953D, 963D and 973D Track-Type Loaders Steering Does Not Function Caterpillar


Steering Does Not Function
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1.1. Probable Causes
2.1. Recommended Actions
3.2. The Oil Level In The Hydraulic Tank Is Low
4.2. The Hystat Pump Controls Are Suspect
5.2. Charge Pump Pressure Becomes Low
6.2. Crossover Relief and Makeup Valves
7.2. Charge Pump Flow Becomes Low
8.2. Piston Pump (Rotating Group) Flow Rate
9.2. Hystat Pump Case Drain Flow Rate
10.2. The Hystat Pump Or The Hystat Motor Has Failed
11.2. The Control Lever Is Not Calibrated Properly
12.2. Failed Brake Components
13.2. The Speed Sensors Are Not Operating Properly

Table 1
Event Codes  
Code and Description  Conditions which Generate this Code  System Response 
E192(3) Steering System Malfunction  There is an active diagnostic code for the steering system.  The code is logged. 
E256(3) Steering Output Detected in Wrong Direction  The machine ECM commands steering in a particular direction but the speed sensor for the hystat motor detects motion in the opposite direction.  The code is logged. 
E257(3) Steering Output Detected with No Command Given  The machine ECM is not commanding any steering, but the speed sensor for the hystat motor detects motor speed.  The code is logged. 
E258(3) No Steering Detected with Command Given  The speed sensor for the hystat motor does not detect motor speed in response to a steering command from the machine ECM.  The code is logged. 

Probable Causes

  • The oil level in the hydraulic tank is low.

  • Hystat pump controls

  • Low charge pump pressure

  • Crossover relief and makeup valves

  • Charge pump flow rate low.

  • Piston pump (rotating group) flow rate

  • Hystat pump case drain flow rate

  • The hystat pump or the hystat motor has failed.

  • The control lever is not calibrated properly.

  • Failed brake components

  • The speed sensors are not operating properly.

Recommended Actions

Note: Care must be taken in order to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting, and repair of the machine. You must be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component that contains fluid. Dispose of all fluids according to local regulations.

The Oil Level In The Hydraulic Tank Is Low

The hydraulic oil tank has a sight gauge on the outside of the tank. Check the hydraulic oil level with the machine on level ground and the engine OFF. If oil is NOT seen in the sight glass, the hydraulic oil level is low.

If the hydraulic oil level is low, check for oil leakage.

Inspect the following items of the hydraulic tank for oil leakage:

  • All of the weld joints

  • All of the bolted covers and components

  • All fittings

  • All hose flange connections

Oil may leak out of the system if the welds are damaged, if covers and components have the seals or gaskets gaskets installed improperly, or if the bolts and fittings are improperly torqued. Correct any of the problems that are found. Fill the hydraulic tank with the correct oil. Monitor the tank in order to ensure that the leaks have been stopped.

Refer to Operation and Maintenance Manual, "Hydrostatic Transmission and Hydraulic System Oil Level - Check" or Operation and Maintenance Manual, "Hydraulic System Oil - Check" for your machine.

Note: Ensure that the strainer is present in the filler tube before adding the recommended hydraulic oil.

The Hystat Pump Controls Are Suspect

With the Monitoring System or AVSpare Electronic Technician (Cat ET), check for event codes associated with the steering lever position sensors.

Any type of failure with the steering position sensors will result in a "Level 3 Warning" that indicates that the machine needs to be shut down immediately in a safe manner and in a safe position before damage to the machine or injury to personnel occurs.

Note: Event codes are listed above.

If an event code for the steering system is logged, continue with the following procedures.

  1. If your machine is equipped with a joystick then check the monitoring system or Cat ET for any fault codes that are related to "Steering Position Sensor 1", "Steering Position Sensor 2", or "Steering Position Sensor 3".

    Ensure that a problem with the steering lever position sensors are not causing the hystat pump to operate improperly. If the signals from the steering position sensors are incorrect, the ECM may not be commanding the solenoid to energize properly, and/or the ECM may not be commanding the solenoid to de-energize properly. Correct any problems with these components, then check to see if the symptom is resolved.

    Perform the calibration routine for the "Steering Control Position Sensors". Refer to Testing and Adjusting, "Calibration" for your machine.

