STEERING SYSTEM ON 797 OFF-HIGHWAY TRUCK CATERPILLAR

797 B STEERING SYSTEM

STEERING SYSTEM

The steering pump will produce flow at high pressure until the steering accumulators are charged with oil and the pressure increases to 24115 ± 345 kPa (3500 ± 50 psi). This pressure is referred to as the CUT-OUT pressure. When the CUT-OUT pressure is reached, the accumulator pressure sensor sends a signal to the Chassis ECM. The
Chassis ECM then increases the amount of current to the steering pump displacement solenoid and the pump destrokes to the LOW PRESSURE STANDBY condition. During LOW PRESSURE STANDBY, the pressure at the pump should be between 1720 and 3445 kPa (250 and 500 psi).


NOTE: Not all of the switches and sensors shown in the visual input directly to the Chassis ECM as shown. Some of these inputs enter other ECMs and enter the Chassis ECM through the Cat Data Link.

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 22050 ± 345 kPa (3200 ± 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 pressure sensor sends a signal to the Chassis ECM. The Chassis ECM then decreases the amount of current to the displacement solenoid and the pump upstrokes to maximum displacement (full flow).
Engine speed is also an input to the Chassis ECM that is used to control the output from the steering pump.
The accumulator pressure sensor can have one of the following values:

1. Really low - Occurs during heavy steering
2. Low - Occurs during light steering, or normal bleed down
3. Normal - Desired pressure range of accumulator
4. High - Occurs when the accumulator pressure has gone above the normal cut-out
    Point.

Whenever the pressure status drops to Low or Really Low, the charging system activates. The steering pump is activated until the steering pressure reads High. At that point, the charging system deactivates. The system stays deactivated until normal bleed down or some amount of steering re-activates the system.

The charging control has 3 modes of operation:

1. Charging off
2. Charging low
3. Charging high

For each of these modes, there is a separate map of engine output speed to charging control current. This map is intended to adjust the pump output to provide more flow at lower engine speeds. The output current has a rate limit check. This will help smooth the current going to the pump when the logic switches between different maps.

There are a couple of alterations to the specified logic:
* Hysteresis for the pressure status. This prevents the charging system from being unstable if the           pressure stays at the decision point.
* Off timer. This keeps the pump off until the engine is running. It also turns the pump off when the       engine is stopped. If the engine is not running, the steering pump is forced off.
* Diagnostic Override. Allows ET to override the steering pump output current. Before the override       will be allowed on, the machine must not be moving .

Lower rear tank

The steering tank is separately located within the large hydraulic tank on the right side of the truck. The steering tank provides an oil supply for the steering system, the brake cooling drive system, and the fan motor drive system.

The oil level for the steering tank is checked at the upper sight gauge (1) when the oil is cold and the engine is stopped. After the engine is started, the oil level will decrease as the oil fills the steering accumulators. After the steering accumulators are filled, the oil level should be checked again at the lower sight gauge (2). When the engine is running and the steering accumulators are fully charged, the oil level should not be below
the ENGINE RUNNING marking of the lower gauge. If the ENGINE RUNNING level is not correct, check the nitrogen charge in both steering accumulators. A low nitrogen charge will allow excess oil to be stored in the accumulators and will reduce the secondary steering capacity.

Before removing the cap to add oil to the lower rear tank, be sure that the engine was shut off with the key start switch, and the steering oil has returned to the tank from the accumulators.

A steering oil level sensor (3) and a steering oil temperature sensor (4) are located on the rear of the steering tank. The sensors provide input signals directly to the VIMS Main Module. The VIMS informs the operator of the steering tank oil level and 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). The steering system on the 797B truck uses a piston-type pump for the oil supply. All piston-type pumps have a small amount of leakage into the case for cooling and lubrication. The case drain oil returns to the steering tank through a case drain oil filter (see Visual No. 53). Check the steering tank breather (5) for plugging.

