Introduction

NOTE: For Specifications with illustrations make reference to the SPECIFICATIONS FOR 768C TRACTOR & 769C TRUCK HYDRAULIC SYSTEM, Form No. SENR7817. If the Specifications in Form SENR7817 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.

Hydraulic System

The 5S5123 Hydraulic Testing Group is used to make the pressure test of hydraulic system. Before making any tests, visually inspect the complete hydraulic system by looking for leakage of oil and for parts that have damage. For some of the tests a timer and a measuring rule (either for inches or for millimeters) are usable tools.

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When testing and adjusting the hydraulic system, move the machine to a smooth horizontal location. Move away from working machines and personnel. There must be only one operator. Keep all other personnel away from the machine or where the operator can see the other personnel.

Visual Checks

A visul inspection of the hydraulic system and its components is the first step when making a diagnosis of a problem. Stop the engine and when the oil is cool so any pressure in the tank will be at a minimum, make the following inspections:

1. Measure the oil level. Slowly turn the filler cap until it is loose to let the tank pressure lower before removing the filler cap. Look for air in the oil in the tank.

2. Check all oil lines and connections for damage or leaks.

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 viscosity of the oil is wrong.

2. The relief valve opens at low oil pressure.

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

4. 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 100 psi (690 kPa) will be larger than the pump flow (gpm) at 1000 psi (6900 kPa).

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

Method of finding flow loss ...

Flow loss when expressed as a percent of pump flow 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. SENR7717, for pump flow of a new pump at 100 psi and 1000 psi.

Test On The Machine

Install a Flow Meter. Run the engine at 2000 rpm. Measure the pump flow at 100 psi (690 kPa) and at 1000 psi (6900 kPa). 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 100 psi (690 kPa) and at 1000 psi (6900 kPa). 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)

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 100° F (38° C).

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.

BODY IN DUMP POSITION

The time for the hoist cylinders to get fully extended and lift the body to dump position is approximately 10.5 seconds. Float down from the dump position is approximately 10 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 100° F (38° C). Make a record of the temperature of the hydraulic oil.

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 12 in. (305 mm), move the control lever to the HOLD position. Measure the distance the head end of the hoist cylinder has lifted. Measure the distance again in 5 minutes. The difference between the two measurements is the drift. Make a comparison of the record of the oil temperature and the drift measurement with the chart for the HOIST CYLINDER DRIFT IN FIVE MINUTES.

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

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 (right). 2. Plug in valve (oil line connection).

Remove plug (2) in the valve (oil line connection) on the rod end of hoist cylinder (1). Some oil will run out when the plug is removed before the 5P3501 Adapter Assembly with the 5S4648 Hose Assembly, the 1S8937 Valve and the 7S8714 Pressure Gauge are installed in the plug opening.

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

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 2200 + 50 or - 0 psi (15 200 + 345 or - 0 kPa).

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 and then put the control lever in the FLOAT position.

Install the 7S8714 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 100° F (38° C).

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 2200 + 50 or - 0 psi (15 200 + 345 or - 0 kPa).

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), the shims under the plug and shims (2) in the plug. Either remove shims for a decrease or add shims for an increase in the pressure setting of the relief valve.

Pilot valve spring (4) 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 80 ± 5 lb. ft. (1 10 ± 7 N·m).

6J3400 RELIEF VALVE (Part of Control Valve)
1. Plug. 2. Shims. 3. Pilot valve. 4. Pilot valve spring. [Note: 2J2866 Shims are under the hex of plug (1)]

Fill Oil Tank Procedure

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 moving. Use the brakes to stop the vehicle.

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

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.

The control lever (1) must be 11.06 in. (281 mm) 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.

Loosen the locknut on rod (3) that is against fitting (5) and disconnect the fitting from the lever. If the measurement for dimension (2) is too much, turn fitting (5) in the direction to make rod (3) longer. Make the length of rod (3) shorter if measurement (2) is not enough.

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. 5. Fitting on end of rod.

The length for rod (3) is approximately 47 in. (1194 mm). The length of rod (4) is 54.12 in. (1374.6 mm).

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.

Move control lever (1) to the FLOAT position. The lower roller on yoke (9) must be near but not against the short lever (not shown in the illustration) between the two rollers on the yoke.

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

If it is necessary to change the position of the yoke, move the control lever to the FLOAT position. Loosen bolt (6) and then move switch and plate (8) until the lower roller on yoke (9) is near but not against the short lever. Tighten the bolt.

If the position of yoke (9) can not be correctly adjusted by moving switch and plate (8) on bracket (7), 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 (9) 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 (9) 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.