  2. With the monitoring system , perform the calibration routine for the "Steering Control Position Sensors" in Messenger. After you enter the "Service Password" , the real time status may be accessed via "Service/Calibrations/Steering Calibrations/Steering Control Position Sensors".

    This procedure will calibrate all three-position sensors so that the machine ECM can properly interpret the signals from the sensors and correctly control the hystat pump control solenoids.

    If these sensors are not properly calibrated, the sensors may be falsely sending signals to the ECM to steer with no steering request, or sending signals in one steering direction but not the other.

  3. With the monitoring system, check the signal "Duty Cycle" from the steering lever position sensors.

    View the status of the steering lever position sensors in order to ensure that the signals are accurate.

    The active real time status of these sensors may be viewed at the following location: "Service/System Status/Steering/Steer Pos Sensor 1 Duty Cycle, Steer Pos Sensor 2 Duty Cycle, Steer Pos Sensor 3 Duty Cycle".

    All three sensors should be sending a relatively close signal, as follows:

    • NO STEER - 49% - 51% Duty Cycle

    • LEFT STEER - 8% - 10% Duty Cycle

    • RIGHT STEER - 88% - 90% Duty Cycle

    If the sensors have been calibrated and are still sending incorrect signals, and if no electrical problems are found in the sensor circuits, one or more of the sensors are bad and must be replaced.

    Note: Note: All three of the sensors are packaged together as an assembly therefore all three of the sensors must be replaced.

    After replacing the sensors, repeat Step 2 then retest for the symptom.

Charge Pump Pressure Becomes Low

The charge pump is a fixed displacement gear-type pump. The charge pressure relief valve must open in order to allow enough oil to flow in order to maintain charge pressure. Oil flow past the charge pressure relief valve will increase as engine RPM increases, however, if the charge pressure relief valve is stuck open too far, the hystat charge pressure cannot be maintained.

The charge pressure relief valve maintains (limits) the charge pressure to approximately 2850 kPa (413 psi) at 2000 engine RPM. Charge pressure oil is sent to the drive loop, the steering control valve, the dual tilt valve (used as pilot oil), and the pump actuator piston.

Test and adjust the Charge Pressure Relief Valve. Refer to Testing and Adjusting, "Charge Relief Valve - Adjust" for your machine.

Note: The Cold Oil Relief Valve can also cause similar symptoms. Ensure that the cold oil relief valve is operating properly prior to performing any test and adjust procedures on the charge pressure relief valve.

Crossover Relief and Makeup Valves

Test and adjust the crossover relief and makeup valves. Refer to Testing and Adjusting, "Crossover Relief Valve - Adjust" for your machine.

Oil from the high pressure side of the steering loop may leak across to the low pressure side of the loop under one of the following conditions:

  • The relief function or the makeup function of one of the valves is stuck open

  • One of the valves springs is weak

  • One of the valves springs is broken

  • One of the valves springs is misadjusted

If one of the conditions listed above exists, the necessary high pressure may not be possible or may not be able to be maintained in one or both sides of the steering loop.

Test the mechanical neutral position for the hystat pump control valve. Refer to Testing and Adjusting, "Piston Pump Neutral - Adjust" for your machine. If the neutral position of the hystat pump control valve is incorrect (off-center while in the NO STEER condition), the hystat pump control valve will command the actuator piston to move off-center when there is no steering request. This condition may cause the machine to stray from straight line travel in a NO STEER condition, and may cause the machine to steer more sharply in one direction than the other.

Charge Pump Flow Becomes Low

If the charge pump becomes too inefficient and cannot produce the required oil flow, steering will be slow to react, turns will not be as sharp, or the machine may not steer at all.

Use a flow meter in order to test the flow rate of the hystat charge pump. The charge pump flow rate should be approximately 61.9 L/min (16.4 US gpm) at 1850 engine RPM.

Piston Pump (Rotating Group) Flow Rate

If the piston pump becomes too inefficient and cannot produce the required oil flow, steering will be slow to react, turns will not be as sharp, or the machine may not steer at all.

Use a flow meter to test flow rate of the hystat pump (flow from the rotating group). The piston pump flow rate should be approximately 276 L/min (73 US gpm) at 1850 engine RPM. At 2200 engine RPM, the flow rate should be approximately 328 L/min (86.6 US gpm).

Hystat Pump Case Drain Flow Rate

Increased or elevated case drain flow rate indicates pump inefficiency.

