Diagnosing Track Drive Failure on the Case 850K Series 3 Dozer

[MikePhua]

The Case 850K Series 3 and Its Hydraulic Drive System
The Case 850K Series 3 is a mid-size crawler dozer designed for grading, site prep, and light earthmoving. With a Tier 3 diesel engine and hydrostatic transmission, it offers precise control and smooth power delivery. Each track is independently driven by a hydraulic motor fed by a variable-displacement pump, allowing for zero-radius turns and fine maneuvering.
The hydrostatic system relies on electronic control modules, solenoids, and CAN bus communication to coordinate movement. When one track fails to engage, the issue can stem from electrical faults, hydraulic pressure loss, or mechanical failure in the pump or motor.
Initial Symptoms and Fault Code Analysis
In one diagnostic case, the right track of an 850K Series 3 failed to respond, while the left track operated normally. The only fault code displayed was 4533F on the instrument cluster. This code typically points to a CAN communication error or voltage irregularity in the control system.
The operator noted that striking the dashboard temporarily restored function, suggesting a loose connection or intermittent short. Replacing the instrument cluster did not resolve the issue, prompting deeper investigation into the wiring harness and connectors.
Electrical Troubleshooting and Harness Inspection
Electrical faults in hydrostatic systems often originate from:

• Loose pins or sockets in connectors
  
• Chafed wires causing intermittent shorts
  
• Corrosion at terminals or ground points
  
• Voltage drop across solenoid connectors
  

Technicians performed continuity tests across the dash harness, flexed wires to simulate vibration, and checked for unexpected resistance between pins. Although readings appeared normal, the behavior suggested a marginal connection—possibly a loose socket or fatigued wire strand.
Replacing the dash harness eliminated the fault code and restored CAN communication, but the right track remained inactive, indicating a deeper hydraulic issue.
Hydraulic Pressure Testing and Component Isolation
To isolate the problem, the solenoid was removed from the park brake valve and pressure tests were conducted on both tracks. The left track showed 7,000 psi, while the right track registered zero. Swapping electrical connectors between the pumps and motors did not change the pressure readings, ruling out control-side faults.
This pointed to a mechanical failure in the right-side pump or motor. Technicians checked:

• Charge pressure on both pumps (confirmed equal)
  
• Pilot pressure at diagnostic ports
  
• Amperage draw on proportional coils during forward command
  
• Manual override lever on the pump’s EDC (electronic displacement control)
  

The EDC’s manual lever allows technicians to stroke the pump mechanically, bypassing electronic control. If movement occurs during manual override, the issue is electrical. If not, the pump or motor is likely damaged internally.
Pump and Motor Behavior in Hydrostatic Systems
In hydrostatic drives, the pump adjusts displacement based on electronic signals. If the pump fails to stroke, no pressure is generated, and the motor remains idle. Common failure points include:

• Stuck swash plate
  
• Damaged servo piston
  
• Internal leakage
  
• Faulty EDC solenoid
  

The technician confirmed that even with manual override, the right track remained dead. This strongly suggested a mechanical failure in the pump, possibly a seized swash plate or blocked internal passage.
Recommendations for Repair and Verification
To confirm pump failure:

• Remove and inspect the pump for physical damage
  
• Check swash plate movement and servo piston response
  
• Test solenoid resistance and voltage under load
  
• Flush hydraulic lines to remove debris or contamination
  

If the pump is replaced, technicians should:

• Calibrate the new unit using diagnostic software
  
• Verify CAN communication and voltage signals
  
• Recheck pressure and flow on both tracks
  
• Test under load to confirm full functionality
  

For fleet managers, maintaining a log of fault codes, pressure readings, and component replacements can help track recurring issues and improve future diagnostics.
Conclusion
Track drive failure on the Case 850K Series 3 dozer can stem from a complex interplay of electrical and hydraulic systems. In this case, the resolution required replacing the dash harness to restore communication, followed by pressure testing and mechanical inspection to isolate a failed pump. With methodical troubleshooting and component-level analysis, even elusive faults can be resolved—bringing the machine back to full productivity.