Troubleshooting Of The 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.

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Sudden movement of the machine or release of oil under pressure can cause injury to persons on or near the machine. To prevent possible injury, do the procedure that follows before testing and adjusting the hydraulic system:

1. Move the machine to a smooth horizontal location. Move away from working machines and personnel.

2. Put the transmission shift lever in NEUTRAL position. Stop the engine.

3. Permit only one operator on the machine. Keep all other personnel either away from the machine or in view of the operator.

4. Activate the parking brakes.

5. Put blocks in front of and behind the wheels.

PINS INSTALLED IN BODY OF TRUCK (Typical Example)

6. On a truck, fully lift the body and install the two pins at the rear of the truck. Disconnect the hoist control linkage as shown (remove the cotter pin and pin).

HOIST CONTROL LINKAGE DISCONNECTED

7. Make sure the transmission rotary selector spool is in NEUTRAL Position.

8. Make sure all hydraulic pressure is released before any fitting, hose or component is loosened, tightened, removed or adjusted.

NOTE: The hydraulic system involves the operation of the torque converter, the brakes and brake cooling and the hoist (or wagon implements). The troubleshooting for the hydraulic system in this section deals for the most part with the hoist hydraulics. For more detailed information on the torque converter and brakes and brake cooling respectively, make reference to:

Form No. SENR2935, 768C Tractor & 769C Truck Power Train, the section on Testing and Adjusting.

Form No. SENR2940, 768C Tractor & 769C Truck Air System and Brakes, the section on Testing and Adjusting.

The 5S5123 and 6V4161 Hydraulic Testing Groups can be used to make the pressure tests of the hydraulic system. Before any tests are made, visually inspect the complete hydraulic system for leakage of oil and for parts that have damage. For some of the tests a timer, magnet and a measuring rule (either for inches or for millimeters) are usable tools.

Visual Checks For The Hydraulic System (Includes Torque Converter, Brake Cooling and Hoist or Wagon Hydraulic Systems)

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Make reference to WARNING on first page of HYDRAULIC SYSTEM TESTING AND ADJUSTING section.

The following checks make up the visual checks part of the troubleshooting section:

Check 1. Oil level in hydraulic oil tank.

Check 2. Leakage.

Check 3. Parking brake release oil filter.

Check 4. Torque converter oil filter.

Check 5. Return oil screen.

Check 6. Torque converter scavenge screen.

Check 7. Oil cooler screens.

Check 8. Parts inside hydraulic oil tank.

Do visual checks first when troubleshooting a problem. Make the checks with the engine OFF and the parking brakes ON. Put the transmissionf shift lever and transmission rotary selector spool in NEUTRAL. During these checks, use a magnet to separate ferrous particles from non-ferrous particles (O-ring seals, aluminum, bronze, etc.).

Check 1.

Check the oil level in hydraulic oil tank (19). Look for air or water in the oil (at the sight gauge). Many problems in the hydraulic system are caused by low oil level or air in the oil. Add oil to the hydraulic oil tank if it is needed. The torque converter, brake cooling and hoist or wagon hydraulic systems use SAE 10W oil.

OUTER RIGHT SIDE OF MAIN FRAME
9. Parking brake release oil filter.

INNER RIGHT SIDE OF MAIN FRAME
10. Torque converter oil filter.

NOTE: Parking brake release oil filter (9) and torque converter oil filter (10) each has an oil filter bypass valve. An oil filter bypass valve lets oil bypass (go around) the oil filter element(s) whenever the pressure difference between inlet oil and outlet oil at the oil filter gets too high. Any oil that does not go through the filter element(s) goes directly into the hydraulic circuit. Dirty oil causes restrictions in valve orifices, sticking valves, etc.

The pressure setting of the bypass valves in the filters is approximately 250 kPa (36 psi).

Problem 1:

The oil level is too high.

PROBABLE CAUSE:

a. Worn or damaged engine crankshaft rear seal lets engine oil into the torque converter cover.

Problem 2:

There is an indication of water in the oil.

PROBABLE CAUSE:

a. Failure of oil cooler (7).

Problem 3:

There is an indication of air in the oil.

PROBABLE CAUSE:

a. Do Visual Check 2.

Problem 4:

The oil level is too low.

PROBABLE CAUSE:

a. Do Visual Check 2. Add oil before any tests are done.

b. Check the oil level in each final drive. Brake cooling oil can leak into the final drive.

