797F Off-Highway Truck Steering System: Piston Pump (Steering) Pressure

The following tests cover the setting of the following valves:

Pressure and flow compensator valve
High-pressure cutoff valve

Table 1
Required Tools
Tool	Part Number	Part Description	Qty
A	8T-0860	Pressure Gauge	1

Steering piston pump
(1) Current adjustment screw
(2) Displacement solenoid
(3) High-pressure cutoff valve(4) Pressure and flow compensator valve

The steering piston pump is part of a double piston pump group. The fan drive pump is the other part of the pump group. Both pumps are variable displacement piston pumps. A charging pump is located between the steering pump and the fan drive pump. The charging pump is used in order to provide both pumps with oil. The steering pump is located at the front bottom section of the pump drive. The pump drive is located on the right-hand frame rail near the rear of the flywheel housing.

The steering piston pump will produce flow at high pressure until the steering accumulators are charged with oil and until the pressure increases to the cutout pressure of 24115 ± 345 kPa (3500 ± 50 psi). When the cutout pressure is reached, the accumulator pressure sensor will send a signal to the Chassis ECM. The Chassis ECM will then increase the amount of current to displacement solenoid (2) in order to move a spool within pressure and flow compensator valve (4) . The pressure and flow compensator valve destrokes the steering piston pump to the low pressure standby condition.

At low-pressure standby, the pressure at the steering pump should be between 1720 kPa (250 psi) and 3445 kPa (500 psi). The steering pump will operate at the minimum swashplate angle in order to supply oil for lubrication and for normal internal leakage. Because of the normal internal leakage within the steering system, the pressure in the accumulators will gradually decrease to the cut-in pressure of 22050 ± 345 kPa (3200 ± 50 psi). When the pressure in the steering accumulators decreases to the cut-in pressure, the accumulator pressure sensor will send a signal to the Chassis ECM. The Chassis ECM then decreases the amount of current to displacement solenoid (2) and the steering pump upstrokes to the maximum displacement.

Note: Both the cutout pressure and the cut-in pressure are controlled by the accumulator pressure sensor.

(5) Pressure tap for the steering pump
(6) Pressure tap for the steering accumulators(7) Solenoid and relief valve

The steering pump pressure can be measured at pressure tap (5) .

Steering accumulator pressure can be measured at pressure tap (6) . At low idle in the NEUTRAL position or in the NO STEER position, the steering pump will cycle between the cutout pressure and the cut-in pressure in intervals of 30 seconds or more.

Pressure and flow compensator valve
(1) Current adjustment screw
(2) Displacement solenoid and valve
(3) High-pressure cutoff valve
(8) Locknut(9) Setscrew

Pressure and flow compensator valve (4) is mounted to the case of the steering pump. Oil flows from the pump that charges the system to the high-pressure cutoff valve (3) , to displacement solenoid (2) , and then to the actuator piston (maximum angle).

Current adjustment screw (1) controls the spring pressure in the displacement valve. The current adjustment screw changes the minimum current that is required to start destroking the steering pump.

Note: Do not adjust current adjustment screw (1) in the chassis. The adjustment should only be performed on a hydraulic test stand.

High-pressure cutoff valve (3) controls the maximum pressure in the steering system if the accumulator pressure sensor should not function properly, or a wire fails to the displacement solenoid. High-pressure cutoff valve (3) controls the maximum pressure by controlling the flow of pump output pressure to the actuator piston (minimum angle). The pressure setting for the high-pressure cutoff valve is 28200 ± 1000 kPa (4090 ± 145 psi). If the system is functioning normally, neither the high presure cutoff valve or the backup relief valve will be active.

Use the following procedure in order to check high-pressure cutoff valve (3) :

Disconnect the wiring harness to displacement solenoid (2) .Note: When the displacement solenoid is disconnected, the steering pump will destroke and the steering pump will operate at the minimum flow and at the maximum pressure.

Install Tool (A) to pressure tap (5) .

Start the engine. Operate the engine at high idle.

Observe Tool (A) . High-pressure cutoff valve (3) will open in order to limit the maximum steering pump pressure.

In order to adjust the setting of the high-pressure cutoff valve, loosen locknut (8) on high-pressure cutoff valve (3) . Rotate setscrew (9) clockwise in order to raise the pressure. Rotate setscrew (9) counterclockwise in order to lower the pressure.