COUNTERBALANCE VALVE

running ahead of the hoist pumps if the load shifts rapidly to the rear and attempts to pull the hoist cylinders. Signal pressure from the head end of the hoist cylinders holds the counterbalance valve open. Oil from the rod end of the hoist cylinders flows unrestricted through the counterbalance valve to the front brake oil cooler. If the head end pressure decreases below 6900 ± 690 kPa (1000 ± 100 psi), the counterbalance valve moves down and restricts the flow of oil from the rod end of the cylinders to the front brake oil cooler.

During LOWER and FLOAT, the counterbalance valve allows unrestricted flow from the pump to the rod end of the hoist cylinders.

RAISE POSITION

HEAD END SIGNAL PRESSURE

FROM HOIST CYLINDER

ROD END

TO FRONT BRAKE OIL

COOLER

LOWER AND FLOAT POSITIONS

TO HOIST CYLINDER

ROD END

FROM PUMP

HOIST

COUNTERBALANCE VALVE

142

• Hoist valve in LOWER (power down)

Shown is a sectional view of the hoist valve in the LOWER (power down) position. The RAISE solenoid is de-energized and pilot oil is vented to the tank. The LOWER and FLOAT solenoids are both energized and direct pilot oil pressure to the lower end of the directional spool.

Supply oil from the pump flows past the directional spool, through the counterbalance valve, to the rod end of the hoist cylinders. Oil in the head end of the hoist cylinders flows to the tank.

The directional spool also vents the passage to the dual stage relief valve signal stem. The dual stage relief valve signal stem allows supply pressure to be limited by the low pressure relief valve.

ON

ROD END VENT SLOT LOAD CHECK

VALVE

If the pressure in the rod end of the hoist cylinders exceeds

3450 + 350 - 0 kPa (500 + 50 - 0 psi), the low pressure relief valve will open. When the low pressure relief valve opens, the dump spool moves to the left and pump oil flows to the front brake oil cooler.

The low pressure hoist relief valve setting is checked at the two pressure taps located on the hoist pump. Check the relief pressures with the hoist lever in the LOWER position and the engine at HIGH IDLE.

• Low pressure relief setting checked during LOWER at HIGH IDLE

143

• Hoist valve in FLOAT Shown is a sectional view of the hoist valve in the FLOAT position. The RAISE and LOWER solenoids are de-energized and pilot oil is vented to the tank. The FLOAT solenoid is energized and directs pilot oil pressure to the lower end of the small diameter spool located below the directional spool. The small diameter spool pushes against the directional spool and moves the directional spool up. Because the pilot pressure is acting on a smaller surface area, the directional spool does not move up as far as during LOWER.

Pump supply oil flows past the directional spool, through the

counterbalance valve, to the rod end of the hoist cylinders. Oil in the head end of the hoist cylinders flows to the tank. The directional valve is in a position that permits the pressure of the oil flowing to the front brake oil cooler to be felt at the rod end of the hoist cylinders.

The rod end pressure helps to hold the body against the frame when traveling. The hoist lever should always be in the FLOAT position while traveling.

ROD END VENT SLOT LOAD CHECK

VALVE

• Operate truck with hoist lever in FLOAT

• Front brake oil cooler filters (arrow)

144

When the hoist valve is in the HOLD or FLOAT position, all the hoist pump oil flows through the front brake oil cooler filters (arrow) located outside the left frame. Excess oil from the parking brake release valve also flows through these filters.

Oil flows from the front brake oil cooler filters to the front brake oil cooler located above the torque converter.

1. Front brake oil cooler diverter valve 2. Front brake oil

cooler

145

The hoist and parking brake release pump oil flows from the front brake oil cooler filters, through the front brake oil cooler diverter valve (1), to the front brake oil cooler (2).

When the service or retarder brakes are applied, air pressure is sent to the front brake oil cooler diverter valve. Normally, front brake cooling oil is diverted around the cooler and goes directly to the front brakes. When air is sent to the diverter valve piston, front brake cooling oil is allowed to flow through the front brake oil cooler. Since the coolers use the coolant from the aftercooler circuit, diverting oil around the coolers provides cooler aftercooler air during high power demands (when climbing a grade with the brakes RELEASED, for example).

1

2

• Two-stage hoist cylinders

146

Shown are the twin two-stage hoist cylinders used to raise the body.

147

• Hoist system circuit The hoist system pumps pull oil from the hydraulic tank through suction screens.

Oil flows from the hoist pump through the hoist screens to the hoist control valve.

The hoist valve uses parking brake release pressure as the pilot oil to shift the directional spool inside the hoist valve. Three solenoid valves are used to direct the pilot oil to the ends of the directional spool. The solenoid valve on the right is energized in the RAISE position. The two solenoid valves on the left are energized in the LOWER or FLOAT