Check 2.

Inspect all oil lines, hoses and connections for damage or leaks. Look for oil on the ground under the machine.

NOTE: If oil can leak out of a fitting or connection, air can leak in. Air in the system can be as bad as not enough oil.

HYDRAULIC SYSTEM SCHEMATIC

1. Valve group (lockup clutch and solenoid).

2. Relief valve (torque converter outlet).

3. Brake valve (emergency and parking).

4. Passage to rear wheel brake.

5. Relief valve (torque converter inlet).

6. Torque converter.

7. Oil cooler for torque converter and rear brakes.

8. Oil pump drive.

9. Oil filter.

10. Oil filter.

11. Lockup clutch.

12. Screen.

13. Oil pump (parking brake release).

14. Oil pump (torque converter charging).

15. Oil pump (torque converter scavenge).

16. Screen.

17. Brake (left rear wheel).

18. Brake (right rear wheel).

19. Hydraulic oil tank.

20. Oil screen.

21. Oil pump (hoist and rear brake cooling).

22. Screen (suction).

23. Relief valve (oil coolant to rear brakes).

24. Control valve.

25. Screen (hoist return oil).

26. Screen (suction).

Check 3.

Remove and inspect (cut apart if necessary) parking brake release oil filter (9) for foreign material.

Problem 1:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 2:

Shiny steel particles are found.

PROBABLE CAUSE:

a. Mechanical failure of parking brake release section (13) of the oil pump.

Check 4.

Remove and inspect (cut apart if necessary) torque converter oil filter (10) for foreign materials.

OUTER LEFT SIDE OF MAIN FRAME
19. Hydraulic oil tank. 20. Return oil screen.

Problem 1:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 2:

Shiny steel particles are found.

PROBABLE CAUSE:

a. Mechanical failure of torque converter charging section (14).

Check 5.

Remove and inspect return oil screen (20) for foreign materials.

Problem 1:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 2:

Shiny steel particles are found.

PROBABLE CAUSE:

a. Mechanical failure of torque converter scavenge section (15). Do Visual Check 6.

Problem 3:

Pieces of red non-ferrous material (part of brake disc) are found.

PROBABLE CAUSE:

a. Failure of wheel brakes (17) and/or (18).

Problem 4:

Aluminum particles are found.

PROBABLE CAUSE:

a. Do Visual Checks 6 and 7.

Check 6.

Drain the cover (housing) of torque converter (6). Measure the amount of oil removed from the cover. Inspect scavenge screen (16) for foreign material and/or a restriction.

BOTTOM OF TORQUE CONVERTER COVER
16. Scavenge screen (inside cover).

Problem 1:

More than 19 liters (5 U.S. gal.) of oil are drained from the cover.

PROBABLE CAUSE:

a. Restriction in scavenge screen (16).

Problem 2:

More than 19 liters (5 U.S. gal.) of oil are drained from the cover and aluminum particles are found.

PROBABLE CAUSE:

a. Mechanical failure of torque converter (6).

Problem 3:

More than 19 liters (5 U.S. gal.) of oil are drained from the cover. During Visual Check 5, shiny steel particles were found in return oil screen (20).

PROBABLE CAUSE:

a. Mechanical failure of torque converter scavenge section (15).

Problem 4:

Iron or steel chips are found.

PROBABLE CAUSE:

a. Worn flywheel or starter gear on engine. Inspect the starter and flywheel gears.

b. Worn drive or driven gear on torque converter oil pump.

c. Mechanical failure of oil pump drive (8). Drain and inspect the oil pump drive.

Problem 5:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 6:

Bronze particles are found.

PROBABLE CAUSE:

a. Worn lockup clutch (11) in torque converter (6).

Check 7.

Remove and inspect screen (12) for foreign materials.

RIGHT SIDE OF ENGINE
12. Screen (between elbows).

Problem 1:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 2:

A large quantity of aluminum particles are found in screen (12).

NOTE: A small amount of aluminum particles from (wear of) oil pump (21) is normal.

PROBABLE CAUSE:

a. Mechanical failure of oil pump (21). Do Visual Check 8.

b. Mechanical failure of torque converter (6).

Check 8.

Drain the oil from hydraulic oil tank (19). Inspect the bottom of the hydraulic oil tank for debris. Remove and clean suction screens (22) and (26). Remove and inspect return oil screen (25) for foreign materials.