Steering pump

The steering pump (1) is part of a double piston pump group.  The fan drive pump (2) is the other part of the pump group.  The pump group is mounted on the front of the pump drive.  The pump drive is located on the inside of the right frame rail near the torque converter.  A charging pump is located between the steering pump and the fan drive pump and is used to keep the pumps supplied with oil.

The steering pump operates only when the engine is running and provides the necessary flow of oil to the accumulators for steering system operation.  The Chassis ECM controls the flow of oil from the steering pump by energizing the displacement solenoid (3).

The Chassis ECM analyzes the accumulator pressure and the engine speed inputs and sends between 0 and 650 milliamps to the solenoid. Between 0 to 100 milliamps the pump is at maximum displacement and the steering pump flow is at maximum. Between 600 to 650 milliamps the pump is at minimum displacement and the steering pump flow is at minimum.  The coil resistance through the solenoid is approximately 24 ohms.

The displacement solenoid moves a spool in the pressure and flow compensator valve (4) to control the flow of pump output pressure to the minimum angle actuator piston.  The minimum angle actuator piston moves the swashplate to the minimum flow position.  The current adjustment screw (5) controls the minimum current required to start destroking the pump.  The present setting is 100 milliamps.

NOTE:  Do not adjust the current adjustment screw in chassis.  This adjustment should only be done on a hydraulic test stand.

The high pressure cut-off valve (6) controls the maximum pressure in the steering system if the accumulator pressure sensor fails.  The high pressure cut-off valve controls maximum pressure by controlling the flow of pump output pressure to the minimum angle actuator piston.  When system pressure is at maximum, the high pressure cut-off valve sends oil to the minimum angle actuator piston and moves the swashplate to the minimum flow position.  The high pressure cut-off valve setting is
26180 ± 345 kPa (3800 ± 50 psi).

The high pressure cut-off valve setting can be checked by disconnecting the displacement solenoid harness and operating the truck at HIGH IDLE. When the solenoid is disconnected, the pump will destroke and operate at minimum flow and maximum pressure (High Pressure Cut-off).  The high pressure cut-off valve opens to limit the maximum pump pressure.  The high pressure cut-off valve adjustment screw can be used to change the setting.

The minimum angle stop screw (7) is located near the pressure and flow compensator valve.  The maximum angle stop screw is located on the other side of the pump.

NOTE: Do not adjust the minimum or maximum angle stop screws in chassis. This adjustment should only be done on a hydraulic test stand.

Steering and fan drive pump.

Shown is a sectional view of the 797B steering pump.  The steering pump is part of a double piston pump group. The fan drive pump is the other part of the pump group..  The pumps are variable displacement piston- type pumps. Oil flows from the steering pump to the steering solenoid and relief valve manifold.

Oil from the steering tank enters the pump group in the port below the charge pump impeller.  The charge pump keeps the two pumps full of oil.

The large spring around the maximum angle actuator piston holds the swashplate at maximum angle.  Pump output pressure is always present on the left side of the steering pump maximum angle actuator piston and also helps to hold the swashplate at maximum angle.  When the swashplate is at maximum angle, pump output is at maximum flow.  This is the position of the pump when the displacement solenoid receives 0 to 100 milliamps from the Chassis ECM.

When the displacement solenoid is receiving between 100 and 650 milliamps from the Chassis ECM, the displacement solenoid moves a spool in the pressure and flow compensator valve.  The spool allows pump output pressure to flow to the minimum angle actuator piston.

The minimum angle actuator piston has a larger diameter than the maximum angle actuator piston.  The minimum angle actuator piston moves the swashplate toward the minimum flow position.  The swashplate angle and pump flow will modulate with the amount of  current at the displacement solenoid.  When the swashplate is at minimum angle, pump output is at minimum flow.  This is the position of the pump when the displacement solenoid receives 600 to 650 milliamps from the Chassis ECM.

Before the swashplate contacts the minimum angle stop, the minimum angle actuator piston will open a small drain port to tank and stop the movement of the swashplate.  Draining the minimum angle actuator piston oil will prevent the swashplate from contacting the minimum angle stop repeatedly which can be noisy and may cause damage to the pump.

