AD 22 Underground Articulated Truck Steering Systems Caterpillar

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Personal injury or death can result from machine articulation or movement. Machine frames can move and a person can be crushed. Connect the steering frame lock link between the front and rear frames before working on machine. Do not disconnect the lock link until the machine is made operational and the commissioning procedure is completed.

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Personal injury or death can result from escaping fluid under pressure. Escaping fluid under pressure, even a very small pin-hole size leak, can penetrate body tissue, causing serious injury, and possible death. Always use a board or cardboard when checking for a leak. Do not check for pin hole leaks in pump or motor hoses during a steer stall condition. If fluid is injected into your skin, it must be treated by a doctor familiar with this type of injury immediately.

Performing tests of the steering system can be used for diagnosis of poor performance and for finding the source of oil leakage inside the hydraulic system.

This section provides probable causes to a known problem. Since there may be several probable causes, specific inspections or specific instrument tests may be recommended. These inspections and tests will help to identify the cause that is most probable.

Note: Make sure that you use hydraulic oil with the correct viscosity for the ambient temperature and for the operating conditions. To determine the correct oil viscosity, see the reference that follows.

Reference: For additional information about the various lubricant viscosity for your machine, refer to the section Operation and Maintenance Manual, "Lubricant Viscosities".

Causes for Failures in the Steering System

Common failures in the steering system consist of the following items:

• Broken oil line or a leak in an oil line connection

• Worn steering piston pump

• Worn steering metering pump

• Wrong pressure setting of steering check and relief valve

• Worn steering check and relief valve

• Leaking steering check and relief valve

• Spools in the steering control valve that do not shift properly

• Worn steering cylinders

• Pressure setting of the pressure and flow compensator valve

• Pressure setting of the steering relief valve

Refer to the following sections before performing troubleshooting checks:

1. Refer to Troubleshooting, "Machine Preparation for Troubleshooting".

2. Perform a visual inspection. Refer to Troubleshooting, "Visual Inspection".

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

• Air in the steering hydraulic circuit

• Loose connection of the oil line on the inlet side of the steering piston pump

• Low oil level in the hydraulic tank

• Wrong oil viscosity

Problem: Turning The Steering Wheel Requires Too Much Force

Probable Cause

• Open backup relief valve

• Low oil level in the hydraulic tank

• Worn steering piston pump

• Tight end cover on the steering metering pump

• Interference of components on steering column with shaft

• Improper operation of steering metering pump

• Improper operation of steering check and relief valves

• Cold oil

• Kinked signal line

• Leaky signal line

Problem: The Machine Does Not Turn When The Steering Wheel Is Turned

Probable Cause

• Air in system

• Malfunction of the steering check and relief valves

• Lack of oil in steering metering pump

• Worn steering metering pump

• Disengaged steering column shaft from steering metering pump

Problem: Erratic Steering

Probable Cause

• Air in system

• Worn steering linkage

• Worn cylinder piston

• Loose cylinder piston

• Malfunction of steering check and relief valves

• The timing between the gerotor and pin (1) on the steering metering pump is not correct.

Illustration 1	g06235946
Timing of the steering metering pump (1) Pin (2) Internal pump gear

• The timing of the steering metering pump drive is not correct. Pin (1) must align with the valley of internal pump gear (2)

Problem: The Machine Turns Too Slowly In Both Directions

Probable Cause

• Not enough oil flow from the steering piston pump

• Malfunction of steering check and relief valves

• The priority spool or the directional spool will only allow partial flow.