Problem 1:

Rubber particles are found.

PROBABLE CAUSE:

a. Seal failure.

b. Hose failure.

Problem 2:

Shiny steel and/or chrome particles are found in return oil screen (25).

COVER REMOVED FROM HYDRAULIC OIL TANK
22. Suction screen (behind plate). 25. Return oil screen. 26. Suction screen (behind plate).

PROBABLE CAUSE:

a. Mechanical failure of a hydraulic cylinder and/or rod (look at cylinder rods for scoring, etc.).

Problem 3:

Large quantity of aluminum particles are found in return oil screen (25).

NOTE: A small amount of aluminum particles from (wear of) oil pump (21) is normal.

PROBABLE CAUSE:

a. Mechanical failure of oil pump (21).

NOTE: If any of the above particles are found during any visual check, all components of the torque converter, brake cooling and hoist (or wagon) hydraulic systems must be cleaned. Do not use any damaged parts. Any damaged parts must be removed and new parts installed.

Performance Test List

Performance tests of the hydraulic system can be used for a diagnosis of poor performance and to find the source of oil leakage inside the hydraulic system.

Problem: Pump makes noise, the cylinder rods do not move smoothly and there are air bubbles in the oil.

PROBABLE CAUSE:

1. The relief valve opens at low oil pressure.

2. Loose connection of the oil line on the inlet side of the pump.

3. The pump has too much wear.

Problem: The temperature of the oil is too hot.

PROBABLE CAUSE:

1. The viscosity of the oil is wrong.

2. The relief valve opens at low oil pressure.

3. The pump has too much wear.

4. There is a restriction in an oil passage.

5. The load of the system is too high.

Problem: The output of the pump is low.

PROBABLE CAUSE:

1. Low level of the oil in the tank.

2. The viscosity of the oil is wrong.

3. The pump has too much wear.

Problem: The pressure of the oil is low.

PROBABLE CAUSE:

1. The relief valve opens at an oil pressure that is lower than the pressure in the SPECIFICATIONS.

2. The pump has too much wear.

3. The failure of an O-ring seal in the system.

Checking Pump Efficiency

For any pump test at a given rpm, the pump flow (gpm) at 690 kPa (100 psi) will be larger than the pump flow (gpm) at 6900 kPa (1000 psi).

The difference between the pump flow of two operating pressures is the flow loss.

Flow loss when expressed as a percent of flow loss is used as a measure of pump performance.

Example of finding percent of flow loss ...

If the percent of flow loss is more than 10% for test on the machine or 15% for test on the bench, pump performance is not good enough.

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^*Numbers in examples are for illustration and are not values for any specific pump or pump condition. See the SPECIFICATIONS FOR 768C TRACTOR & 769C TRUCK HYDRAULIC SYSTEM Form No. SENR2941 for pump flow of a new pump at 100 psi and 1000 psi.

Test On The Machine

Install a 9S2000 Flow Meter. Run the engine at 2000 rpm. Measure the pump flow at 690 kPa (100 psi) and at 6900 kPa (1000 psi). Use these values in Formula I.

Test On The Bench

If the test bench can be run at 1000 psi and at full pump rpm, find the percent of flow loss using Formula I.

If the test bench can not be run at 1000 psi at full pump rpm, run the pump shaft at 1000 rpm. Measure the pump flow at 690 kPa (100 psi) and at 6900 kPa (1000 psi). Use these values in the top part of Formula II. For the bottom part of the formula, run the pump shaft at 2000 rpm. Measure the pump flow at 100 psi.

Cylinder Performance (Truck)

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Make reference to WARNING on first page of HYDRAULIC SYSTEM TESTING AND ADJUSTING section.

Lift Time for Hoist Cylinders

Start the engine and run it at high idle speed. Move the control lever to the RAISE position. Hold the lever in RAISE position for 5 seconds after the body has lifted to dump position, then lower the body. Do this lift and lower cycle many times to get the temperature of the hydraulic oil to more than 38°C (100°F).

BODY IN DUMP POSITION (Typical Example)

Lower the body to the frame and start the test. With the engine running at high idle speed use the timer to measure the time for the body to lift from the frame to dump position.

The time for the hoist cylinders to get fully extended and lift the body to dump position is approximately 10.5 seconds.

Cylinder Drift

Start the engine and lift and lower the body many times to get an increase in the temperature of the hydraulic oil.

