Diagnosing Charge Pressure Loss and Tracking Drift in the CAT 953C

[MikePhua]

The CAT 953C and Its Hydrostatic Drive System
The Caterpillar 953C track loader was introduced in the early 2000s as part of CAT’s evolution toward electronically controlled hydrostatic drive systems. Built for versatility in earthmoving, demolition, and site prep, the 953C featured a fully hydrostatic transmission, pilot-operated controls, and a robust undercarriage. With an operating weight of approximately 34,000 pounds and a bucket capacity of 2.5 cubic yards, it became a staple in mid-size fleets across North America.
The 2ZN serial prefix identifies a specific production run of the 953C, with multiple sub-variants depending on emissions compliance, hydraulic configuration, and electronic control modules. The hydrostatic system uses two variable displacement pumps feeding two drive motors, allowing independent control of each track and precise maneuvering.
Tracking Left in Forward but Straight in Reverse
A common issue in hydrostatic machines is directional drift—where the machine veers to one side during travel. In this case, the 953C tracked left when moving forward but remained straight in reverse. This asymmetry suggests a calibration or sensor issue rather than mechanical failure.
Initial diagnostics revealed:

• Rebuilt drive pumps with new piston blocks
  
• Charge pump deemed functional by dealer
  
• Persistent charge pressure loss under load
  
• Tracking sensor contamination on the left side block
  

Cleaning the tracking sensor resolved the directional drift, confirming that sensor feedback was skewing the electronic control logic. However, the charge pressure issue remained unresolved.
Terminology and Hydraulic Concepts
- Charge Pressure: The baseline hydraulic pressure supplied to the closed-loop hydrostatic system to maintain fluid volume and prevent cavitation.
- Servo Valve: A proportional valve that controls pump displacement based on joystick input and sensor feedback.
- Make-Up Valve: A valve that supplements fluid to the drive loop when internal leakage exceeds charge pump capacity.
- Case Drain Pressure: The pressure in the motor housing return line, used to detect internal leakage.
Charge Pressure Fluctuation and Stall Testing
Under stall conditions—with the parking brake locked out and the machine under load—charge pressure dropped below spec by up to 30 psi. This fluctuation was isolated to forward travel, suggesting directional sensitivity in the hydraulic loop.
Tests confirmed:

• Good charge pressure at neutral (420 psi)
  
• Pressure drop during forward travel
  
• Servo lines capped individually during diagnostics
  
• Filters replaced and cut open with no debris found
  

The technician suspected internal leakage in one of the drive motors, despite dealer claims that the motors were good. The logic was that make-up valves were compensating for fluid loss, causing the charge pump to struggle under load.
Case Drain Testing and Leakage Diagnosis
To confirm motor integrity, case drain pressure should be measured using low-pressure gauges at the quick-connect fittings. Normal case drain pressure is below 3 psi. Anything higher indicates excessive internal leakage, which can overwhelm the charge pump and cause pressure loss.
If one motor leaks internally only in forward travel, it may still perform adequately in reverse, explaining the directional discrepancy. This behavior is consistent with wear in the motor’s rotating group or valve plate.
Calibration and Electronic Adjustments
After pump installation, engine-on calibration is essential to synchronize pump displacement, joystick input, and sensor feedback. Failure to perform this step can result in tracking drift, sluggish response, and uneven pressure distribution.
Calibration involves:

• Accessing the electronic control module under the right armrest
  
• Setting pump pressures and neutral bias
  
• Verifying sensor alignment and feedback accuracy
  

In this case, calibration was attempted but deemed ineffective due to excessive tracking offset. Only after cleaning the tracking sensor did calibration become viable.
Recommendations for Final Diagnosis and Repair
To resolve the remaining charge pressure issue:

• Measure case drain pressure on both motors during forward and reverse travel
  
• Inspect make-up valves for continuous flow or leakage
  
• Confirm servo valve integrity and response time
  
• Recheck pump bench test results, especially piston block tolerances
  
• Perform full engine-on calibration after sensor cleaning
  

If case drain pressure exceeds 3 psi, motor replacement or rebuild may be necessary. While pumps were rebuilt, motors may still exhibit wear not detectable through basic flow testing.
Field Anecdotes and Practical Advice
One technician working on a similar 963C noted that charge pressure dropped to 265 psi under load, barely within spec. After replacing a leaking motor, pressure stabilized and tracking improved. Another operator emphasized the importance of isolating each hydraulic loop during testing, as cross-contamination can mask the true source of leakage.
In older hydrostatic machines, even minor internal leakage can cascade into performance issues. Charge pressure is the lifeblood of the system—when it falters, everything else follows.
Conclusion
The CAT 953C’s tracking and charge pressure issues highlight the complexity of hydrostatic diagnostics. While sensor cleaning resolved directional drift, persistent pressure loss under load points to internal leakage—likely in one of the drive motors. With precise testing, calibration, and component inspection, the machine can be restored to full operational integrity. In hydrostatic systems, pressure tells the story—and every psi counts.