797F OFF-HIGHWAY TRUCK STEERING SYSTEM OPERATION PART 1

Hydraulic Schematic

Basic steering hydraulic schematic with "No Turn"
(A) To fan motor
(B) To fan pump
(1) Steering accumulators
(2) Pressure sensor (steering pump)
(3) Solenoid and relief vale manifold
(4) Steering pump
(5) Case drain oil filters
(6) Hydraulic tank (steering section)
(7) Return oil filter
(8) Oil cooler
(9) Pressure reducing valve
(10) Hand metering unit
(11) Steering control valve (12) Steering cylinders

Location of Components


Illustration 2
Typical bottom view of 797F LAJ1-151 (6) Steering cylinders (7) Steering control valve (8) Steering accumulators (18) Steering and fan drive filter (23) Steering solenoid and backup relief valve



Illustration 3
Typical bottom view of 797F XQ WSP1-Up and 797F LAJ152-Up. (6) Steering cylinders (7) Steering control valve (8) Steering accumulators (18) Steering and fan drive filter (23) Steering solenoid and backup relief valve



Illustration 4
(7) Steering control valve (19) Backup relief valve (23) Steering solenoid and backup relief valve



Illustration 5
Inside of the RH side of the frame (1) Accumulator pressure sensor (12) Pressure reducing valve (23) Steering solenoid and backup relief valve



Illustration 6
Inside of the RH side of the frame (14) Steering piston pump (21) Hydraulic tank (24) Pump drive



Illustration 7
(22) Steering metering pump



Illustration 8
Bottom view of the steering accumulators (1) Accumulator pressure sensor (10) High oil pressure switch

Operation

The steering system is hydraulic and the steering pump is electronically controlled. There is no mechanical connection between the steering wheel and steering cylinders (6) that move the front wheels. Oil for the steering system is stored in hydraulic tank (21). Steering piston pump (14) is a variable displacement piston pump. Accumulator pressure sensor (1) sends the pressure reading of the accumulators to chassis ECM (2). Chassis ECM (2) controls the flow of oil from the steering pump by energizing displacement solenoid (4).

Illustration 9	g01987913
(14) Steering piston pump (24) Pump drive

Pump drive (24) drives the following pumps: the steering piston pump, the piston pump for the fan drive, the brake actuation pump, the gear pump for the motor drive for the rear axle oil pump, the gear pump for hoist and brake cooling and the brake cooling motor drive. Pump drive (24) is on the right-hand frame rail near the rear of the flywheel housing. Pump drive (24) is driven by a drive shaft. The drive shaft is connected between the pump drive and the engine. Steering piston pump (14) operates only when the engine is running.

The steering piston pump will produce flow at high pressure until steering accumulators (8) are charged with oil and the pressure increases to the cutout pressure. When the cutout pressure is reached, accumulator pressure sensor (1) will send a signal to chassis ECM (2). Chassis ECM (2) will then increase the amount of current to displacement solenoid (4) in order to move a spool within pressure and flow compensator valve (5). The pressure and flow compensator valve destrokes the steering piston pump to the low pressure standby condition.

At low-pressure standby, the steering pump operates at the minimum swashplate angle in order to supply oil for lubrication and normal internal leakage. Because of the normal internal leakage within the steering system, the pressure in the accumulators will gradually decrease until the cut-in pressure is reached. When the pressure in steering accumulators (8) decreases to the cut-in pressure, accumulator pressure sensor (1) sends a signal to chassis ECM (2). Chassis ECM (2) then decreases the amount of current to displacement solenoid (4) and the steering pump upstrokes to the maximum displacement.

Engine speed is also an input to chassis ECM (2) that is used to control the output from the steering pump. The engine speed is monitored by engine speed sensor (3) .

The accumulator pressure sensor can have one of the following values:

Table 1
Really low	This value occurs during heavy steering.
Low	This value occurs during light steering or normal internal leakage.
Normal	The desired pressure range of the accumulator.
High	This value occurs when the accumulator pressure has gone above the normal cutout pressure.

Whenever accumulator pressure sensor (1) senses the system pressure to be low or really low, the charging system will activate. The steering pump is activated until the system pressure is high. At that point, the charging system deactivates. The system stays deactivated until normal internal leakage or some amount of steering reactivates the charging system.

