Yesterday, 04:37 PM
A CAT 973 track loader that veers left during straight travel—without steering input—likely suffers from hydraulic imbalance, drive motor wear, or steering valve drift. This behavior is not considered normal, even for older machines, and can be corrected through targeted inspection and adjustment.
CAT 973 Background and Drive System Design
The Caterpillar 973 is a high-production track loader introduced in the 1980s, built for heavy-duty excavation, loading, and site preparation. It features a hydrostatic drive system, meaning each track is powered independently by a hydraulic motor controlled via steering pedals. The 973’s steering system uses pilot-operated valves to modulate flow to each side, allowing precise directional control.
The 66G series, referenced in this case, was part of the earlier generation of 973s. These machines are known for their robust undercarriage and powerful breakout force, but like all hydrostatic systems, they rely heavily on fluid balance and valve integrity to maintain straight-line travel.
Terminology Note
Root Causes and Diagnostic Strategy
In the Netherlands, an operator reported persistent left pull on a 973 with clean filters and normal fluid levels. After consulting a CAT technician, the issue was traced to a worn spool in the steering valve. Replacing the valve restored straight travel without pedal compensation.
In Texas, a contractor noted similar behavior on a 973C. The right drive motor had excessive case drain flow, indicating internal leakage. After rebuilding the motor, the machine tracked straight under all conditions.
Conclusion
A CAT 973 that pulls to one side during straight travel is not operating as intended. The cause is typically hydraulic imbalance due to valve wear, motor degradation, or pilot signal drift. With proper diagnostics and component inspection, the issue can be resolved, restoring full control and reducing operator fatigue. Even older machines like the 66G series can maintain precise tracking with regular maintenance and calibration.
CAT 973 Background and Drive System Design
The Caterpillar 973 is a high-production track loader introduced in the 1980s, built for heavy-duty excavation, loading, and site preparation. It features a hydrostatic drive system, meaning each track is powered independently by a hydraulic motor controlled via steering pedals. The 973’s steering system uses pilot-operated valves to modulate flow to each side, allowing precise directional control.
The 66G series, referenced in this case, was part of the earlier generation of 973s. These machines are known for their robust undercarriage and powerful breakout force, but like all hydrostatic systems, they rely heavily on fluid balance and valve integrity to maintain straight-line travel.
Terminology Note
- Hydrostatic Drive: A propulsion system using hydraulic motors to drive each track independently.
- Steering Valve Drift: A condition where internal leakage or wear causes unintended flow to one side.
- Drive Motor Wear: Degradation of internal components in the hydraulic motor, leading to uneven torque output.
- Pilot Pressure: Low-pressure hydraulic signal used to control main valve functions.
- Straight Drive Bias: A tendency for the machine to pull left or right during neutral travel.
- The machine pulls left when driving forward with no steering input.
- Slight pressure on the right pedal is required to maintain a straight path.
- The behavior is mirrored in reverse, though less pronounced.
- Engine and transmission temperatures are normal.
- Hydraulic oil levels and filters are verified and clean.
Root Causes and Diagnostic Strategy
- Steering valve imbalance: Internal wear or contamination can cause one side to receive more flow even when centered.
- Drive motor wear: A worn motor may produce less torque, requiring compensation from the opposite side.
- Track tension mismatch: Uneven track tension can cause drag, though this typically affects turning, not straight travel.
- Pilot control drift: A misadjusted or leaking pilot valve may send unintended signals to one side.
- Hydraulic pump output variance: If one pump section is weaker, it may affect drive balance.
- Check pilot pressure and valve centering. Use gauges to verify that both sides receive equal signal pressure at neutral.
- Inspect drive motors for internal leakage. This may require flow testing or removal for bench inspection.
- Verify track tension and adjust to spec. Uneven tension can amplify minor hydraulic imbalances.
- Flush and replace hydraulic fluid if contamination is suspected. Debris can cause valve sticking.
- Consult service manual for steering valve calibration. Some models allow centering adjustments via set screws or shims.
In the Netherlands, an operator reported persistent left pull on a 973 with clean filters and normal fluid levels. After consulting a CAT technician, the issue was traced to a worn spool in the steering valve. Replacing the valve restored straight travel without pedal compensation.
In Texas, a contractor noted similar behavior on a 973C. The right drive motor had excessive case drain flow, indicating internal leakage. After rebuilding the motor, the machine tracked straight under all conditions.
Conclusion
A CAT 973 that pulls to one side during straight travel is not operating as intended. The cause is typically hydraulic imbalance due to valve wear, motor degradation, or pilot signal drift. With proper diagnostics and component inspection, the issue can be resolved, restoring full control and reducing operator fatigue. Even older machines like the 66G series can maintain precise tracking with regular maintenance and calibration.
