797F Off-Highway Truck: SOLENOID AND RELIEF VALVE OPERATION

Location of the steering solenoid and relief valve

The steering solenoid and relief valve is located on the front frame rail under the engine.

View A of the steering solenoid and relief valve
(1) Pump supply port
(2) Pressure tap for measuring pump supply pressure
(3) Pressure tap for measuring steering accumulator pressure (4) Pressure tap for the Auxiliary Power Unit (APU)

Oil supply flows from the steering pump through port (1) and check valve (8) to the steering solenoid and relief valve. Check valve (8) prevents oil in the accumulators from flowing back to the steering pump when the pump destrokes to the low pressure standby condition.
Pump supply pressure can be measured at pressure tap (2). Pump supply pressure will increase until the pump outlet pressure is approximately the same value as the cutout pressure. Then, the pump supply pressure will decrease to the setting of the low pressure standby condition.
Note: If steering pressure is measured at pressure tap (2) during the low pressure standby condition, then a suitable gauge must be used in order to measure the steering system pressure when the steering pump upstrokes to the maximum displacement.
Steering accumulator pressure can be measured at pressure tap (3). Accumulator pressure will increase until the cutout pressure is reached. Due to normal leakage within the steering system, the pressure in the accumulators will gradually decrease to the cut-in pressure.
Pressure tap (4) is for the Auxiliary Power Unit (APU). Refer to "Secondary Steering" for more information.

Steering solenoid and relief valve
(1) Pump supply port
(5) Steering accumulator port
(6) Cavity
(7) Steering accumulator port
(8) Check valve
(9) Port to steering control valve
(10) Orifice
(11) Port for return oil from fan drive
(12) Passage
(13) Steering solenoid valve
(14) Port for return oil from steering control valve
(15) Return port to steering hydraulic tank (16) Backup relief valve




Illustration 4
Side view (8) Check valve (13) Steering solenoid valve




Illustration 5
Section A-A (13) Steering solenoid valve (17) Passage (18) Cavity (19) Plunger (20) Valve (21) Oil outlet openings (22) Spring




Illustration 6	g02008694
Section view of the steering solenoid and relief valve (1) Pump supply port (2) Pressure tap for measuring pump supply pressure (5) Steering accumulator port (7) Steering accumulator port (8) Check valve (9) Port to steering control valve (13) Steering solenoid valve (14) Port for return oil from steering control valve (16) Backup relief valve

The pressure oil from the steering piston pump is routed through port (1) in order to fill cavity (6). Cavity (6) connects the following components: port (1) from the steering piston pump, ports (5) and (7) for the steering accumulators, port (9) for the steering control valve, passage (12) for steering solenoid valve (13) and backup relief valve (16) for the steering system.
Return port (15) drains oil to the steering hydraulic tank. Cavity (18) connects the following components: return port (15), port (14) for return oil from the steering control valve, port (11) for return oil from the fan drive system, passage (12) for steering solenoid valve (13) and backup relief valve (16) for the steering system.

Steering Solenoid Valve

Steering solenoid valve (13) is used to drain pressure oil from the accumulators when the machine is not being used.
The oil pressure of the steering accumulators is in cavity (6). This oil flows through orifice (10) into passage (12). When steering solenoid valve (13) is deactivated, this oil is blocked by plunger (19). When the engine start switch is moved to the OFF position, steering solenoid valve (13) is activated. When steering solenoid valve (13) is activated, plunger (19) moves up. Pressure oil goes through orifice (10) in passage (12), past plunger (19) and out return port (15) to the steering hydraulic tank. Orifice (10) limits the oil pressure at return port (15). The oil pressure at return port (15) will be lower than the pressure setting of the filter bypass valve in the steering hydraulic tank.

View behind the operator's station (23) Location of the chassis electronic control

The steering solenoid valve is activated for 70 seconds. The steering solenoid valve is activated by a relay for the purging of the steering accumulators. The relay is located in relay panel (24). The relay is activated by the chassis electronic control module (23) when the engine start switch key is moved to the OFF position. The steering solenoid valve is not activated when the battery disconnect switch or the engine shutdown control is used.

Backup Relief Valve for the Steering System

Backup relief valve (16) for the steering system will protect the steering system from pressure spikes if the pressure and flow compensator valve fails to destroke the steering pump. Also, the backup relief valve will limit the maximum pressure if the high pressure cutoff valve for the steering pump does not open. The oil in cavity (6) enters passage (17) in backup relief valve (16). If the pressure of the oil is within limits, valve (20) will block the oil. If oil pressure reaches the relief pressure, valve (20) compresses spring (22). The oil from cavity (6) flows past valve (20) through oil outlets (21). This oil is now open to return port (15) and the oil drains to the steering hydraulic tank.
When the oil in the steering circuit can go through passage (17) and oil outlets (21), the pressure of the oil cannot increase to a pressure higher than the setting of backup relief valve (16). The pressure setting of backup relief valve (16) can be changed by turning the setscrew. A bench test must be performed before any adjustments are made.

Secondary Steering

The machine does not have a secondary steering pump. If the steering piston pump fails, an Auxiliary Power Unit can be used in order to operate the steering circuit on a disabled machine. The Auxiliary Power Unit connects to port (4) on the steering solenoid and relief valve and to a suction port on the steering hydraulic tank. This arrangement will provide pressure oil in order to charge the steering accumulators. Steering capability is then available to tow the machine.

See You Soon!!!

MARYGAR