Oil that leaks past the pistons into the pump housing provides lubrication for the rotating components.  This oil leakage is referred to as case drain oil.  Case drain oil flows through the case drain port and a case drain oil filter (see Visual No. 53) to the steering tank.

Steering pump pressure and flow compensator valve.

Shown is the pressure and flow compensator valve for the 797B steering pump.  The charge pump pulls oil from the steering tank and keeps the steering and fan drive pumps full of oil.  Oil flows from the charge pump to the high pressure cut-off valve, the displacement valve, and the maximum angle actuator piston

The pump output oil and the spring around the maximum angle actuator piston holds the swashplate at maximum angle.  This is the position of the pump when the displacement solenoid receives 0 to 100 milliamps from the Chassis ECM and pump output pressure is low.

When the displacement solenoid is receiving between 100 and 650 milliamps from the Chassis ECM, the displacement solenoid moves the valve spool to the right.  The spool allows pump output pressure to flow to the minimum angle actuator piston.  The minimum angle actuator piston has a larger diameter than the maximum angle actuator piston.  The minimum angle actuator piston moves the swashplate toward the minimum flow position.

The current adjustment screw controls the spring pressure in the displacement valve and changes the minimum current required to start destroking the pump.  The present setting is 100 milliamps.

NOTE:  Do not adjust the current adjustment screw in chassis.  This adjustment should only be done on a hydraulic test stand.

The high pressure cut-off valve controls the maximum pressure in the steering system if the accumulator pressure sensor fails.  The high pressure cut-off valve controls the flow of pump output pressure to the minimum angle actuator piston.  When system pressure is at maximum, the high pressure cut-off valve sends oil to the minimum angle actuator piston and moves the swashplate to the minimum flow position.  The high pressure cut-off valve setting is 26180 ± 345 kPa (3800 ± 50 psi).

The maximum accumulator pressure during normal operation should be 24115 ± 345 kPa (3500 ± 50 psi).  This pressure is referred to as the CUT-OUT pressure. When the CUT-OUT pressure is reached, the accumulator pressure sensor sends a signal to the Chassis ECM.  The Chassis ECM then increases the amount of current to the steering pump displacement solenoid and the pump destrokes to the LOW PRESSURE STANDBY condition.  During LOW PRESSURE STANDBY, the pressure at the pump should be between 1720 and 3445 kPa (250 and 500 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 22050 ± 345 kPa
(3200 ± 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 pressure sensor sends a signal to the Chassis ECM. The Chassis ECM then decreases the amount of current to the displacement solenoid and the pump upstrokes to maximum displacement (full flow).

Steering system oil supply hose

Steering pump supply oil flows through a hose (1) and a check valve in the solenoid and relief valve manifold (2). The solenoid and relief valve manifold connects the steering pump to the accumulators and the steering control valve (3). The solenoid and relief valve manifold also provides a path to drain for the steering oil.

Steering system oil supply hose

Steering pump supply oil flows through a hose (1) and a check valve in the solenoid and relief valve manifold (2). The solenoid and relief valve manifold connects the steering pump to the accumulators and the steering control valve (3). The solenoid and relief valve manifold also provides a path to drain for the steering oil

Steering oil supply hose

Steering oil flows through the hose (1) to the pressure reducing valve (2). The pressure reducing valve reduces the steering pressure to a signal pressure of 6200 kPa (900 psi).  Excess steering oil flows to tank through a hose (3).  The reduced signal oil flows through the hose (4) to the fan drive pump and the brake cooling drive pump.

The fan drive pump and the brake cooling drive pump use the signal oil pressure to destroke the pumps to minimum flow at start-up and during cold temperatures.

Two pressure switches monitor the condition of the steering system.  The low steering pressure switch (5) monitors the output of the steering pump. This switch monitors pump supply pressure during LOW PRESSURE STANDBY. The VIMS refers to this switch as the "low steering pressure" switch.

The other steering pressure switch (see Visual No. 52) monitors the steering system accumulator pressure.  The VIMS refers to this switch as the "high steering pressure" switch.