Problem: Machine Turns Too Slowly In One Direction

Probable Cause

• The directional spool does not shift fully due to restriction at one end

• Restriction in steering metering pump

• Malfunction of the steering 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 the 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

• Restriction in the line circuit

Problem: Low Oil Pressure

Probable Cause

• Low-pressure setting of the flow compensator valve in the pressure and flow compensator valve

• Worn steering piston pump

Problem: Steering Wheel Turns With No Operator Effort

Probable Cause

• The sleeve in the steering metering pump sticks due to foreign material

• The spool in the steering metering pump sticks due to foreign material

• Broken centering springs in the steering metering pump

• Damaged centering springs in the steering metering pump

Problem: Soft Steering

Probable Cause

• Air in the steering hydraulic circuit

• Low oil level in the steering hydraulic tank

Problem: The Steering Piston Pump Does Not De-Stroke To Standby Pressure

Illustration 2	g06235946
Timing of the steering metering pump (1) Pin (2) Internal pump gear

Probable Cause

• The timing between the gerotor and pin (1) on the steering metering pump is not correct.

• The timing of the steering metering pump drive is not correct. Pin (1) must align with the valley of internal pump gear (2).

The Machine Pauses Before the Machine Starts to Steer

Probable Causes

• The hydraulic oil is too thick or the type of oil is incorrect

• Steering charge pressure or flow is low

• The steering pump controls are suspect

• The orifice in the pump control is partially blocked

• The neutral setting for the steering pump is improperly adjusted

Steering Speed Slow in Both Directions and Steering Pump Output is Low

Probable Causes

• Steering charge pressure or flow is low

• The steering pump controls are suspect

• The pressure setting for the pump cutoff valve is low

• The steering pump has failed or the pump has wear

Steering Speed Slow in Both Directions with Low Steering Pump Oil Output

Probable Causes

• Steering charge pressure or flow is low

• The steering pump controls are suspect

• The pressure setting for the pump cutoff valve is low

• The steering pump has failed or the pump has wear. The steering motor has failed or the motor has wear

Steering Speed Slow in One Direction and Steering Pump Oil Output Low in One Direction

Probable Causes

• The steering pump control is improperly adjusted or the control is suspect

Low Turning Force, and Low Steering Pump Output Pressure, in Both Directions

Probable Causes

• The cutoff pressure setting for the steering pump is too low

• Both pressure settings of the crossover relief and makeup valves are low or the valves are suspect

• Steering charge pressure is too low

• The steering pump has too much wear

Low Turning Force, and Low Steering Pump Pressure Output, in One Direction

Probable Causes

• One pressure setting of a crossover relief and makeup valve is low or the valve is suspect

• The steering pump has excessive wear or excessive leakage

The Machine Will Not Turn in Either Direction

Probable Causes

• The steering pump pressure or flow is low

• The pump cutoff valve is suspect

• The steering pump or the steering motor has failed

• The steering pump or the steering motor is loose. The steering pump or the steering motor has backed out of the drive splines

• The parking brake is not releasing. Check the following causes:

