Usage:
Hydraulic System
NOTE: For Specifications with illustrations, make reference to the SPECIFICATIONS FOR 768C TRACTOR & 769C TRUCK HYDRAULIC SYSTEM, Form No. SENR2941. If the Specifications in Form SENR2941 are not the same as in the Systems Operation and the Testing and Adjusting, look at the printing date on the back cover of each book. Use the Specifications given in the book with the latest date.
NOTE: For Electrical and Hydraulic Systems schematics, make reference to the following:
SCHEMATIC FOR 768C TRACTOR & 769C TRUCK ELECTRICAL SYSTEM, Form No. SENR2948.
SCHEMATIC FOR 768C TRACTOR & 769C TRUCK HYDRAULIC SYSTEM, Form No. SENR2978.
1. Return line for rear brake cooling oil.
2. Tube for brake makeup oil.
3. Makeup line for front brake master cylinder.
4. Makeup line for rear brake master cylinder.
5. Makeup line for retarder brake master cylinder.
6. Bleeder line for retarder brake master cylinder.
7. Bleeder line for rear brake master cylinder.
8. Makeup oil tank for brake master cylinders.
9. Return line for transmission scavenge oil.
10. Return line for torque converter scavenge oil.
11. Oil tank for transmission.
12. Hydraulic oil tank for brake cooling, hoist cylinders and torque converter.
13. Check valve (in the control valve).
14. Detent for FLOAT position.
15. Main relief valve (part of control valve).
16. Drain line from emergency and parking brake valve.
17. Outlet for torque converter charging oil and parking brake release oil.
18. Relief valve for brake cooling oil.
19. Line to brake cooling oil system for rear brakes.
20. Line to head end of truck hoist cylinders.
21. Line to the rod end of truck hoist cylinders.
22. Line from screen in tank to (hoist or wagon) pump.
23. Drain line from torque converter inlet relief valve.
24. Line to control valve (in tank) from pump.
25. Line from transmission oil tank to transmission.
26. Hoist cylinder (left side).
27. Hoist cylinder (right side).
28. Hydraulic hoist (implement on 768C Tractor) pump.
A. RAISE position.
B. HOLD position.
C. FLOAT position.
D. LOWER position.
HYDRAULIC SYSTEM SCHEMATIC (TRUCK)
Hydraulic And Transmission Oil Tanks
The hydraulic system involves the following systems:
- Hydraulic and Transmission TanksHoistTorque ConverterBrake & Brake Cooling
This service manual covers the hydraulic and transmission oil tank and the hoist system.
For Systems Operation on the torque converter, see Form No. SENR2934, 768C Tractor & 769C Truck Power Train.
For Systems Operation/Testing and Adjusting on the brake and brake cooling, see Form No. SENR2940, 768C Tractor & 769C Truck Air System and Brakes.
The tank is split into two separate tanks (11) and (12). The smaller tank (11) provides SAE 30 oil for the transmission part of the power train. The larger tank (12) provides SAE 10W hydraulic oil for brake operation and cooling, hoist (or wagon hydraulic system) operation and torque converter operation.
Line (9) is a return line for transmission scavenge oil to the filter (30) located on top of the transmission tank (11). Transmission oil leaves the tank through line (25).
Mounted on top of the hydraulic tank (12) is the makeup oil tank (8) for the front, rear and retarder brake master cylinders. Lines (3), (4) and (5) respectively are the makeup lines from makeup tank (8) to each brake master cylinder. Lines (6) and (7) are the bleeder lines for the retarder and rear brake master cylinders respectively. Makeup oil flows through tube (2) from screen assembly (31) to brake oil makeup tank (8).
