Diagnosing Steering Failure in the Caterpillar 973 Track Loader

[MikePhua]

The Evolution of the CAT 973 Series
The Caterpillar 973 track loader was introduced in the late 1980s as part of Caterpillar’s push to modernize its crawler loader lineup. Built on the success of the 955 and 977 series, the 973 offered increased horsepower, hydrostatic drive, and improved operator ergonomics. With an operating weight of over 50,000 lbs and a bucket capacity exceeding 4 cubic yards, the 973 became a staple in demolition, land clearing, and heavy excavation.
By the early 2000s, the 973 had evolved into the 973C and later the 973D, incorporating electronic controls and emissions-compliant engines. However, the original 973—especially pre-electronic models like the 86G prefix—remains widely used due to its mechanical simplicity and field serviceability.
Symptoms of Steering Malfunction
Operators encountering steering issues on the 973 often report:

• Delayed response when pressing the right or left steering pedal
  
• Complete loss of steering on one side
  
• Machine reversing instead of turning when engaging a pedal
  
• Inconsistent behavior after several hours of operation
  
• Increased engine load or bogging during attempted turns
  

These symptoms suggest a failure in the hydrostatic drive system, which uses variable displacement pumps and motors to control track speed and direction independently. Steering is achieved by varying flow to each track motor, allowing differential speed and torque.
Hydrostatic Drive and Steering Logic
The 973’s hydrostatic transmission relies on:

• Dual variable displacement pumps
  
• Track drive motors with case drain and shift ports
  
• Brake lines and pressure regulators
  
• Mechanical or electronic control linkages
  

When a steering pedal is pressed, the transmission control adjusts pump output to one motor, increasing or decreasing flow to create a turn. If one motor fails to respond, the machine may reverse or stall due to imbalance in torque distribution.
Common Causes of Steering Failure
Several mechanical and hydraulic issues can lead to steering loss:
Case Drain Blockage

• Each track motor has a case drain port to relieve internal pressure
  
• If blocked, pressure builds and prevents motor rotation
  
• Spec: Case drain pressure should be 0–2 psi during testing
  

Brake Line Interference

• Capping the brake line is sometimes used to lock brakes during diagnostics
  
• On the 973, this can cause the machine to move unexpectedly when placed in gear
  
• Caution: Always test with transmission lever fully back and brakes engaged
  

Motor Shift Port Confusion

• Two taps on each motor: one for shifting, one for case drain
  
• Using the wrong port during testing can yield false results
  
• Correct tap: The one closer to the center of the machine
  

Transmission Stall Damage

• Holding the transmission at stall for more than 30 seconds can damage port plates
  
• Always allow 30 seconds in neutral between stall tests
  

Hydraulic Fluid Contamination

• Dirty or degraded fluid can cause valve sticking and pump inefficiency
  
• Recommended fluid: Caterpillar HYDO Advanced 10 or equivalent
  
• Change interval: Every 1,000 hours or annually
  

Control Linkage Wear

• Mechanical linkages under the seat armrest may wear or misalign
  
• Check for three switches under the right-hand armrest—some models use them for control logic
  

Diagnostic Procedure
To isolate the issue:

• Cap the brake line carefully and engage the transmission in gear
  
• Locate the case drain tap and connect a pressure gauge
  
• Pull the transmission lever fully back and observe pressure
  
• If pressure exceeds 2 psi, the motor may be internally leaking or blocked
  
• Inspect fluid condition and check for metal particles
  
• Test both motors independently to compare response
  

A technician in Georgia once diagnosed a steering issue on a 973 that reversed instead of turning left. After testing case drain pressure, he found the right motor had 8 psi—well above spec. Replacing the motor resolved the issue, and the machine returned to normal operation.
Preventive Maintenance and Recommendations
To maintain steering performance:

• Change hydraulic filters every 500 hours
  
• Inspect control linkages quarterly
  
• Test case drain pressure annually
  
• Avoid prolonged stall testing
  
• Use OEM-grade fluid and monitor for contamination
  
• Keep a service log with fault codes and pressure readings
  

Operators should also acquire the correct manuals:

• Operation & Maintenance Manual: SEBU5848
  
• Parts Manual: SEBP1387
  

These documents provide detailed procedures and specifications for troubleshooting and repair.
Field Anecdotes and Lessons Learned
A farm crew in Alberta used a 2000-model 973 for land clearing. After 10 hours of intermittent use, the right-side steering failed. The machine reversed when the left pedal was pressed. After capping the brake line and testing case drain pressure, they discovered a blocked relief valve. Cleaning the valve and replacing the fluid restored steering.
In another case, a contractor in Ohio bypassed the electronic throttle actuator due to a leak. This caused erratic steering behavior. After installing a mechanical throttle cable and recalibrating the transmission control, the issue was resolved.
Conclusion
Steering issues in the Caterpillar 973 track loader are often rooted in hydrostatic drive imbalances, case drain pressure anomalies, or control linkage wear. With methodical diagnostics and proper maintenance, these problems can be resolved without major overhaul. The 973, built during a transitional era of mechanical and hydraulic integration, continues to serve reliably when its systems are understood and respected. Whether clearing land or loading debris, precise steering remains essential—and achievable—with the right approach.