The hoist system on the 777F Update trucks is electronically controlled by the Transmission/Chassis ECM. The hoist control system operates similar to the 777D trucks.
The main components in the hoist system are:
- Hoist control lever and position sensor (in cab)
- Hoist pump (1)
- Hoist control valve (2)
- Hoist cylinders (3)
- Hydraulic oil tank (4)
The operator controls the hoist lever (arrow). The four positions of the hoist lever 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 lower solenoid 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 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 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.
The hoist lever (1) controls a position sensor (2). The PWM sensor sends duty cycle input signals to the Transmission/Chassis ECM. The hoist lever position sensor is a Hall-Effect position sensor and operates the same as the transmission shift lever sensor (3) previously described. 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
Shown is the hoist, converter, and brake hydraulic tank. The oil level is checked by opening the small door (1) and looking at the 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 (2) can be used to fill the tank when the hoist cylinders are 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.
Check the hoist, converter, and brake hydraulic tank breather (3) for restriction. Clean the filter if it is restricted.
Shown is the rear of the hoist, converter, and brake hydraulic tank. The hoist pump pulls oil from the tank through the suction screen (1) located in the rear of the tank. Oil returns from the hoist valve through the port (2).
Brake cooling oil returns to the hydraulic tank through the three upper ports (3).
Other ports located on the hydraulic tank are:
- Transmission charging pump suction (4)
- Transmission return (5)
- Torque converter pump suction (6)
- Brake cooling pump suction (7)
- Torque converter inlet relief valve return (8)
The hoist pump (1) is a gear type pump that is attached to the drive gear at the rear of the engine. Mounted to the hoist pump is the brake cooling pump and the brake charging pump.
Oil flows from the hoist pump to the hoist control valve.
The hoist system relief pressures are different in the RAISE and LOWER positions.
The body up switch must be in the RAISE position before the LOWER relief valve setting can be tested. Move a magnet past the body up switch until the body up alert indicator on the dash turns ON. If the body up switch is in the LOWER position, the Transmission/Chassis ECM will hold the hoist valve in the SNUB position and the LOWER relief valve will not open.
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.
Hoist pump pressure can be checked at the pressure tap (2) on the pump.
The hoist control valve (1) is located behind the engine on the right side of the frame. The hoist valve is same as the hoist control valve on the 777D.
The hoist valve uses torque converter lockup clutch pump oil as the pilot oil to shift the directional spool inside the hoist valve. Lockup clutch pump oil enters the hydraulic actuators (2) on both ends of the hoist valve.
Pilot oil 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. The solenoid on the right is the RAISE solenoid valve (1), and the solenoid on the left is the LOWER solenoid valve (2).
The left pressure tap (3) is used to check the pilot pressure of the hoist lower solenoid. The right pressure tap (4) is used to check the pilot pressure of the raise solenoid.
When the Transmission/Chassis ECM receives an input signal from the hoist lever sensor, the 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 how much pilot oil is drained from the end of the directional spool and, therefore, how far the directional spool travels toward the solenoid.
An oil cooler relief valve is located in the hoist control valve behind the large plug (5). The relief valve limits the brake oil cooling pressure when the hoist valve is in the HOLD, FLOAT or SNUB position.
The hoist system relief pressures are controlled by the two relief valves located on top of the hoist valve. The RAISE relief valve (6) limits the pressure in the hoist system during RAISE. The LOWER relief valve (7) limits the pressure in the hoist system during LOWER.
The hoist cylinder lower circuit pressure tap (1) and raise circuit pressure tap (2) are located on the cross-tube between the lower hoist cylinder mounts.
The relief valve pressure setting is tested with the engine at HIGH IDLE and the hoist valve in the RAISE or LOWER position.
The body up switch at the frame near the body pivot pin must be in the RAISE position before the LOWER relief valve setting can be tested. Move a magnet past the body up switch until the body up alert indicator on the dash turns ON. If the body up switch is in the LOWER position, the Transmission/Chassis ECM will hold the hoist valve in the SNUB position and the LOWER relief valve will not open.
An orifice plate is installed between the upper hose and the rod end port on both hoist cylinders. The orifice plate restricts the flow of oil from the rod end of the hoist cylinders.
The orifice plate also prevents cavitation of the cylinders when the body raises faster than the pump can supply oil to the cylinders (caused by a sudden shift of the load).
NOTE: If the snub feature is not adjusted correctly, residual pressure will exist in the head side of the cylinders and the body will not rest on the frame. The raise circuit pressure tap should be used to ensure there is no residual pressure in the head side of the cylinders.
Otherwise, when checking the raise (high) circuit pressure, the pressure tap on the hoist pump is easier to access.
This illustration shows a sectional view of the hoist control valve in the HOLD position. Pilot oil pressure is present at both ends of the directional spool. 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 to the tank. All the hoist pump oil flows through the brake oil coolers to the rear brakes.
The position of the directional spool blocks the oil in the head end and rod end of the hoist cylinders.
A gauge connected to a pressure tap at the pump 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. The maximum pressure in the circuit should correspond to the setting of the brake oil cooler relief valve.
When the directional spool is initially shifted, the load check valve remains closed until the supply pressure is higher than the pressure in the hoist cylinders. The load check valve prevents 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. The dual stage relief valve signal stem moves down and blocks the supply pressure from opening the low pressure relief valve.
Oil flowing from the rod end of the hoist cylinders flows freely through the brake oil cooler to the brakes.
