MINIMUM PUMP FLOW

From Pump From

137

The pressure of the oil from the compensator valve passage moves the control piston, which rotates the swashplate toward the minimum angle. The pistons now have very little movement in and out of the barrel as the retraction plate and slippers follow the minimum angle of the swashplate.

While the accumulators are filled, this small movement of the pistons maintains the pressure at the setting of the pressure compensator valve. The compensator spool will remain open to provide pressure oil behind the control piston. Excess oil from the pump outlet goes into the pump case for cooling and lubrication. The oil then goes through a drain line to the case drain oil filter and steering hydraulic tank.

As the steering wheel is turned and oil is taken from the accumulators, the pressure at the pump outlet will decrease. When accumulator pressure decreases, the pressure compensator valve will allow the swashplate to move toward maximum angle and increase pump output.

Swashplate Piston Supply Passage for Pump Output Passage From Pump Pressure Compensator Valve Control Piston

MINIMUM FLOW

The 789C is equipped with a load sensing, pressure compensated, piston-type pump (1). The steering pump is mounted to the pump drive. The pump drive is located on the inside of the right frame rail near the torque converter.

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 steering pump contains a load sensing controller (2) 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 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 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 16470 ± 350 kPa

(2390 ± 50 psi). This pressure is referred to as the CUT-IN pressure.

138

1

2 3

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).

A pressure tap (3) is located on the pump pressure switch manifold. 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 789C. One switch (4) 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 is mounted on the bottom of one of the steering accumulators (see Visual No. 153). This switch 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 Transmission/Chassis ECM. The Transmission/Chassis ECM sends signals to the VIMS, which 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.

On the 789C truck, steering pump supply oil flows through a check valve (1) to the solenoid and relief valve manifold (2). The solenoid and relief valve manifold connects the steering pump to the accumulator charging valve (3), the accumulators and the steering directional valve (4). The solenoid and relief valve manifold also provides a path to drain for the steering oil.

When checking the steering system CUT-OUT and CUT-IN pressures, a gauge can be connected at the pressure tap (5).

Steering system oil samples can be taken at the steering system Scheduled Oil Sampling (S•O•S) tap (6). 139 1 2 3 4 5 6

140

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

As pressure increases in the accumulators, pump supply pressure is sensed in the accumulator charging valve and on both ends of the flow compensator. 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.

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.

Pump Output Load Sensing Pressure Accumulator Charging Valve To Accumulators From Accumulators Load Sensing Controller Flow Compensator Swashplate Piston Actuator Piston MAXIMUM FLOW

141

Pump supply pressure will increase until the accumulator pressure acting on the accumulator charging valve shifts the spool, and the load sensing signal pressure 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. Pump oil (at low pressure standby pressure) flows past the lower end of the displaced flow compensator spool to the actuator piston. The actuator piston has a larger surface area than the swashplate piston. 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 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).

Pump Output Load Sensing

Pressure