HOIST SYSTEM ON HAULTRUCK 789D

789D Off-Highway Truck 

HOIST SYSTEM

Identify the hoist system components

INTRODUCTION
The hoist system on the 789D truck is electronically controlled by the Transmission/ Chassis ECM the same as the 789C truck. The four operating positions are: RAISE, HOLD, FLOAT, and LOWER.
The hoist valve has a fifth position referred to as the SNUB position. The operator is unaware of the SNUB position because a corresponding lever position is not provided. When the body is being lowered, just before the body contacts the frame, the Transmission/Chassis ECM signals the hoist solenoids to move the hoist valve spool to the SNUB position. In the SNUB position, the body float speed is reduced to prevent the body from making hard contact with the frame.


The hoist system can be enabled or disabled using Cat ET. All trucks shipped from the factory without bodies installed are set at the Hoist Enable Status 2. The Hoist Enable Status 2 is a test mode only and will prevent the hoist cylinders from accidentally being activated. After the body is installed, change the Hoist Enable Status to 1 for the hoist system to function properly.
The main components in the hoist system are:
* Hoist pump (1)
* Hoist screens (2)
* Hydraulic tank (3)
* Hoist cylinders (4)
* Hoist control valve (5)

Explain hoist system operation in hold.

The hoist system pumps pull oil from the hydraulic tank through suction screens (1).
Oil flows from the hoist pump (2) through the hoist screens (3) to the hoist control valve (4).
The hoist valve uses parking brake release valve (5) pressure as pilot oil to shift the directional spool inside the hoist valve. Two solenoid valves are used to drain the pilot oil from the ends of the directional spool. The raise solenoid valve (6) on the left is energized in the RAISE position. The lower solenoid valve (7) on the right is energized in the LOWER or FLOAT position.

When the hoist valve is in the HOLD or FLOAT position, all the hoist pump oil flows through the rear brake oil coolers (8) to the rear brakes (9). An oil cooler relief valve is located in the hoist valve. The relief valve limits the rear brake oil cooling pressure when the hoist valve is in the HOLD or FLOAT position.

Two hydraulic cylinders are used to raise the body away from the frame of the truck. When the hoist lever is held in the RAISE position, supply oil flows to the head end of the hoist cylinders (10) and moves the two stage cylinders to their extended lengths. The oil from the rod end of the cylinders (11) flows through the hoist valve to the rear brake oil cooling circuit.

When the hoist lever is moved to the LOWER or FLOAT position and the cylinders are extended, supply oil enters the rod end of the hoist cylinders and lowers the second stage of the cylinders. The oil from the head end of the cylinders flows through the hoist valve to the hydraulic tank.

Identify hoist lever

The operator controls the hoist lever (arrow). The four positions of the hoist lever are RAISE, HOLD, FLOAT, and LOWER.
The truck should normally be operated with the hoist lever in the FLOAT position. Traveling with the hoist in the FLOAT position will make sure the weight of the body is on the frame and body pads and not on the hoist cylinders. The hoist control valve will actually be in the SNUB position.

If the transmission is in REVERSE when the body is being raised, the hoist lever sensor is used to shift the transmission to NEUTRAL. The transmission will remain in NEUTRAL until:
The hoist lever is moved into the HOLD or FLOAT position; and • The shift lever has been cycled into and out of NEUTRAL.
NOTE: If the truck is started with the body raised and the hoist lever in FLOAT, the lever must be moved into HOLD and then FLOAT before the body will lower.

Identify Pulse width modulated (Pwm ) position sensor

The hoist lever controls a Pulse Width Modulated (PWM) position sensor (arrow). The PWM sensor sends duty cycle input signals to the Transmission/Chassis ECM. Depending on the position of the sensor and the corresponding duty cycle, one of the two solenoids located on the hoist valve is energized.
The four positions of the hoist lever are RAISE, HOLD, FLOAT, and LOWER, but since the sensor provides a duty cycle signal that changes for all positions of the hoist lever, the operator can modulate the speed of the hoist cylinders.
The hoist lever sensor performs three functions:
* Raises and lowers the body •
* Neutralizes the transmission in REVERSE •
* Starts a new TPMS cycle
• The hoist lever position sensor receives 24 volts from the Transmission/Chassis ECM. To check the supply voltage of the sensor, connect a multimeter between Pins A and B of the sensor connector. Set the meter to read “DC Volts.” To check the output signal of the hoist lever position sensor, connect a multimeter between Pins B and C of the hoist lever position sensor connector. Set the meter to read “Duty Cycle.” The duty cycle output of the sensor should be approximately 5% to 95% between full RAISE to full LOWER.