1. Return line for rear brake cooling oil.
2. Inlet tube for makeup oil.
3. Makeup line for front brake master cylinder.
4. Makeup line for rear brake master cylinder.
5. Makeup line for retarder brake master cylinder.
6. Bleeder line for retarder brake master cylinder.
7. Bleeder line for rear brake master cylinder.
8. Makeup oil tank for brake master cylinders.
9. Return line for transmission scavenge oil.
16. Drain line from emergency and parking brake valve.
17. Outlet for torque converter charging oil and parking brake release oil.
19. Line to brake cooling oil system for rear brakes.
20. Line to head end of truck hoist cylinders.
21. Line to rod end of truck hoist cylinders.
22. Line from screen in tank to (hoist or wagon) pump.
23. Drain line from torque converter inlet relief valve.
24. Line to control valve (in tank) from pump.
25. Line from transmission oil tank to transmission.
INNER (RIGHT) SIDE OF OIL TANKS
HYDRAULIC AND TRANSMISSION TANK
1. Return line for rear brake cooling oil. 2. Inlet tube for makeup oil. 3. Makeup line for front brake master cylinder. 4. Makeup line for rear brake master cylinder. 5. Makeup line for retarder brake master cylinder (not shown in this view). 6. Bleeder line for retarder brake master cylinder. 7. Bleeder line for rear brake master cylinder. 8. Makeup oil tank for brake master cylinders. 9. Return line for transmission scavenge oil. 10. Return line for torque converter scavenge oil. 11. Oil tank for transmission. 12. Hydraulic oil tank for brake cooling, hoist cylinders and torque converter. 29. Line to breather for hydraulic oil tank, torque converter and oil pump drive. 30. Filter assembly for transmission return oil. 31. Screen assembly for return oil from torque converter and brake cooling system. 32. Breather line to transmission case. 33. Line to breather for transmission oil tank and transmission case.
Screen assembly (31) is also mounted on top of hydraulic tank (12). Oil to the torque converter and the parking brake release leaves tank (12) through line (17) and returns to tank through line (10) to screen assembly (31).
RIGHT SIDE - REAR OF ENGINE
34. Breather for hydraulic oil tank, torque converter and oil pump drive. 35. Breather for transmission oil tank and transmission case.
Line (32) vents the transmission case to the transmission tank (11). Line (33) vents the transmission tank to breather (35). Line (29) vents the hydraulic tank (12) to breather (34). Breathers (34) and (35) are located near the rear and right side of the engine.
HYDRAULIC COMPONENT LOCATION
11. Oil tank for transmission. 12. Hydraulic tank for brake cooling. 15. Relief valve (part of control valve inside tank). 18. Relief valve for the oil to the oil cooler (inside tank). 26. Hoist cylinder (left side). 27. Hoist cylinder (right side). 28. Hydraulic hoist (implement on 772B Tractor) pump. 36. Transmission pump (under hoist pump). 37. Pump drive. 38. Steering pump.
The hydraulic hoist pump (28), transmission pump (36) and steering pump (38) are driven by the pump drive (37) which is driven by the engine flywheel. The location of hoist pump (28) is on the back of the pump drive (37), installed on the right side of the vehicle frame. The hoist pump operates the hoist cylinders (26) and (27) for the truck dump body or the hydraulic implements on a unit pulled by the tractor. The hoist pump gets oil through a screen in the hydraulic tank and line (22). The pump oil goes through line (24) to the control valve in the hydraulic tank. When the valve spool in the control valve is in HOLD position (B), the pump oil goes through the control valve and through line (19) to the oil coolers, to the brakes on the rear wheels and then to the tank through line (1).
PUMP DRIVE GROUP
28. Hoist pump. 36. Transmission pump. 37. Pump drive. 38. Steering pump.
The relief valve (18), in line (19) to the oil cooler for the brakes on the rear wheels, opens when the pressure of the oil is approximately 580 kPa (85 psi).
The relief valve (15), in the control valve, opens when the pressure of the pump oil in either line (20) or (21) gets to approximately 17 250 kPa (2500 psi).
Lines (20) and (21), for the 768C Tractor, go to a junction on the frame near the hydraulic tank. The junction has connections for the oil lines to the hydraulic implements on the unit pulled by the tractor.
Control Lever Positions
The control lever in the cab of the vehicle is connected to the valve spool, in the control valve, in the hydraulic tank. The positions of the control valve for the dump body of the 769C Truck are RAISE position (A), HOLD position (B), FLOAT position (C) and LOWER position (D).
The control valve for the 768C Tractor has the same positions and controls the hydraulic components on the unit pulled by the tractor.
A spring on the end of the valve spool, in the control valve, moves the valve spool and the control lever from either RAISE position (A) or LOWER position (D) when the control lever is released. The spring moves the valve spool from RAISE position (A) to HOLD position (B) or the spring moves the valve spool from LOWER position (D) to FLOAT position (C). Detent (3) on the valve spool holds it in FLOAT position (C).
