Diagnosing Steering Reversal in the CAT 973 Loader: Hydraulic Imbalance, Drive Motor Leakage, and Field Testing

[MikePhua]

Introduction: When Steering Sends You Backward
The Caterpillar 973 track loader is a powerful earthmoving machine, but like many hydrostatic drive systems, it can develop steering anomalies that defy intuition. One particularly puzzling issue occurs when the machine reverses direction unexpectedly during steering input—especially after the hydraulic oil warms up. This article explores the mechanics behind this behavior, the role of drive motor leakage, and how to test and resolve the issue using pressure diagnostics and brake loading techniques.
Symptoms of the Steering Issue

• Steering functions normally when cold.
  
• After warming up, pressing the right steer pedal causes the machine to reverse.
  
• Pressing the left steer pedal causes the machine to stop.
  
• The issue is most pronounced at high idle in forward gear.
  

These symptoms suggest an imbalance in hydraulic flow between the left and right drive systems, likely caused by excessive internal leakage in one of the drive motors.
Understanding Hydrostatic Steering in the CAT 973
The CAT 973 uses a hydrostatic transmission system with two independent hydraulic circuits—one for each track. Steering is achieved by varying the speed and direction of each track through hydraulic modulation.
Key components include:

• Hydrostatic Pumps: Deliver pressurized fluid to each drive motor.
  
• Drive Motors: Convert hydraulic energy into mechanical rotation.
  
• Steering Pedals: Modulate flow to each side, enabling turning.
  
• HPCU (Hydrostatic Power Control Unit): Coordinates pressure and flow between circuits.
  

When one side of the system develops excessive leakage—especially internally within the motor casing—it disrupts the balance of pressure and flow, causing unintended directional changes.
Root Cause: Left Drive Motor Leakage
In this case, the left drive motor is suspected of having high internal leakage. When the right steer pedal is pressed, the right side slows down. If the left side is weak, the right side can overpower it and reverse direction. This behavior is consistent with a hydrostatic imbalance caused by a leaking motor.
Diagnostic Procedure: Pressure Testing and Brake Loading

1. Lock the Brakes
   • Disconnect and plug the hose from the left side of the HPCU to the cab support area.
     
   • These light lines control the brakes; locking them allows for load testing.
     
2. Connect Pressure Gauges
   • Use high-pressure gauges to monitor the servo pressure (typically around 345 psi).
     
   • Attach a gauge to the case drain nipple on each motor to measure internal leakage.
     
3. Load the Drive System
   • With brakes locked, attempt to move the machine in both directions.
     
   • A healthy motor will build pressure without excessive case drain.
     
   • A leaking motor will show elevated case pressure—above 5 psi is a red flag.
     
4. Compare Both Sides
   • Measure and record case pressure on both motors under load.
     
   • Significant disparity indicates which motor is leaking internally.
     
5. Relieve Case Pressure
   

• Before testing, open the filler cap to relieve any residual case pressure.
  

Repair Recommendations

• If case pressure is high on the left motor, it likely needs to be rebuilt or replaced.
  
• Inspect seals, bearings, and internal surfaces for wear or scoring.
  
• Flush the hydraulic system and replace filters after motor replacement.
  
• Recheck pressure balance after repairs to confirm resolution.
  

Field Anecdote: Ontario Mechanic’s Method
A technician in Ontario shared a similar case involving a CAT 973 with erratic steering. After isolating the left motor and performing a brake load test, he found case pressure spiking to 8 psi—well above normal. Replacing the motor restored proper steering behavior, and the machine returned to full service without further issues.
Preventive Maintenance Tips

• Monitor steering response during warm-up and full operation.
  
• Check hydraulic fluid condition and temperature regularly.
  
• Perform pressure tests annually or when steering anomalies appear.
  
• Replace drive motor seals every 3,000–4,000 hours or as needed.
  
• Keep detailed service logs to track performance trends.
  

Conclusion: Steering by Pressure, Not Guesswork
Hydrostatic steering systems rely on precise pressure balance between drive motors. When one side leaks internally, it can cause unexpected reversals, stalls, or sluggish response. By understanding the mechanics of the CAT 973’s steering system and applying targeted pressure diagnostics, operators and technicians can pinpoint the problem and restore control. In machines where every turn matters, steering should never be a guessing game.