Both steering pressure switches provide input signals to the VIMS Main Module. 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 transmission gear is not in NEUTRAL.

Steering system oil supply hose

Steering pump supply oil flows through a hose (1) and a check valve in the solenoid and relief valve manifold. The check valve (not shown) prevents accumulator oil from flowing back to the steering pump when the pump destrokes to LOW PRESSURE STANDBY.

Pump supply pressure can be measured at the tap (2).  Pump supply pressure will increase until the pump outlet pressure is approximately 24115 ± 345 kPa (3500 ± 50 psi) at LOW IDLE (CUT-OUT).  Then the
pump supply pressure will decrease to the LOW PRESSURE STANDBY setting. The LOW PRESSURE STANDBY setting is between
1720 and 3445 kPa (250 and 500 psi).

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.

Steering accumulator pressure can be measured at the tap (3). Accumulator pressure will increase until the pressure is approximately 24115 ± 345 kPa (3500 ± 50 psi) at LOW IDLE (CUT-OUT).  Because of
the normal leakage in the steering system, the pressure in the accumulators will gradually decrease to the CUT-IN pressure, which is 22050 ± 345 kPa (3200 ± 50 psi).

At LOW lDLE in the NEUTRAL or NO STEER position, the pump will cycle between the CUT-OUT and CUT-IN conditions in intervals of 30 seconds or more.  Connecting a pressure gauge to the accumulator pressure tap will indicate these steering system pressures.

To operate the steering circuit on a disabled truck, an Auxiliary Power Unit (APU) connects to the supplemental steering connector (4) on the solenoid and relief valve manifold.  The APU will provide supply oil to charge the accumulators.  Steering capability is then available to tow the truck.

Check valve

Steering pump supply oil flows through the check valve (1). The check valve prevents accumulator oil from flowing back to the steering pump when the pump destrokes to LOW PRESSURE STANDBY.

The accumulator bleed down solenoid (2) drains pressure oil from the accumulators when the truck is not in operation.  The accumulator bleed down solenoid is activated by the Chassis ECM when the key start switch is moved to the OFF position.  The Chassis ECM holds the solenoid open for 70 seconds.

The back-up relief valve (3) protects the system from pressure spikes if the pump cannot destroke fast enough or limits the maximum pressure if the steering pump high pressure cut-off valve does not open. The setting of the back-up relief valve is 27560 ± 345 kPa (4000 ± 50 psi).

Steering system oil samples can be taken at the steering system Scheduled Oil Sampling (S•O•S) tap (4).

Solenoid and relief valve manifold

Shown is a sectional view of the solenoid and relief valve manifold. Steering pump supply oil flows through a check valve.  The check valve prevents accumulator oil from flowing back to the steering pump when the pump destrokes to LOW PRESSURE STANDBY.

The accumulator bleed down solenoid is activated by the Chassis ECM when the key start switch is moved to the OFF position.  The Chassis ECM holds the solenoid open for 70 seconds.

Pressure oil from the accumulators is sensed by the bleed down solenoid. When the solenoid is ENERGIZED, the plunger moves and connects the pressure oil to the drain passage.  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.  When the solenoid is
DE-ENERGIZED, spring force moves the plunger and pressure oil cannot go to drain

The back-up relief valve protects the steering system from pressure spikes if the pump cannot destroke fast enough or limits the maximum pressure if the steering pump high pressure cut-off valve does not open.  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 oil pressure reaches approximately 27560 ± 345 kPa (4000 ± 50 psi) at a flow of
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 only be adjusted 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 the pressure setting, or counterclockwise to decrease the pressure setting.  One revolution of the setscrew will change the pressure setting approximately 4730 kPa (685 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.

Steering accumulators

Three steering accumulators (1) provide the supply oil during normal operation and temporary secondary steering if a loss of pump oil flow occurs.  Inside the accumulators is a rubber bladder that is charged with nitrogen.  The nitrogen charge provides energy for normal steering and secondary steering capability if steering pump flow stops. The nitrogen charge pressure is 10335 kPa (1500 psi) at 21°C (70°F).