CONTROL LEVER IN HOLD POSITION
A. RAISE position. C. FLOAT position. D. LOWER position.
Control Valve
A control lever in the cab of the vehicle is connected to the valve spool of the control valve in the hydraulic tank. The control lever has four positions. Two of the positions, HOLD and FLOAT are the same for the 768C Tractor and the 769C Truck. The hydraulic components on the unit pulled by the 768C Tractor can either open and close doors or lift and lower a dump body or for some other function. The other two positions for the control lever in the 768C Tractor will not have a position name in this manual. The explanation will be the operation of the control valve in the four positions for the hoist cylinders on the 769C Truck.
Hold Position
The oil from the pump goes into the control valve through inlet (2). With the valve spool (9) in HOLD position (B), the pump oil goes around the valve spool and through passage (6) to the oil coolers, to the brakes on the rear wheels and then to the hydraulic tank.
The HOLD position of the valve spool keeps the oil both in passage (7) and (8) and in the ends of the hoist cylinders. The cylinders cannot be moved. The pump oil is also in the spring chamber of the pump relief valve (1).
1. Relief valve.
2. Inlet for pump oil.
3. Check valve.
4. Spring on valve spool.
5. Detent ball (four).
6. Passage (to oil cooler for brakes and to tank).
7. Passage (for rod ends of hoist cylinders).
8. Passage (for head ends of hoist cylinders).
9. Valve spool.
A. RAISE position.
B. HOLD position.
C. FLOAT position.
D. LOWER position.
CONTROL VALVE IN HOLD POSITION
Float Position
Valve spool (9) is held in FLOAT position (C) by four detent balls (5) in the detent groove around the end of the valve spool.
The position of the valve spool in FLOAT position (C) lets some of the pump oil go through grooves in the valve spool and into passage (6) to the oil coolers, to the brakes on the rear wheels and then to the hydraulic oil tank. The grooves in the valve spool are restrictions and there is an increase in the pressure of the pump oil in the control valve. The pressure of the pump oil opens check valve (3) enough to let the pump oil go through the check valve, through the check valve, through passage (7), through the rod ends of the hoist cylinders and into the second stage oil chambers. The weight of the dump body on the hoist cylinders pushes the oil from the head ends of the cylinders through passage (8) in the control valve. The oil goes from passage (8) into the hydraulic tank.
Once the second stage cylinder is retracted, the weight of the dump body becomes the only force acting to retract the first stage cylinder. This completes the lowering of the dump body.
1. Relief valve.
2. Inlet for pump oil.
3. Check valve.
4. Spring on valve spool.
5. Detent ball (four).
6. Passage (to oil cooler for brakes and to tank).
7. Passage (for rod ends of hoist cylinders).
8. Passage (for head ends of hoist cylinders).
9. Valve spool.
A. RAISE position.
B. HOLD position.
C. FLOAT position.
D. LOWER position.
CONTROL VALVE IN FLOAT POSITION
Raise Position
The oil from the pump goes into the control valve through inlet (2). With valve spool (9) held in RAISE position, passage (6) is not open to pump oil. When the pump oil cannot go out through passage (6), there is an increase in the pressure. The pressure of the pump oil opens check valve (3). The pump oil goes through the open check valve, around valve spool (9) through passage (8) and to the head ends of the cylinders. The pump oil in the head ends of the cylinder moves the cylinders toward their extended lengths and the dump body lifts. When the pump oil in the head ends of the cylinders moves the cylinders, the oil in the rod ends of the cylinders is pushed through passage (7) around valve spool (9) and through passage (6) to the oil coolers, to the brakes on the rear wheels and then to the tank.
When the control lever is released, spring (4) on the end of valve spool (9) moves the valve spool and the control lever to the HOLD position.
1. Relief valve.
2. Inlet for pump oil.
3. Check valve.
4. Spring on valve spool.
5. Detent ball (four).
6. Passage (to oil cooler for brakes and to tank).
7. Passage (for rod ends of hoist cylinders).
8. Passage (for head ends of hoist cylinders).
9. Valve spool.
A. RAISE position.
B. HOLD position.
C. FLOAT position.
D. LOWER position.
CONTROL VALVE IN RAISE POSITION
Lower Position
With the valve spool held in the LOWER position (no illustration), the pump oil goes through the inlet into the control valve. The LOWER position of the valve spool in the control valve closes the passages to the oil cooler and there is an increase in the pressure of the pump oil. The high pressure of the pump oil opens the check valve and the oil goes through the check valve, around the valve spool and through the passage to the rod ends of the cylinders. The pump oil in the rod ends of the cylinders and the weight of the body move the cylinders to their retracted positions. The oil in the head ends of the cylinders is pushed from the cylinders through the control valve and through the passage to a screen assembly in the tank.