The temperature of the hydraulic oil must be no less than 38°C (100°F).

With the body empty and the engine running, move the body control lever to the RAISE position. When the body has lifted enough for the first stage of the hoist cylinders to be extended 305 mm (12 in.), move the control lever to the HOLD position. Make a record of the time for the weight of the empty body to push the head end (first stage) 12.7 mm (.50 in.) onto the second stage of the hoist cylinders. The acceptable time for hoist cylinder drift is in a relation to the temperature of the hydraulic oil for the cylinders.

Too much hoist cylinder drift is caused by:

1. Leaks in the lines and connections between the hydraulic tank and the hoist cylinders.

2. Worn control valve spool.

3. Worn seals in the hoist cylinders.

Relief Valve

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Make reference to WARNING on first page of HYDRAULIC SYSTEM TESTING AND ADJUSTING section.

The relief valve is part of the control valve. The control valve is below the level of the oil in the hydraulic tank. When an adjustment is needed for the relief valve, hydraulic oil must be drained from the tank.

Relief Valve Test (Truck)

Lower the body and stop the engine. After the engine stops, move the control lever to RAISE and LOWER positions, then put the lever in the FLOAT position.

LOCATION FOR RELIEF VALVE TEST
1. Hoist cylinder (left). 2. Quick disconnect nipple.

Connect a hose assembly with a 28 000 kPa (4000 psi) pressure gauge to the quick disconnect nipple assembly (2) on the rod end of hoist cylinder (1).

Start the engine and lift and lower the body many times to get the temperature of the hydraulic oil to more than 38°C (100°F).

Run the engine at high idle speed and move the control lever to the RAISE position. After the body has been lifted to dump position, look at the gauge and move the control lever and hold it in the RAISE position. The high indication on the gauge is the pressure setting of the relief valve. The correct setting for the relief valve is 17 250 ± 170 kPa (2500 ± 25 psi).

Relief Valve Test (Tractor)

Move the control lever to the FLOAT position until the hydraulic component, on the unit pulled by the tractor, stops its movements. Stop the engine, move the control lever to each position then put the control lever in the FLOAT position.

Install a 28 000 kPa (4000 psi) pressure gauge on one end of the 5S4648 Hose Assembly and the 5S4965 Pressure Plate on the other end.

Disconnect one of the hoses from the hydraulic tank at the hose junction for the pulled unit and install the pressure plate (hose and gauge). It is necessary to use longer bolts through the flange when the pressure plate is between the hose flange and the junction.

Start the engine and move the control lever to each position again and again to get the temperature of the hydraulic oil to more than 38°C (100°F).

Run the engine at high idle speed and move the control lever to the position that puts hydraulic oil through the hose where the pressure gauge is installed. Hold the lever in this position and look at the pressure gauge. The high indication on the gauge is the pressure setting of the relief valve. The correct setting of the relief valve is 17 250 ± 170 kPa (2500 ± 25 psi).

NOTE: Do not keep the control lever in the RAISE position, at relief valve pressure setting, for longer than 5 seconds.

If the setting of the relief valve is not correct, make a record of the indication on the gauge and make an adjustment for the relief valve.

Adjustment for Relief Valve

Start and run the engine and move the control lever to the FLOAT position for approximately 20 seconds. Stop the engine.

COVER ON FRONT OF HYDRAULIC TANK

Drain the oil from the hydraulic tank until the oil level is below the cover of the tank. Remove the cover.

Remove plug (1) and adjust as follows:

To increase pressure setting either add shims (2) in the plug (1) or remove shims (4) from under plug (1).

To decrease pressure setting either remove shims (2) from inside the plug (1) or add shims (4) to plug (1).

Pilot valve spring (5) and pilot valve (3) must be in position when plug (1) and the shims are installed on the relief valve. The torque for plug (1) is 108 ± 7 N·m (80 ± 5 lb. ft.).

6J3400 RELIEF VALVE
1. Plug. 2. Shims. 3. Pilot valve. 4. Shims. 5. Pilot valve spring.

Fill Oil Tank Procedure

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Make reference to WARNING on first page of HYDRAULIC SYSTEM TESTING AND ADJUSTING section.

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NOTICE
Use two different oils when filling or adding to the hydraulic tank. Use SAE 30 when filling the transmission oil tank and SAE 10W when filling the hydraulic oil tank.

