John Deere 850 Dozer Troubleshooting and Transmission Recovery

[MikePhua]

The JD 850 and John Deere’s Mid-Size Dozer Heritage
The John Deere 850 dozer was introduced in the late 1970s as part of Deere’s push into the mid-size crawler market. With an operating weight around 40,000 pounds and powered by a naturally aspirated or turbocharged six-cylinder diesel engine, the 850 was designed for road building, site prep, and forestry work. It featured a torque converter drive, power shift transmission, and hydraulic blade control—making it a versatile machine for contractors and municipalities alike.
John Deere, founded in 1837, has sold millions of machines globally. The 850 series evolved through multiple generations, with the original 850 eventually giving way to the 850B, C, and later J and K models. The early 850 remains a favorite among operators for its mechanical simplicity and rugged build, though its age brings unique maintenance challenges.
Terminology Notes

• Power Shift Transmission: A hydraulic transmission that allows gear changes without clutching, using pressurized fluid and planetary gear sets.
  
• Torque Converter: A fluid coupling between the engine and transmission that multiplies torque and allows smooth acceleration.
  
• Clutch Pack: A set of friction discs and steel plates used to engage gears in a power shift transmission.
  

Common Symptoms and Field Observations
Operators may encounter:

• Dozer moves forward but not in reverse
  
• Transmission engages intermittently or slips under load
  
• Engine revs but machine fails to respond
  
• Hydraulic pressure drops during gear changes
  
• Audible grinding or hesitation when shifting
  

A grading crew in Tennessee reported that their JD 850 would move forward reliably but refused to engage reverse. After checking fluid levels and linkages, they discovered a failed clutch pack in the reverse gear set, which had worn down due to contaminated fluid and excessive heat.
Root Causes and Diagnostic Strategy
Transmission issues on the JD 850 often stem from:

• Worn Clutch Packs
  • Friction material degrades over time, especially under heavy load
    
  • Diagnosed via pressure testing and inspection of transmission sump
    
• Low or Contaminated Hydraulic Fluid
  • Causes poor clutch engagement and overheating
    
  • Confirmed by fluid sampling and filter inspection
    
• Faulty Control Valve or Solenoid
  • Prevents proper fluid routing to gear sets
    
  • Detected by manual valve testing and electrical continuity checks
    
• Linkage Misadjustment
  • Gear selector may not fully engage detents
    
  • Diagnosed by inspecting cable tension and lever alignment
    
• Torque Converter Failure
  

• Internal vanes or stator may be damaged, reducing torque multiplication
  
• Confirmed by stall testing and converter pressure readings
  

Inspection steps:

• Check transmission fluid level, color, and smell
  
• Inspect filters for metal particles or clutch debris
  
• Test hydraulic pressure at clutch ports during gear engagement
  
• Verify gear selector movement and detent engagement
  
• Remove transmission cover and inspect clutch packs if needed
  

Repair Procedure and Component Recommendations
To restore transmission function:

• Drain and flush transmission fluid and replace filters
  
• Remove transmission cover and inspect clutch packs for wear
  
• Replace worn friction discs and steel plates with OEM-rated components
  
• Inspect control valve for sticking or wear and clean or replace as needed
  
• Test solenoids and replace faulty units
  
• Refill with manufacturer-recommended hydraulic fluid and test under load
  

Recommended components:

• JD 850 transmission clutch pack kit
  
• Hydraulic filter set with bypass valve
  
• Control valve rebuild kit with seals and springs
  
• Torque converter pressure test kit
  
• SAE 10W hydraulic transmission fluid with anti-wear additives
  

A technician in Alberta rebuilt a JD 850 transmission after discovering that the reverse clutch pack had delaminated. After replacing the pack and flushing the system, the machine returned to full operation and completed a season of road building without further issues.
Preventive Maintenance and Long-Term Solutions
To prevent future transmission failures:

• Change transmission fluid every 500 hours or annually
  
• Replace filters at each fluid change and inspect for debris
  
• Adjust gear selector linkage quarterly
  
• Monitor hydraulic pressure during operation and log anomalies
  
• Avoid prolonged idling in gear or excessive gear changes under load
  

Some fleet managers retrofit older JD 850s with temperature sensors and pressure gauges to monitor transmission health in real time. Others install upgraded cooling systems to reduce fluid breakdown in high-duty cycles.
Recommendations for Operators and Technicians

• Maintain a transmission service log with fluid changes, pressure readings, and clutch inspections
  
• Train operators to shift smoothly and avoid gear changes under full throttle
  
• Stock spare clutch packs, filters, and control valve components for field repairs
  
• Partner with John Deere dealers or transmission rebuild shops for overhaul support
  
• Consider full transmission teardown during engine rebuild or undercarriage service
  

For machines operating in grading, forestry, or landfill applications, transmission integrity is essential for productivity and safety.
Conclusion
Transmission problems in the John Deere 850 dozer can bring work to a halt, but with careful diagnostics, quality repairs, and preventive care, the machine can return to full strength. In the rhythm of earthmoving, gear engagement is more than mechanics—it’s momentum, control, and confidence. When the clutch packs grip and the fluid flows clean, the 850 pushes forward with purpose.