When the control lever is released, the spring on the end of the valve spool moves the valve spool and the control lever to the FLOAT position.
Relief Valve
The relief valve is part of the control valve. The relief valve limits the pressure in the hydraulic system. The pump oil (6) from the control valve goes through orifice (5) into chamber (3). When the relief valve is closed, the pressure of the oil on both sides of dump valve (4) is the same. Spring (9) holds the dump valve closed.
The relief valve is closed when the pressure of the pump oil is less than the pressure setting of the valve. The pressure setting of the relief valve is approximately 17 250 kPa (2500 psi). When the pressure of the pump oil is more than the setting of the relief valve, the pressure of the oil in chamber (3) opens pilot valve (2). The oil in chamber (3) now goes through the open pilot valve and through passage (10) to the tank. The pressure of the oil on the spring side of dump valve (4) is now less than the pressure of the oil in passage (6). The force of the high pressure oil in passage (6). The force of the high pressure oil in passage (6) is now more than the force of spring (9) and the pressure of the oil opens dump valve (4). The pump oil in passage (6) goes through the open dump valve and through passage (10) to the tank.
RELIEF VALVE
1. Shims. 2. Pilot valve. 3. Chamber. 4. Dump valve. 5. Orifice. 6. Pump oil from control valve. 7. Spring (on pilot valve). 8. Shims. 9. Spring (in dump valve). 10. Passage (to tank).
The pressure setting of the relief valve can be changed. Add to shims (1) in the plug or remove shims (8) from under the plug if the pressure setting is low. Remove shims (1) from inside the plug or add shims (8) under the plug if the pressure setting is high.
Hoist Cylinders (Truck)
Two hydraulic cylinders are used to move the body away from the frame of the truck. When the control lever is held in the RAISE position, the pump oil moves the two stage cylinders to their extended lengths and the body is in dump position.
The hoist cylinders lift the dump body in two stages: The pump oil goes into opening (6) for the head ends of the cylinders. The oil goes through a tube in rod assembly (1) into chamber (5) for the head end of cylinder (3). The pressure of the pump oil in chamber (5) moves cylinder (3).
CROSS SECTION OF A HOIST CYLINDER
1. Rod assembly. 2. Cylinder (second stage). 3. Cylinder (first stage). 4. Grooves (in end of cylinder). 5. Chamber (in head end of cylinder). 6. Opening to head end of cylinder and flow control valve. 7. Opening to rod end of cylinder. 8. Chamber (between first stage and second stage cylinders). 9. Chamber (between rod assembly and second stage cylinder). 10. Orifices in rod assembly.
HOIST CYLINDERS
1. Rod assembly. 2. Cylinder (second stage). 3. Cylinder (first stage).
The oil in chamber (8) must now go through grooves (4) in the end of second stage cylinder (2). When cylinder (3) moves against the end of second stage cylinder (2), cylinder (2) moves. The oil in chamber (9) now goes through orifices (10), through hollow rod assembly (1) and out through opening (7) to the hydraulic tank. When cylinder (3) is against the end of cylinder (2) and cylinder (2) is against the end of rod assembly (1), the hoist cylinder is at its extended length.
HOIST CYLINDER SCHEMATIC (First Stage Cylinder to Extended Length) (Nomenclature same as for cross section of hoist cylinder)
HOIST CYLINDER SCHEMATIC (Extended Cylinder) (Nomenclature same as for cross section of hoist cylinder)
Move the control lever to either the LOWER or FLOAT position and the body goes down to the truck frame.
When the cylinders are at extended length and the control lever is moved to the LOWER position, pump oil retracts only the second stage cylinder (2). When the second stage gets to retracted length, gravity moves the first stage of the cylinder until the body is on the truck frame.
HOIST CYLINDER SCHEMATIC (Second Stage Cylinder Retracted) (Nomenclature same as for cross section of hoist cylinder)
The body goes down when the pump oil goes through opening (7) into rod assembly (1), through orifices (10), into chamber (9) between the rod and first stage cylinder (2). The oil pushes cylinder (2) and cylinder (3) down as it fills chamber (9) with pump oil. The weight of the body (gravity) moves cylinder (3) down to its retracted length. The oil in chamber (5) goes through the tube in rod assembly (1), through the flow control valve and through opening (6) to the control valve and to the hydraulic tank. Flow control valve in opening (6) is a restriction to the oil flow from chamber (5). This restriction, to the flow of oil from the cylinder, lets the dump body lower at a slow speed.