HYDRAULIC AND TRANSMISSION TANK
1. Hydraulic oil filler cap. 2. Transmission oil filler cap.

1. Put oil in hydraulic tank until the oil can be seen in the gauge.

2. Start and run the engine at low idle rpm. With the engine running, add oil to the tank to keep the level of the oil in the gauge.

3. When no more oil is needed to keep oil in the gauge, raise the body until the first stage cylinders are half extended. Lower the body and add oil if needed.

4. Raise the body until the first stage cylinder is extended. Lower the body and add oil if needed.

5. Raise the body until the second stage cylinder is one half extended. Lower the body and add oil if needed. Raise the body until it is in full dump position. Lower the body and add oil if needed.

NOTE: If the unit pulled by the tractor has no hoist cylinders, hold the control lever in a position until the hydraulic component is completely activated, then move the control lever to the HOLD position. Add oil to the hydraulic tank if necessary. Hold the control lever in the other position, then to the HOLD position and add oil if necessary.

6. Raise the body to full dump and lower it three times. Put on the brakes and release them three times. Add oil needed to see the oil in the gauge.

Adjustment For Control Linkage

The hoist control linkage must be correctly adjusted when the machine is equipped with a transmission neutralizer.

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The transmission on this machine will neutralize when the transmission shift lever is in REVERSE and the hydraulic control lever is in either RAISE or LOWER position. The neutralizer will not stop the vehicle from rolling. Use the brakes to stop. Make reference to WARNING on first page of HYDRAULIC SYSTEM TESTING AND ADJUSTING section.

If the truck has a transmission neutralizer and the transmission does not go from REVERSE to NEUTRAL when control lever (1) is moved to either the RAISE or LOWER position, a linkage adjustment may be necessary. An electric failure in the neutralizer switch or wires could also be the cause.

HYDRAULIC CONTROL LEVER
1. Control lever. 2. 281 mm (11.06 in.) dimension (lever in FLOAT position, to doorsill).

The control lever (1) must be 281 mm (11.06 in.) from the doorsill of the cab when the lever is in the FLOAT position. If the measurement for dimension (2) is not correct, the control linkage needs an adjustment.

To adjust the linkage, first measure the length of rod (4) between the centers of the screws on joints (6). The measured length must be 1185 mm (46.7 in.). Then loosen the locknut on rod (3) that is against yoke (5) and disconnect the yoke from the lever. If the measurement for dimension (2) is too much, turn yoke (5) in the direction to make rod (3) longer. Make the length of rod (3) shorter if measurement (2) is not enough. The length for rod (3) will be approximately 1195 ± 5 mm (47.05 ± .20 in.) as measured in a straight line between the centers of the rod end bolt hole and yoke (5) pin holes.

It may be necessary to adjust the location of the neutralizer switch if the linkage dimensions are correct. Remove the cover from the back of the operator's cab and then remove the four bolts which hold the switch cover (box) to the inside wall of the cab. Remove the cover (box) from over the switch in the cab.

CONTROL LINKAGE RODS
3. Rod from the shaft for the control lever in the cab. 4. Rod to the lever on the hydraulic control in the hydraulic oil tank (shown disconnected). 5. Yoke on end of rod. 6. Joints.

Move control lever (1) to the FLOAT position. The lower roller on yoke (10) must be near but not against the short lever (not shown in the illustration) between the two rollers on the yoke.

If it is necessary to change the position of the yoke, move the control lever to the FLOAT position. Loosen bolt (7) and then move switch and plate (9) until the lower roller on yoke (10) is near but not against the short lever. Tighten the bolt.

If the position of yoke (10) can not be correctly adjusted by moving switch and plate (9) on bracket (8), it will be necessary to make either one of the next two adjustments.

One of the adjustments is to make the length of rod (3) either longer or shorter until the lower roller on yoke (10) is very near the short lever when control lever (1) is in FLOAT position.

NOTE: When rod (3) is made longer, be sure control lever (1) can be easily moved to the LOWER position.

The second adjustment is for the location of yoke (10) on the switch. Put the control lever in the FLOAT position and loosen the setscrew in the yoke. Move the yoke so that the lower roller, on the yoke, is near but is not against the short lever between the rollers on the yoke. Tighten the setscrew in the yoke.

NEUTRALIZER SWITCH
7. Bolt (hidden). 8. Bracket. 9. Switch and plate. 10. Yoke.