Hydraulic Troubleshooting and System Behavior in the John Deere 850J WLT Bulldozer

[MikePhua]

Introduction to the John Deere 850J WLT
The John Deere 850J WLT (Wide Long Track) is a mid-to-large-class crawler dozer designed for pushing, grading, and land-clearing operations. Renowned for its electronic control systems and hydrostatic drive, it offers precise maneuverability and responsive hydraulics. However, like all modern heavy equipment, it is not immune to electronic and hydraulic system glitches, particularly those involving the pilot control circuits, main pumps, or electrohydraulic controllers.
Terminology Explained

• Hydrostatic Drive: A system using hydraulic pumps and motors to transmit power to the tracks without gears or a traditional transmission.
  
• WLT (Wide Long Track): A track configuration that offers increased stability and lower ground pressure, ideal for soft or uneven terrain.
  
• Pilot Control System: A low-pressure hydraulic circuit that operates the main control valves via joysticks or levers.
  
• SCV (Selective Control Valve): A valve that directs hydraulic fluid to specific functions like blade tilt, lift, or angle.
  

Symptoms of Hydraulic Malfunction
In the case of the 850J WLT, the operator experienced a complete loss of blade function after start-up. The following conditions were observed:

• Blade would not lift, tilt, or angle
  
• Transmission and steering were functional
  
• No warning codes were present on the monitor
  
• The machine had previously operated normally
  

Such symptoms suggest either an electrical or hydraulic control failure specific to the blade’s control system, rather than a total hydraulic failure.
Diagnostic Path and Insights
To resolve this issue, the following steps were taken:

• Check Fuses and Relays: The first step is to inspect electrical components related to hydraulic control, including the fuse for the SCV circuit and control relays. Corrosion or poor contact can cause intermittent or complete loss of signal to solenoids.
  
• Monitor SCV Control Input: Operators verified that the joystick or switch input was being received by the controller. If inputs are not detected, the issue could be in the wiring harness or controller.
  
• Hydraulic Pilot Pressure Test: Pilot pressure was measured at the control valve block. A lack of pilot pressure pointed to a malfunction in the pilot pump or a related solenoid failing to open.
  
• Hydraulic Oil Level and Quality: A low reservoir level or aerated fluid can starve the pilot circuit and cause spongy or delayed hydraulic response. However, in this case, oil level and quality were verified as acceptable.
  
• Solenoid Coil and Spool Function: It was determined that one or more of the control valve solenoids might be stuck or non-responsive. Removal and bench-testing confirmed one solenoid was not energizing despite input voltage.
  

Resolution and Restoration
Once the faulty solenoid was identified, it was replaced with an OEM component. Upon restart, the blade function returned immediately. Operators also cleaned out the connector pins and applied dielectric grease to prevent future moisture intrusion. The issue highlighted the vulnerability of exposed electrical components to corrosion and vibration over time.
Lessons from the Field
This situation reflects a common theme in modern electronically controlled hydraulic systems: when systems fail silently without error codes, the issue often lies in components outside the monitoring capability of the onboard diagnostics. For older machines, hydraulic faults were more mechanical. In newer machines like the 850J, electrical and hydraulic systems must be viewed together.
In another case on a forestry site, a similar 850J suffered intermittent blade drop due to a cracked pilot pressure hose. It only revealed itself under certain angles and temperatures. Field technicians used dye and UV light to detect the hairline leak, demonstrating the creative methods sometimes required to pinpoint elusive failures.
Preventive Tips for 850J Operators

• Regularly inspect and clean electrical connectors, especially those related to hydraulic valves
  
• Use dielectric grease on terminals to prevent corrosion
  
• Keep hydraulic oil clean and at proper levels, and change filters on schedule
  
• Listen for changes in sound when operating controls—delays or quiet whines can signal low pilot pressure
  
• Operate blade functions briefly at startup to confirm normal behavior before engaging in full work cycles
  

Conclusion
The John Deere 850J WLT is a powerful and intelligent dozer, but its complexity demands a systematic approach when diagnosing failures. Issues like complete blade function loss, even without fault codes, can often be traced to electrical control components such as solenoids and relays. A solid understanding of how pilot control systems interact with hydraulic logic valves is critical for operators and technicians alike. The ability to troubleshoot across systems—hydraulic, electrical, and mechanical—remains one of the most valuable skills on any job site.