• The wiring has a fault

• The brake valve spool is stuck

• The brake valve has excessive internal leakage

In One Direction, the Pump Remains at Neutral and the Machine Does Not Turn

Probable Causes

• The pump controls in the steering pump are suspect

• The zero position for the steering pump is improperly adjusted

• The steering pump controls have excessive leakage

Steering Pump Pressures in NEUTRAL Are Too High or Too Low

Probable Causes

1. The pressure setting of the charging valve for steering is too high or too low.

2. The setting for the pressure control valve for steering is low.

Steering Speed Changes with a Step or Steering Modulation is Poor

Probable Causes

1. The hydraulic oil is too thick or the oil is the wrong type. Warm the oil to operating temperature.

2. The steering supply flow is partially blocked.

3. The pump control is suspect or the control needs adjustment.

The Machine Turns in the Wrong Direction

Probable Causes

1. The high-pressure hoses are connected incorrectly between the steering pump and the steering motor.

The Steering is Jerky or Unstable

Probable Causes

1. The pump cut-off valve and the crossover relief and makeup valves have an undesirable interaction.

2. The hydraulic oil is too thick or the oil is the wrong type. Warm the oil to operating temperature.

3. The charging valve for steering is unstable.

The Machine Continues to Turn When the Steering Control is Returned to Neutral

Probable Causes

1. The adjustment of the zero position in the steering pump is not correct.

2. Adjustment of the zero position at neutral is not correct.

3. The pump control is suspect. Check these causes:

   1. A broken spring in the pump control

   2. A sticking valve spool

4. The pump has failed.

The Pressure or the Flow From the Steering Pump is Low

Probable Causes

1. The oil level in the hydraulic tank is too low.

2. The pump inlet or the suction line is blocked or crushed.

3. The steering pump has failed or the pump is worn.

4. The pressure control valve is suspect or the valve setting is too low.

The Machine Will Not Turn or the Steering Radius is Very Large

Probable Causes

1. The steering pump is not connected to the pump drive or the pump drive shaft is broken.

2. The oil level in the hydraulic tank is low.

3. The steering pump has failed.

4. The system pressure is low.

5. The parking brake is ON. Check the following causes:

   1. Low oil pressure in the power train

   2. Excessive internal leakage in the brake valve

6. A mechanical problem in the drive train is interfering with the steering.

The Turning Radius is Too Large in Both Directions

Probable Causes

1. The system pressure in the steering circuit is low. Check the following causes:

   1. The steering pump is faulty or worn. See Step 2.

   2. The bypass valve is suspect.

   3. The pressure control valve is set too low.

   4. The pressure control valve has excessive leakage.

   5. The steering motor is failing. See Step 3.

2. The steering pump is malfunctioning. Check the following causes:

   1. The crossover relief and makeup valve is leaking excessively.

   2. The pump controls are misadjusted or damaged.

   3. The stops are incorrect or worn.

3. The steering motor is malfunctioning. Check the following causes:

   1. The steering motor is worn or failed.

   2. The pressure control valve is suspect.

The Turning Radius is Too Large in One Direction

Probable Causes

1. The system pressure in the steering circuit is low. Check the following causes:

   1. The pressure control valve has excessive leakage.

   2. The steering pump or the steering motor is failing.

      Refer to Step 2 and Step 3.

2. The steering pump is malfunctioning. Check the following causes:

   1. The crossover relief and makeup valve is leaking.

   2. The pump controls are misadjusted or damaged.

   3. The pump is worn or failed.

3. The steering motor is malfunctioning. Check the following causes:

   1. The steering motor is worn or failed.

   2. The pressure control valve is suspect.

The electric drive pump (secondary steering) does not stay on.

Possible Causes

1. There is a problem with the pressure switch which activates the electric drive pump.

   • Refer to Systems Operation, Testing and Adjusting, "Steering System" for information about the pressure switch.

2. There is a problem with the electrical connections between the following components and the machine ECM.

   • There is a problem with the electric drive pump.

   • Secondary steering pump relay.

The electric drive pump (secondary steering) does not supply the correct pressure.

Possible Causes

1. There is a problem with the electric drive pump.

   1. The relief valve, which is inside the electric drive pump, is stuck open.

   2. The motor is faulty.

2. A check valve in the load sensing line is stuck open. This check valve is located in the main control valve LS3 port.

   • Refer to Systems Operation, Testing and Adjusting, "Steering System" for the location of the check valve.

3. There is a problem with the metering pump.

   1. The metering pump may be directing oil to the tank instead of directing oil to the steering cylinders. This may result in reduced system pressure.

      • Refer to Systems Operation, Testing and Adjusting, "Piston Pump (Steering, Hoist) - Test and Adjust" for information on testing the system pressure.

   2. The metering pump may be blocking oil between the main control valve and the steering cylinders.

   3. The metering pump may not be producing the correct flow into the load sensing line. This will affect the flow from the electric drive pump.

4. There is a problem with the steering cylinders.

   1. There may be mechanical damage to one or both of the steering cylinders. This could include damage to the seals.

   2. A relief valve, which is attached to a steering cylinder, may be stuck open. This may result in reduced system pressure.