To check the secondary steering system, the engine must be shut off with the manual shutdown switch while leaving the key start switch in the ON position.  When the manual 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.

Steering oil from the solenoid and back-up relief valve manifold returns to the steering tank through the steering and fan drive oil return filters (2). The hydraulic fan drive oil also returns to the tank through these filters

WARNING
High pressure oil remains in the accumulators if the manual 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.
Steering and fan drive system oil samples can be taken at the Scheduled Oil Sampling (S•O•S) tap (3) located in the filter base.  The steering system (S•O•S) tap located below the solenoid and back-up relief valve manifold is the preferred location to obtain the steering system oil samples .

An oil filter bypass switch (4) is also located on the filter base. The bypass switch provides an input signal to the Brake/Cooling ECM.  The Brake/Cooling ECM sends the signal to the VIMS, which informs the operator if the filter is restricted.  An oil filter bypass valve is located in the filter base. The bypass valve will open to protect the systems if the filter is restricted.

Two pressure switches monitor the condition of the steering system.  The high steering pressure switch (5) monitors the steering system accumulator pressure.  The VIMS refers to this switch as the "high steering pressure" switch.  The other steering pressure switch (see Visual No. 48) monitors the output of the steering pump.  This switch monitors pump supply pressure during LOW PRESSURE STANDBY.  The VIMS refers to this switch as the "low steering pressure" switch.

Both steering pressure switches provide input signals to the VIMS Main Module. 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 transmission gear is not in NEUTRAL.

The accumulator pressure sensor (6) is the main component input to the Chassis ECM for controlling the steering system.  The sensor is used to determine when the accumulators need to be charged.  The accumulator pressure sensor also controls the cut-out pressure and the cut-in pressure. The accumulator pressure sensor can have one of the following values:
1. Really low - Occurs during heavy steering
2. Low - Occurs during light steering, or normal bleed down
3. Normal - Desired pressure range of accumulator
4. High - Occurs when the accumulator pressure has gone above the normal cut-out point


Whenever the pressure status drops to Low or Really Low, the charging system activates.  The steering pump is activated until the steering pressure reads High.  At that point, the charging system deactivates.  The system stays deactivated until normal bleed down or some amount of steering re-activates the system.

Steering and fan drive oil cooler

Steering and fan drive oil flows from the steering and fan drive filter through the steering and fan drive oil cooler (1) to the tank. The steering and fan drive oil cooler is the smaller diameter oil cooler on the right side. The two larger diameter oil coolers on the left are for front brake oil cooling.

Located on the front of the hydraulic tank are two case drain oil filters. There are several piston-type pumps and motors used on the 797B truck. All piston-type pumps and motors have a small amount of leakage into the case for cooling and lubrication. The case drain oil then returns to the lower rear hydraulic tank through the case drain oil filters.

Oil returns to the hydraulic tank through the bottom case drain filter (2) from the following pumps and motors:
* Brake cooling drive pump

* Brake cooling motor

* Steering pump

Oil returns to the hydraulic tank through the top case drain filter (3) from the following pumps and motors:
* Fan drive motor

* Fan drive pump.

Steering control valve (arrow)

The steering control valve (arrow) is pilot operated from the Hand Metering Unit (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 control 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.

Steering control valve components

Shown is a sectional view of the steering control valve. The main components of the steering control valve are:  the priority spool, the amplifier spool with internal combiner/check spool, the directional spool, the relief/makeup valves, and the back pressure valve.

Pressure oil from the accumulators 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. 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 (supply, tank, right turn, and left turn) are vented to the tank through the HMU. The directional spool is held in the center position by the centering springs.  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 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 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 170 kPa (25 psi) in the passage behind the makeup valve.  A pressure higher than 170 kPa (25 psi) will open the back pressure valve to the tank.

The steering control valve must be removed and tested on a hydraulic test bench to accurately check the setting of the relief/makeup valves.