If the piston pump becomes too inefficient and cannot produce the required oil flow, steering will be slow to react, turns will not be as sharp, or the machine may not steer at all.

Use a flow meter to test the flow rate of the hystat pump case drain. With the flow meter in place and the engine set to 1850 RPM, case drain flow in a steering stall condition should be no more than approximately 27.6 L/min (7.3 US gpm). This is approximately 10% of the piston pump flow rate. If the case drain flow rate from the hystat pump is higher than this 10% specification, the pump should be rebuilt or replaced.

The Hystat Pump Or The Hystat Motor Has Failed

Ensure that the hystat motor is not worn to the point that the steering system cannot maintain the correct pressures. Wear to the hystat motor rotating group may cause excessive internal (case drain) leakage. This condition may cause the hystat motor to operate inefficiently, which results in wider (less sharp) turns, excessive noise while steering, low steering or charge pressures, and hesitation prior to turns.

  1. Start the engine and run the engine.

    ENSURE THAT THE SERVICE BRAKES ARE FULLY APPLIED THROUGHOUT THIS ENTIRE PROCEDURE.

    WARNING: The machine may move during this procedure if the service brakes are released or if the service brakes are weak. When you move the steering control lever during this procedure, ensure that all personnel are positioned so that no injury can occur due to machine movement.

    Warm the hydraulic oil to 65°± 3°C (150°± 5°F).

  2. Stop the engine and disconnect hydraulic hose from the fitting on the manifold of the case drain for the hystat motor.

  3. Place the end of the hose in a suitable container with a known volume such as a 5 gallon bucket.

  4. Install a plug at the fitting on the case drain manifold in the same place that the line for the case drain was attached to the hystat motor.

  5. Start the engine and set engine speed to HIGH IDLE (2200 rpm).

  6. Move the steering lever to the full LEFT TURN position and hold it long enough to determine the case drain flow rate. Repeat this step in the full RIGHT TURN position.

    The motor's case drain flow rate in each direction should be approximately 4 L/min (1 gpm) in either direction.

    1. If the motor's case drain flow rate is no more than 20% of the specification, the motor is capable of attaining the speed/pressure specifications.

    2. If the motor's case drain flow rate is significantly greater when steering one direction than the other, either a cross port leak (on the pressurized side) is present or the flushing valve is stuck in one position. (A cross port leak can cause the machine to drift from straight travel.)

    3. Replace the motor if the flow rate is more than 20% above this specification. The motor is worn and leaking too much. Retest the system after the motor is replaced.

  1. Check The Hystat Motor Output Shaft Seal For Damage Or Wear.

    Damage to or excessive wear of the seal around the hystat motor output shaft can allow hydraulic oil (case drain) to leak past the shaft seal.

    1. Remove the hystat motor from the bore in which it is installed and wiggle the shaft to see if there is any movement.

      If the shaft does not wiggle, disconnect and remove the hystat motor from the machine and bench test the motor.

    2. Operate the motor at 425 psi in both directions. Observe the shaft seal during bench test operation and look for signs of leakage. If leakage is observed replace the shaft seal. Carefully examine the shaft for damage where the seal and shaft contact each other. After repair and assembly, test the motor again and look for leakage at the shaft before reinstalling the motor.

      If the shaft is loose, worn or failed bearings have allowed the shaft to cause wear to the shaft seal.

    3. Disassemble The Hystat Motor And Inspect For Damage

      If it has been determined that the hystat motor has excessive internal wear or damage, disconnect the hydraulic lines and the electrical harness from the motor and remove the motor from the machine. Disassemble the motor and carefully examine all internal parts (port plates, rotating group, bearings, etc.). Replace any worn or damaged parts as necessary and bench test, or replace the worn/damaged motor with a new motor.

The Control Lever Is Not Calibrated Properly

Be sure that the control lever position sensor is calibrated properly. Refer to Testing and Adjusting, "Hystat Operator Control Calibrations" for your machine.

Failed Brake Components

Disassemble the brakes and check the following components of the brakes for failure:

  • seals

  • splines

  • friction discs

  • brake plates

Replace any damaged components, if necessary.

Calibrate the brakes. Refer to Testing and Adjusting, "Calibration" for your machine.

The Speed Sensors Are Not Operating Properly

Check the status of the speed sensors in ET. Refer to Testing and Adjusting, "Speed Sensor - Adjust" for your machine. Replace the sensors, if necessary.

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