The charging system has three modes of operation:

Off
Low
High

For each of the three charging modes, there is a separate map of engine output speed to charging control current. The map is intended to adjust the steering pump output in order to provide more flow at lower engine speeds. The output current has a rate limit check.

Hydraulic Tank (Brake, Hoist, Steering)



(13) Case drain filters (21) Hydraulic tank

Internal leakage from steering piston pump (14) is returned to hydraulic tank (21) through case drain filters (13). Case drain filters (13) are located between hydraulic tank (21) and the front brake oil coolers. Return oil from the steering system flows through steering filter (18). Steering filter (18) is located between the steering solenoid and relief valve (23) and hydraulic oil cooler (16). Normally, oil flows through these filters and clean oil is returned to hydraulic tank (21). Case drain filters (13) and steering filter (18) will trap any debris that is present in the oil.

If any filter element becomes full of debris, the restriction to flow causes a pressure increase. Each of the filters has a bypass valve. The bypass valve will open when the pressure increases to the bypass pressure. When the bypass valve opens, oil is allowed to bypass the filter element. The oil goes directly to hydraulic tank (21). When the oil bypasses the filter element, debris in the oil can damage other system components. Proper maintenance must be used to ensure that the filter element does not stop the flow of clean oil to the hydraulic system.



Illustration 11
View of the standard hydraulic tank (1) Hydraulic tank for the shallow brake arrangement (A) Steering and hydraulic fan section (B) Hoist and brake cooling section (C) Section for the brake and rear axle lubrication

Hydraulic tank (1) for the shallow brake arrangement for the 797F is located on the outside of the right side of the frame. Hydraulic tank (1) has three sections.



Illustration 12
View of the Hydraulic tank for the shallow brake arrangement (1) Hydraulic tank (2) Fill port for the steering tank (3) Sight gauge (4) Breather port for the hydraulic tank (5) Return port for the rear brake cooling (6) Sight gauge (7) Fast fill for the hydraulic tank (8) Return port for the brake pump (9) Top off return port for the brake and rear axle lubrication (10) Return port for the brake and rear axle (11) Suction port for the brake and rear axle (12) Drain port for the brake tank (13) Sight gauge (14) Return port for the relief for the brake cooling (15) Sight gauge (16) Sight gauge

Fill port (2) is for steering section (A) of hydraulic tank (1). Fill port (7) is for section (B) and section (C) of hydraulic tank (1). Section (C) supplies the oil for brake actuation and for the rear axle lubrication. Section (C) is an internal tank that is used to keep the brake actuation oil free of contaminants that return with the brake cooling oil. Steering section (A) for hydraulic tank (1) contains oil for the steering system and the piston pump that drives the fan drive motor. An air breather for hydraulic tank (1) is located on the vertical frame member on the right side of the truck.

Hydraulic tank (1) has five sight gauges. All of the sight gauges are located on the front of hydraulic tank (1). Sight gauge (3) and sight gauge (13) are for the hoist and brake cooling section (B) of hydraulic tank (1). Sight gauge (15) and sight gauge (16) are for the steering and hydraulic fan section (A) of hydraulic tank (1). Sight gauge (6) is for section (C) of hydraulic tank (1) .

The oil level in the sight gauges will drop when the engine is operating and the accumulators are fully charged. Refer to Operation and Maintenance Manual, "Lubricant Viscosities and Refill Capacities" for lubrication viscosities and for refill capacities for the hydraulic tank.


Illustration 13
View of the standard hydraulic tank (17) Return ports for the front brake cooling (18) Return port for the hoist (19) Suction port for the steering and fan drive (20) Return port for the fan, brake cooling, and steering (21) Port for the fast fill warm switch (22) Port for the fast fill cold switch (23) Breather port for the steering tank (24) Port for the level switch for the fast fill dead engine (25) Port for the level sensor for steering (26) Port for the steering oil temperature sensor (27) Suction port for the secondary steering (28) Ecology drain for the steering tank (29) Suction port for the pump drive for the brake cooling (30) Fast fill port for the steering tank (31) Ecology drain for the hydraulic tank (32) Ports for the case drain (33) Suction ports for the brake cooling (34) Suction port for the hoist