Identify hoist system components

Shown is the hoist, converter, and brake oil hydraulic tank (1) and the oil level sight gauges (2). The oil level is normally checked with the upper sight gauge. The oil level should first be checked with cold oil and the engine stopped. The level should again be checked with warm oil and the engine running.

The lower sight gauge is used when filling the hydraulic tank with the hoist cylinders in the RAISED position. When the hoist cylinders are lowered, the hydraulic oil level
will increase. After the hoist cylinders are lowered, check the hydraulic tank oil level with the upper sight gauge as explained above. The hydraulic tank breather (3) and the transmission tank breather (4) are mounted on the right frame upright. Check the breathers for restriction.

Identify hydraulic tank suction screens

Shown is the rear of the transmission hydraulic tank, and the hoist, converter, and brake oil hydraulic tank. The hoist system pumps pull oil from the hydraulic tank through the suction screens (arrows) located in the rear of the tank.

Identify hoist system pump and pressure taps

The hoist system oil is supplied by a two-section pump (1) located at the top rear of the pump drive. Oil flows from the hoist pump through two screens to the hoist valve. The hoist system pressure can be tested at the two pressure taps (2).
When the body is in the DOWN position, the hoist valve will be in the SNUB position. The body position sensor rod must be disconnected from the body and the sensor must be rotated to the RAISE position before the LOWER relief pressure can be  tested. In the HOLD, FLOAT, and SNUB positions, the gauge will show the brake cooling system pressure, which is a result of the restriction in the coolers, brakes, and hoses (normally much lower than the actual oil cooler relief valve setting). The maximum pressure is limited by the oil cooler relief valve, which has a setting of approximately 790 ± 20 kPa (115 ± 3 psi).

Identify hoist screens and hoist screen bypass switches

Oil flows from the hoist pump through the hoist screens (1) to the hoist control valve. Two hoist screen bypass switches (2) provide input signals to the Transmission/ Chassis ECM. The Transmission/Chassis ECM sends signals to the VIMS, which informs the operator if the hoist screens are restricted.

Identify components on right side of hoist valve

Oil flows from the hoist pump through two ports (1) (only one visible in this view) to the hoist control valve located inside the right frame next to the hoist cylinder. Two load check valves, one for each pump port, are located below the two plugs (2). The load check valves remain closed until the pump supply pressure is higher than the pressure in the hoist cylinders. The load check valves prevent the body from dropping before the RAISE pressure increases.

The hoist system relief pressures are different in the RAISE and LOWER positions. The RAISE relief valve (3) controls the pressure in the hoist system during RAISE. The LOWER relief valve (4) controls the pressure in the hoist system during LOWER. The relief valve housing must be removed to install shims (see Visual No. 164).
Oil flows through the drain port (5) to the hydraulic tank. When the hoist valve is in the HOLD, FLOAT, or SNUB position, all the hoist pump oil flows through two
ports (6), one on each side of the hoist valve, to the two rear brake oil coolers located on the right side of the engine.

Identify components on left side of hoist valve

A counterbalance valve (1) is mounted on the left side of the hoist valve. The counterbalance valve prevents cavitation of the cylinders when the body raises faster than the pumps can supply oil to the cylinders (caused by a sudden shift of the load). The counterbalance signal pressure is equal to the RAISE pressure.
An oil cooler relief valve is located behind the large plug (2). The oil cooler relief valve limits the rear brake oil cooling pressure when the hoist valve is in the HOLD, FLOAT, or SNUB position. The setting of the oil cooler relief valve is approximately 790 kPa (115 psi).

The hoist valve uses parking brake release pressure as the pilot oil to shift the directional spool inside the hoist valve. The parking brake release pressure is approximately 4700 ± 200 kPa (680 ± 30 psi).
Pilot pressure is always present at both ends of the directional spool. Two solenoid valves are used to drain the pilot oil from the ends of the directional spool, which then allows the spool to move. On the left is the RAISE solenoid valve (3), and on the right is the LOWER solenoid valve (4).
The RAISE and LOWER solenoid valves are always receiving approximately 300 millivolts at a frequency of 80 Hz when they are in any position except HOLD. The
excitation, referred to as “dither,” is used to keep the solenoids in a ready state for quick response.

When the Transmission/Chassis ECM receives an input signal from the hoist lever sensor, the Transmission/Chassis ECM sends an output signal current between 0 and 1.9 amps to one of the solenoids. The amount of current sent to the solenoid determines the amount of pilot oil that is drained from the end of the directional spool and, therefore, the distance that the directional spool travels toward the solenoid.
Oil flows through two upper ports (5), one on each side of the hoist valve, to RAISE the hoist cylinders. Oil flows through two lower ports (6), one on each side of the hoist valve, to LOWER the hoist cylinders.