If the pressure in the head end of the hoist cylinders exceeds the relief valve settings, the high pressure relief valve will open. When the high pressure relief valve opens, the dump valve moves to the left and pump oil flows to the tank.
The high pressure hoist relief valve setting is checked at the hoist pump pressure tap or the head end pressure tap. Check the relief pressure with the hoist lever in the RAISE position and the engine at HIGH IDLE.
In the LOWER (power down) position, the LOWER solenoid is energized and drains pilot oil from the lower end of the directional spool. The directional spool moves down.
Supply oil from the pump flows past the load check valve and the directional spool to the rod end of the hoist cylinders. Oil in the head end of the hoist cylinders flows to the tank through holes in the directional spool. 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 up switch sends a signal to the Transmission/Chassis ECM to move the directional spool to the SNUB position. In the SNUB position, the directional spool moves slightly to restrict the flow of head end oil through only some of the holes in the spool which allows the body to lower gradually.
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.
If the pressure in the rod end of the hoist cylinders is too high, the low pressure relief valve will open. When the low pressure relief valve opens, the dump valve moves to the left and pump oil flows to the tank.
The low pressure hoist relief valve setting is checked at the rod end pressure tap. Check the relief pressures with the hoist lever in the LOWER position and the engine at HIGH IDLE.
The body up switch must be in the RAISE position before the LOWER relief valve setting can be tested. Move a magnet past the body up switch until the body up alert indicator on the dash turns ON. If the body up switch is in the LOWER position, the Transmission/Chassis ECM will hold the hoist valve in the SNUB position and the LOWER relief valve will not open.
In the FLOAT position, the LOWER solenoid is partially energized and drains some of the pilot oil at the lower end of the directional spool to the tank. The directional spool moves down.
Because the pilot oil is only partially drained, the directional spool does not move down as far as during LOWER (power down).
Pump supply oil flows past the load check valve and the directional spool to the rod end of the hoist cylinders. Oil in the head end of the hoist cylinders flows to the tank. The position of the directional spool permits the pressure of the oil flowing to the brake oil cooler 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 on the hoist cylinders. The hoist valve will actually be in the SNUB position.
The operator does not control the SNUB position. When the hoist lever is in the LOWER or FLOAT position and the body up switch is in the DOWN position, the hoist control valve is in the SNUB position.
A gauge connected to the rod end pressure tap while the hoist control valve is in the SNUB position will show the brake cooling system pressure, which is a result of the restriction in the coolers, brakes and hoses. The maximum pressure in the circuit should correspond to the setting of the brake oil cooler relief valve.
Two-stage hoist cylinders (1) are used to raise the body. Oil flows from the hoist control valve to the two hoist cylinders when the directional spool in the hoist control valve is not in HOLD.
Check the condition of the body pads (2) for wear or damage.
Hoist pilot pressure is required to lower the body with a dead engine. The towing pump can be used to provide the hoist pilot oil.
This illustration shows the hoist hydraulic system in the HOLD position. The hoist pump pulls oil from the hydraulic tank through the suction screen located in the rear of the tank. Oil flows from the hoist pump to the hoist control valve.
When the hoist control valve is in the HOLD, FLOAT or SNUB position, all the hoist pump oil flows through the brake oil coolers located on the right side of the engine. Oil flows from the oil coolers, through the brakes, and returns to the hydraulic tank.
NOTE: If the truck is equipped with the optional caliper type front brake system, the brake cooling pump is not installed and oil from the hoist pump will flow to only the rear brakes.
A brake cooling relief valve is located in the hoist control valve. The relief valve limits the brake oil cooling pressure when the hoist control valve is in the HOLD, FLOAT or SNUB position.
The hoist valve uses torque converter lockup clutch pump oil as the pilot oil to shift the directional spool inside the hoist control valve. Oil flows from the lockup clutch pump to both ends of the hoist control valve.
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 centering springs and the pressure on the opposite end of the spool to move the spool. When the RAISE solenoid is energized, the directional spool will move toward the RAISE solenoid. The RAISE and LOWER solenoid valves constantly receive approximately 300 millivolts at a frequency of 80 Hz from the Transmission/Chassis ECM 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 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 how much pilot oil is drained from the end of the directional spool and, therefore, the distance that the directional spool travels.
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 valve will actually be in the SNUB position.
When the hoist control valve is in the RAISE position, pump supply oil flows to the head end of the hoist cylinders. Pump supply oil also flows to the dual stage signal spool and moves the spool to the left. When the dual stage signal spool moves to the left, pump supply oil is blocked from the LOWER relief valve, and the RAISE relief valve will limit the hoist system pressure.
When the hoist control valve is in the LOWER (power down), FLOAT or SNUB position, pump supply oil flows to the rod end of the hoist cylinders. Pump supply oil is blocked from the dual stage signal spool and the spring holds the spool in the right position. When the dual stage signal spool is in the right position, pump supply oil can flow to the LOWER relief valve, and hoist system pressure is controlled by the LOWER relief valve.
An orifice plate is installed between the upper hose and the rod end port on both hoist cylinders. The orifice plate prevents cavitation of the cylinders when the body raises faster than the pump can supply oil to the cylinders (caused by a sudden shift of the load).
See You Soon!!!
MARYGAR
Hi my truck is not dumping the loads ,it only dumps when empty what is the cause
ReplyDeleteDid the hoist pump fail
ReplyDeletehi my truck is not dumping the loads,it only dumps when empty what the cause
ReplyDelete