To functionally test the right relief/makeup valve on the machine, install two Tees with pressure taps in the right turn steering hose at the steering cylinders.  Steer the truck all the way to the right against the stops and shut off the engine.  An external pump supply must be connected to one of the pressure taps on the right turn hose. Connect a pressure gauge to the other pressure tap on the right turn hose. Pressurize the steering system and the reading on the gauge will be the setting of the right relief/makeup valve.

To test the left relief/makeup valve on the machine, install two Tees with pressure taps in the left turn steering hose at the steering cylinders.  Steer the truck all the way to the left against the stops and shut off the engine. An external pump supply must be connected to one of the pressure taps on the left turn hose.  Connect a pressure gauge to the other pressure tap on the left turn hose.  Pressurize the steering system and the reading on the gauge will be the setting of the left relief/makeup valve

Steering control valve during a RIGHT TURN

When the steering wheel is turned to the RIGHT, the HMU "thermal bleed" and venting of the four work ports (supply, tank, right turn, and left turn) to the tank is stopped.  Right turn pilot oil flows into the left side of the directional spool through a stabilizing orifice and moves the directional spool to the right.  Movement of the directional spool allows pilot oil to flow to the amplifier and combiner/check spools.

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 accumulator 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.

Steering resistance increases the supply (cylinder) pressure to the HMU and the load sensing pilot line.  The load sensing pilot line directs cylinder pressure to the priority spool.  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.

Return oil from the cylinders flows across the directional spool, around the relief/makeup valve, forces the back pressure valve open, and 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 accumulator 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

HMU

The 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 control 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 control 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

A fifth port is on the side of the HMU.  The fifth port is the load sensing signal line to the steering control valve.

Accumulators direct oil to steering control valve

Pump supply oil from the accumulators flows through the steering control valve to the Hand Metering Unit (HMU).

If the steering wheel is not turned, the oil flows through the HMU 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 control valve.  The steering control valve directs oil to the steering cylinders.  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 control valve and returns to the tank

Steering System ET Screens

Shown is the 797B Chassis ECM Configuration Screen as seen with Electronic Technician (ET). One of the parameters shown on the screen is "Steering Accumulator Charging System Installation."  If a truck has the Electronically Controlled Steering System installed, the Value of this parameter must be set to "Installed."

Trucks with the two pump hydraulically controlled steering system should set this Value to "Not Installed."

Chassis ECM Override Parameter Screen

Shown is the 797B Chassis ECM Override Parameter Screen as seen with ET.  The last parameter shown on the screen is "Steering Accumulator Charging Sol Current Override."  If a truck has the Electronically Controlled Steering System installed, the value of this parameter can be changed from 0 to 100%, which represents a current between 0 to
1000 milliamps.  If the value entered is 0%, the ECM will send 0 milliamps of current to the steering pump solenoid.  At 0 milliamps the pump is at maximum displacement and the steering pump flow is at maximum. If the value entered is 60% or greater, the ECM will send 600 or more milliamps of current to the steering pump solenoid.  At 600 milliamps the pump is at minimum displacement and the steering pump flow is at minimum.

The steering pump high pressure cut-off valve setting can be checked by setting this value to 0% and operating the truck at HIGH IDLE.  The Engine Speed and Steering Accumulator Oil Pressure can be observed at the top of the screen.

Chassis ECM Status Screen Group

Shown is the 797B Chassis ECM Status Screen Group 6 as seen with ET. One of the parameters shown on the screen is "Steering Accumulator Charging Solenoid Current."  If a truck has the Electronically Controlled Steering System installed, the value of this parameter will change from 0 to 65.0%, which represents a current between 0 to 650 milliamps.

If the value displayed is 0%, the ECM is sending 0 milliamps of current to the steering pump solenoid. At 0 milliamps the pump is at maximum displacement and the steering pump flow is at maximum.  If the value displayed is 65.0%, the ECM is sending 650 milliamps of current to the steering pump solenoid.  At 650 milliamps the pump is at minimum displacement and the steering pump flow is at minimum.


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