Explain operation of hoist control valve in the HOlD position

Shown is a sectional view of the hoist valve in the HOLD position. Pilot oil pressure is directed to both ends of the directional spool (1). The spool is held in the centered position by the centering springs and the pilot oil. Passages in the directional spool vent the dual stage relief valve signal stem (2) to the tank (3). All the hoist pump oil flows through the rear brake oil coolers (4) to the rear brakes.
The position of the directional spool blocks the oil in the head end of the hoist cylinders (5). Oil in the rod end of the hoist cylinders (6) is connected to the rear brake cooling oil by a small vent slot (7) cut in the directional spool. A gauge connected to the hoist system pressure taps while the hoist valve is in the HOLD position will show the brake cooling system pressure, which is a result of the restriction in the coolers, brakes, and hoses (normally much lower than the actual oil cooler relief valve setting). The maximum pressure in the circuit should correspond to the setting of the rear brake oil cooler relief valve (8). The setting of the oil cooler relief valve is approximately 790 kPa (115 psi).

Explain operation of hoist control valve in the raise position

Shown is a sectional view of the hoist valve in the RAISE position. The RAISE solenoid (12) is energized and drains pilot oil pressure from the lower end of the directional spool (1). The directional spool moves down. Pump oil flows past the directional spool to the head end of the hoist cylinders (5).
When the directional spool is initially shifted, the two load check valves (one shown) (17) remain closed until the pump supply pressure is higher than the pressure in the hoist cylinders. The load check valves prevent the body from dropping before the RAISE pressure increases.
The directional spool also sends hoist cylinder raise pressure to the dual stage relief valve signal stem (2) and the counterbalance valve (11). The dual stage relief valve signal stem moves down and blocks the supply pressure from opening the low pressure relief valve (16).

The counterbalance valve is held open by the hoist cylinder raise pressure. Oil from  the rod end of the hoist cylinders (6) flows freely to the rear brake oil coolers (4). If the body raises faster than the pump can supply oil to the hoist cylinders (caused by a sudden shift of the load) and the raise pressure drops below approximately 2275 kPa (330 psi), the counterbalance valve starts to close and restricts the flow of oil from the rod end of the hoist cylinders. Restricting the flow of oil from the rod end of the hoist cylinders will slow down the cylinders and prevent cavitation. Cavitation in the hoist cylinders can cause the body to drop suddenly when the hoist lever is moved from the RAISE position to the LOWER position.

The high pressure relief valve (15) will open if the pressure is too high. When the high pressure relief valve opens, the dump spool (14) moves to the left, and pump oil is directed to the rear brake oil coolers. The high 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 RAISE position and the engine at HIGH IDLE.

Explain operation of hoist counterbalance valve

During RAISE, the counterbalance valve prevents the dump body from running ahead of the hoist pumps if the load shifts rapidly to the rear of the body and attempts to pull the hoist cylinders. Signal pressure from the head end of the hoist cylinders (1) holds the counterbalance valve open. Oil from the rod end of the hoist cylinders (2) flows unrestricted through the counterbalance valve to the tank (3). If the head end pressure decreases below approximately 2270 kPa (330 psi), the
counterbalance valve moves down and restricts the flow of oil from the rod end of the cylinders to the tank. If no head end signal pressure is present, rod end pressure can still open the counterbalance valve. If the rod end pressure exceeds approximately 6900 ± 690 kPa (1000 ± 100 psi) at the rod end pressure piston (4), the valve will move up and allow rod end oil to flow from the cylinders to the tank. During LOWER and FLOAT, the counterbalance valve allows unrestricted flow from the pump through a check valve (5) to the rod end of the hoist cylinders.

Explain operation of hoist control valve during lower with power down

Shown is a sectional view of the hoist valve in the LOWER (power down) position. The LOWER solenoid (9) is energized and drains pilot oil pressure from the upper end of the directional spool (1). The directional spool moves up.
Supply oil from the pump flows past the directional spool, through the counterbalance valve (11), to the rod end of the hoist cylinders (6). Oil in the head end of the hoist cylinders (5) flows to the tank (3). The supply oil in the rod end of the cylinders and the weight of the body move the cylinders to their retracted positions.

Just before the body contacts the frame, the body position sensor sends a signal to the Transmission/Chassis ECM to move the valve spool to the SNUB position. In the SNUB position, the valve spool moves slightly to restrict the flow of oil and lower the body gently.
The directional spool also vents the passage to the dual stage relief valve signal stem (2). The dual stage relief valve signal stem allows supply pressure to be limited
by the low pressure relief valve (16). If the pressure in the rod end of the hoist cylinders exceeds approximately 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 (14) moves to the left and pump oil flows to the rear brake oil coolers (4). 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.

