48 kPa (7 psi) between the tank passage and the low pressure cylinder port causes the makeup valve to open. The excess oil flows into the low pressure cylinder port to prevent cavitation of the cylinder. The back pressure valve also prevents cavitation of the cylinders by providing a positive pressure of approximately 170 kPa (25 psi)
in the passage behind the makeup valve. A pressure higher than approximately 170 kPa (25 psi) will open the back pressure valve to the tank. The steering directional valve must be removed and tested on a hydraulic test bench to accurately check the setting of the relief/makeup valves. A functional test of the relief/makeup valves can be performed on the machine by connecting a manual hydraulic pump and installing blocker plates to prevent oil from flowing to the steering cylinders. See the service manual for more detailed information.

NOTE: Using the functional test procedure to adjust the relief/makeup valves will provide only an approximate setting. Accurate setting of the relief/makeup valves can only be performed on a hydraulic test bench.

Explain operation of steering directional valve during a right turn

When the steering wheel is turned to the RIGHT, the “thermal bleed” and venting of the four work ports to the tank is stopped. The increased supply pressure flows to the HMU (8) and the load sensing pilot line through the load sensing port (14). The load sensing pilot line directs cylinder pressure to the priority spool in the directional valve. Cylinder pressure is present in the HMU because pilot oil combines with accumulator oil in the combiner/check valve spool in the steering directional valve. The increased pressure in the load sensing line causes the priority spool to move to the right and allows more oil flow to the HMU through the supply line. The load sensing port supply pressure varies with the steering load. The priority spool moves proportionally, allowing sufficient oil flow to meet the steering requirements.

Pilot oil flows through a stabilizing orifice to the right turn pilot port (12) of the steering directional valve and moves the directional spool. Movement of the directional spool allows pilot oil to flow to the amplifier spool (2) and the combiner/check spool.
The pilot oil divides at the amplifier spool. Pilot oil flows through a narrow groove around the combiner/check spool. The pilot oil is momentarily blocked until the amplifier spool moves far enough to the right to allow partial oil flow through one of eight orifices. Pilot oil also flows through a connecting pin hole and a stabilizing orifice to the left end of the amplifier spool and causes the amplifier spool to move to the right. Accumulator oil at the spring end (right end) of the amplifier spool flows through a mid-connecting pin to the left end of the amplifier spool and also causes the amplifier spool to move to the right.

When the amplifier spool moves to the right, accumulator oil flows to the inner chamber, forcing the combiner/check spool to the left. Accumulator oil then flows through seven of the eight orifices. Pilot and accumulator oil combine. Oil flows across the directional spool (which has already shifted) for a RIGHT TURN.

The faster the steering wheel is turned, the farther the directional spool and the amplifier spool are shifted. A higher flow rate is available, which causes the truck to turn faster. The ratio of pilot and pump supply oil that combine is always the same
because one orifice is dedicated to pilot flow and seven orifices are dedicated to accumulator supply flow. Return oil from the cylinders flows across the directional spool, around the relief/ makeup valve, and forces the back pressure valve to open and oil returns to the tank. During a turn, if a front wheel strikes a large obstruction that cannot move, oil pressure in that steering cylinder and oil line increases.

Oil flow to the cylinder is reversed. This pressure spike is felt in the amplifier spool. The combiner/check spool moves to the right and blocks the seven pump supply oil orifices to the steering cylinders. The amplifier spool moves to the left and blocks the pilot oil orifice. Pilot oil flow to the steering cylinders stops. The pressure spike is not felt at the HMU. If the pressure spike is large enough, the relief/makeup valve drains the pressure oil to the tank as previously described.

Identify hand metering unit (Hmu )

The 789D Hand Metering Unit (HMU) (arrow) is located at the base of the steering column, behind a cover at the front of the cab. The HMU is connected to the steering wheel and controlled by the operator.
The HMU meters the amount of oil sent to the steering directional valve by the speed
at which the steering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steering cylinders from the steering directional valve, and the faster the wheels will change direction. On the front of the HMU are four ports: •

• Return to tank                               Left turn 
• Pump supply                                 Right turn 

The HMU also has a fifth port on the side of the HMU. The fifth port is the load sensing signal line to the steering directional valve.

Identify steering accumulators

A third accumulator has been added to the 789D truck. The three accumulators (arrow) provide the supply oil during normal operation and temporary secondary steering if a loss of pump flow occurs.
The accumulators are piston-type accumulators that are charged with nitrogen. The nitrogen charge provides energy for normal and secondary steering capability if steering pump flow stops.
To check the secondary steering system, the engine must be shut off with the engine shutdown switch while leaving the key start switch in the ON position. When
the shutdown switch is used, the bleed down solenoid is not energized and the accumulators do not bleed down. The truck can then be steered with the engine stopped.
NOTE: High pressure oil remains in the accumulators if the shutdown switch is used. To release the oil pressure in the accumulators, turn the key start switch to the OFF position and turn the steering wheel left and right until the oil is drained from the accumulators (steering wheel can no longer be turned).

Lab 1: Steering System Component Identification
Instructions: Write the number of the component next to the name.
1. Case drain oil filter ---------
2. Solenoid and relief valve manifold------------
3. Steering accumulator ------------------
4. Steering tank ---------------------
5. Steering pump --------------------
6. Hand metering unit ----------------
7. Steering directional valve ---------
8. Steering cylinder ----------------

Lab 1: Steering System Component Identification (continued)
Instructions: Write the number of the component next to the name.

…..Solenoid and relief valve manifold
….Steering directional valve
…..Secondary cylinders
….Steering accumulators
…..Hand Metering Unit (HMU)
….Steering tank
….Steering pump
….Accumulator charging valve

Lab 1: Steering System Component Identification (continued)
Instructions: Write the number of the component next to the name.
 ------ Combiner/check spool
 ------ Directional spool
 ------ Load sensing port
 ------ to tank
------  HMU
------  Back pressure valve
------  Right turn cylinder
------ Right turn pilot oil
------ Accumulators
------ Priority spool
------ Left turn pilot oil
-----  Left turn cylinder
 ----- Amplifier spool
 -----Relief/makeup valve




789D Off-Highway Trucks Steering System Post-Assessment
Instructions: Select the correct answer(s) for each question.

1. The steering system hydraulic tank has how many oil level gauges?
     a. 3.
     b. [2].
     c. 1.
     d. 4.
2. To enable the steering system when the pump fails or the engine will not start, the operator ______________.
    a. sets the machine in neutral and manually turns the wheel.
    b. uses the economy mode.
    c. activates the secondary steering pump.
    d. [uses an Auxiliary Power Unit (APU) to charge the accumulators].
3. The 789D has ______________ steering accumulators.
    a. [3].
    b. 2                                                                  .
    c. 1.
    d. 4.
4. The steering system condition is monitored through the ____________________.
     a. diverter valve.
     b. load sensing controller.
     c. steering pump flow compensator.
     d. [low and high pressure switches].
5. Oil circulates through the HMU while the steering wheel is stationary to prevent ____________________.
     a. accumulator discharge.
     b. pressure spikes.
     c. filter failures.
     d. [sticking steering wheel]

6. A steering signal warning is displayed ____________.
    a. during a thermal bleeda.
    b. only when the machine is movingb.
    c. whenever there is a problemc.
     d. [only if ground speed is above 8 km/h (5 mph) or the machine is not in NEUTRAL]d.
7. If the steering pump pressure cycles ____________ during standby conditions,         leakage is in the system and must be corrected.
    a. in 25 secondsa.
    b. [in less than 25 seconds]b.
     c. in 60 secondsc.
    d. onced