769D Off-Highway Truck: Steering System Troubleshooting PART 1

When a problem with the steering system is defined, use the following procedure:

1. Perform a visual inspection. Refer to Testing and Adjusting, "Visual Inspection".

2. If the problem has not been identified, do "Operation Checks".

3. If the problem is still undetermined, do "Performance Checks".

4. If the problem is still undetermined, do "Check The Warning System (Steering System)".

This procedure will help to identify the steering hydraulic system problems.

Operation Checks

The front wheels must be on a dry, smooth hard surface and the hydraulic oil in the steering system must be warm.

Start the machine. In first gear, slowly drive the machine while you turn the steering wheel. Drive the machine until the hydraulic oil temperature is approximately 38 °C (100 °F).

Test the steering system with the engine at high idle. Rotate the steering wheel. Measure the time that is required to turn the front wheels from the full right turn position to the full left turn position and back to the full right turn position. If the time is more than 6.0 seconds, there could be a problem in the steering hydraulic system. If the time is less than 5.0 seconds, there could be a problem in the steering hydraulic system.

Failures in the steering system can be at least one of the following items:

• Broken oil line or a leak in an oil line connection
• Worn steering piston pump
• Pressure setting of the flow compensator valve in the pressure and flow compensator valve
• Pressure setting of the high pressure cutoff valve in the pressure and flow compensator valve
• Low pressure setting of the primary steering backup relief valve
• Open secondary steering check valve
• Worn steering metering pump
• Leaking steering cylinder check and relief valves
• Worn steering cylinders

Performance Checks

Performance checks of the steering system can be used for the following purposes:

• Diagnosis of poor performance
• Source of oil leakage inside the hydraulic system

Problem : Noisy steering piston pump.

Probable Cause

• Failed steering piston pump
• Air in the steering hydraulic circuit
• Loose connection of the oil line on the inlet side of the steering piston pump
• Loose connection of the oil line at the port on the steering hydraulic tank
• Low oil level in the steering hydraulic tank
• Wrong oil viscosity

Problem : Turning the steering wheel requires too much force.

Probable Cause

• Worn spool or sticking spool in the flow compensator valve
• Open primary steering backup relief valve
• Low oil in the steering hydraulic tank
• Worn steering piston pump
• Tight end cover on the steering metering pump
• Interference of components on steering column with shaft
• Overloaded steering axle
• Improper operation of steering metering pump
• Improper operation of steering cylinder check and relief valves
• Kinked load sensing line
• Leaky load sensing line
• Turning the steering wheel faster than 70 rpm at low idle
• Turning the steering wheel faster than 242 rpm at high idle
• Thermal shock in the steering metering pump

Note: Thermal shock occurs when the temperature of the oil in the steering metering pump is 10 °C (50 °F) colder than the temperature of the oil in the steering hydraulic tank.

Problem : The truck does not turn when the steering wheel is turned.

Probable Cause

• Air in system
• Malfunction of the steering cylinder check and relief valves
• Lack of oil in steering metering pump
• Worn steering metering pump
• Disengaged steering column shaft from steering metering pump
• Worn steering cylinders

Problem : Erratic steering.

Probable Cause

Timing of metering pump drive (1) Pin (2) Drive (3) Timing Mark (4) Gerotor star valleys (A) Reference Line (B) Reference Line(C) Reference Line

• Air in system
• Worn steering linkage
• Worn cylinder piston
• Loose cylinder piston
• Malfunction of the steering cylinder check and relief valves
• The timing of the gerotor and the timing of the pin (1) in the steering metering pump is not correct.

Problem : The truck turns too slowly in both directions.

Probable Cause

• Not enough oil flow from the steering piston pump
• Low pressure setting of the flow compensator valve in the pressure and flow compensator valve
• Low pressure setting of the high pressure cutoff valve in the pressure and flow compensator valve
• Malfunction of the steering cylinder check and relief valves

Problem : The truck turns too slowly in one direction.

Probable Cause

• Restriction in steering metering pump
• Malfunction of the steering cylinder check and relief valves

Problem : The steering wheel does not return to the center position correctly.

Probable Cause

• Tight cover on steering metering pump
• Interference of components on steering column with shaft
• Restriction between spool and sleeve in steering metering pump
• Damaged centering springs in steering metering pump
• Broken centering springs in steering metering pump

Problem : Hot oil temperature.

Probable Cause

• The steering piston pump does not go to standby pressure.
• Wrong oil viscosity
• Air in the oil
• Low pressure setting of the high pressure cutoff valve
• Low pressure setting of the primary steering backup relief valve
• Open secondary steering check valve
• Restriction in the line circuit

Problem : Low oil pressure.

Probable Cause

• Worn steering piston pump
• Low pressure setting of the flow compensator valve
• Low pressure setting of the high pressure cutoff valve
• Low pressure setting of the primary steering backup relief valve
• Open secondary steering check valve

Problem : The steering wheel turns with no operator effort.

Probable Cause

• The sleeve in the steering metering pump jams and the spool in the steering metering pump jams due to foreign material.
• Wrong oil line connections on the steering metering pump
• Broken centering springs in the steering metering pump
• Damaged centering springs in the steering metering pump

Timing of metering pump drive (1) Pin (2) Drive (3) Timing Mark (4) Gerotor star valleys (A) Reference Line (B) Reference Line(C) Reference Line

• Timing mark (3) that is on drive (2) must fall on the parallel line between any two gerotor star valleys (4) .

Note: Reference lines (A), (B) and (C) are parallel.

MARYGAR