When the body is in the DOWN position, the hoist valve will be in the SNUB position. The body position sensor rod must be disconnected from the body, and the sensor must be rotated to the RAISE position before the LOWER relief pressure can be tested.

Explain operation of hoist control valve in the floaT position

Shown is a sectional view of the hoist valve in the FLOAT position. The LOWER solenoid (9) is partially energized and drains part of the pilot oil pressure above the directional spool (1) to the tank (3). The directional spool moves up. Because the pilot pressure is only partially drained, the directional spool does not move as far up as during LOWER.
Pump supply oil flows past the directional spool, through the counterbalance valve (11), to the rod end of the hoist cylinders (6). Oil in the head end of the hoist cylinders (5) flows to the tank. The directional valve is in a position that permits the pressure of the oil flowing to the rear brake oil coolers (4) to be felt at the rod end of  the hoist cylinders. The truck should normally be operated with the hoist lever in the FLOAT position.

Traveling with the hoist in the FLOAT position will make sure the weight of the body is on the frame and body pads and not the hoist cylinders. The hoist valve will actually be in the SNUB position. Just before the body contacts the frame, the body position sensor sends a signal to the Transmission/Chassis ECM to move the valve spool to the SNUB position. In the SNUB position, the valve spool moves slightly to restrict the flow of oil and lower the body gently.

Identify body pads

Shown are the twin two-stage hoist cylinders used to raise and lower the body.
Check the condition of the body pads (arrow) for wear or damage.
To LOWER the body with a dead engine, hoist pilot pressure is required. The towing pump can be used to provide the hoist pilot oil. To lower the body with a dead engine:
* Turn ON the key start switch so the towing motor and the hoist • solenoids can be energized.
* Move the hoist lever to the RAISE position for 15 seconds, then to the • FLOAT position.
* Depress the brake retraction switch on the dash.
To RAISE the body with a dead engine, connect an Auxiliary Power Unit (APU) to the hoist cylinders. Follow the same procedure used to lower the body with a dead engine, except keep the hoist lever in RAISE after the 15 second interval.

LAB1: Hoist System Component Identification
Instructions: Identify the numbered components in the above illustration.
1. Hoist pump-----
2. Hoist screen------
3. Hydrolic oil tank----
4. Hydrolic cylinders ----
5. Hoist control valve -----

LAB1: Hoist System Component Identification (continued)
Instructions: Identify the numbered components in the above illustration.

LAB1: Hoist System Component Identification (continued)
Instructions: Identify the numbered components in the above illustration.

789D Off-Highway Trucks Hoist System Post-Assessment
Instructions: Select the correct answer(s) for each question.
1. The hoist system on the 789D truck is electronically controlled by the:
    a. Brake ECMa.
    b. [Transmission/Chassis ECM]b.
    c. Machine ECMc.
    d. VIMS Main moduled.
2. All trucks shipped from the factory without bodies installed are set at the _____       mode.
    a. [Hoist Enable Status 2.
    b. Hoist Enable Status 1.
    c. FLOAT.
    d. HOLD.
3. Pilot oil is supplied to the hoist valve from the:
    a. Pilot pumpa.
    b. [Parking brake release valve]b.
    c. Hoist pumpc.
    d. Towing valved.
4. Which of the following is not a hoist lever position?
    a. [SNUB].
    b. FLOAT.
    c. RAISE.
    d. LOWER
5. The hoist system shares a section of the hydraulic tank with which other system?
    a. Steering system.
    b. [Brake system].
    c. Transmission.
    d. Fan system
6. The hoist valve controls the hoist cylinders and _________________.
    a. [supplies oil to the brake cooling system].
    b. supplies oil to the TCS valve check ball.
    c. supplies oil to the brake actuation system.
    d. all of the above.
7. What prevents the body from dropping before the RAISE pressure increases?
    a. Counterbalance valve.
    b. Low pressure relief valve.
    c. High pressure relief valve.
    d. [Load check valves].
8. What will happen if the pressure in the head of the hoist cylinders is too high?
    a. Counterbalance valve opens.
    b. Low pressure relief valve opens.
    c. [High pressure relief valve opens].
    d. Pump high pressure cutoff valve opens.
9. Technician A says that when the hoist pilot solenoid is energized, pilot oil is directed to the end of       the directional spool. Technician B says that when the hoist pilot solenoid is de-energized, pilot           oil  moves the directional spool. Who is right?
     a. Technician A.
     b. [Technician B].
     c. Both Technician A and Technician B.
     d. Neither Technician A nor Technician B.