John Deere 670B Transmission Failure and Internal Shaft Damage

[MikePhua]

The JD670B and Its Role in Road Grading
The John Deere 670B motor grader was introduced in the late 1980s as part of Deere’s push to modernize its road construction lineup. With an operating weight of approximately 30,000 lbs and a 140 hp diesel engine, the 670B was designed for precision grading, ditch shaping, and finish work. It featured a six-wheel drive option, hydraulic controls, and a modular transmission system that allowed for easier servicing compared to earlier models.
John Deere, founded in 1837, has long been a leader in agricultural and construction machinery. The 670B was part of a successful series that included the 670A and later the 670C, with thousands of units sold across North America and exported to developing infrastructure markets worldwide.
Terminology Note

• Transmission Input Shaft: The rotating shaft that delivers engine torque into the transmission assembly.
  
• Splines: Grooved ridges on a shaft that interlock with mating components to transfer rotational force.
  
• Planetary Gear Set: A gear system that allows multiple gear ratios in a compact space, common in heavy equipment transmissions.
  
• Torque Converter: A fluid coupling between the engine and transmission that multiplies torque and allows smooth acceleration.
  

Symptoms Leading to Failure Diagnosis
Operators of the JD670B reported a sudden loss of drive in all gears. The engine ran normally, hydraulic functions remained active, and the gear selector appeared to engage—but the machine would not move. Initial inspections ruled out external linkage issues and hydraulic pressure loss. The fault was traced to the transmission input shaft, which had sheared internally at the spline interface.
This type of failure is catastrophic but not uncommon in older machines subjected to heavy loads, abrupt directional changes, or improper maintenance intervals.
Root Cause and Mechanical Analysis
The input shaft failure was caused by spline wear and eventual fracture. Over time, the splines—designed to transfer torque between the engine and transmission—can wear down due to:

• Misalignment between engine and transmission housings
  
• Lack of lubrication at the spline interface
  
• Excessive shock loads from abrupt gear changes or downhill braking
  
• Metal fatigue from thousands of load cycles
  

Once the splines wear past a critical threshold, torque transfer becomes uneven, leading to vibration, heat buildup, and eventual breakage.
In one case from 2021, a county road crew in Montana experienced similar symptoms. Their 670B lost drive while grading a gravel shoulder. After towing the machine to a shop, technicians discovered the input shaft had fractured cleanly at the spline root. The shaft was replaced, and the mating coupler was upgraded to a hardened steel variant.
Inspection and Repair Strategy
To confirm input shaft failure:

• Remove the transmission access panel and inspect shaft rotation
  
• Check for metal shavings or spline debris in the housing
  
• Use a borescope to inspect internal coupler wear
  
• Verify engine-to-transmission alignment using dial indicators
  

Repair involves:

• Removing the transmission assembly
  
• Replacing the input shaft and coupler
  
• Inspecting planetary gear sets for collateral damage
  
• Reinstalling with proper torque and alignment specs
  
• Flushing transmission fluid and replacing filters
  

Technicians should use OEM parts or certified aftermarket replacements with hardened splines and improved metallurgy.
Preventive Measures and Operator Practices
To prevent future failures:

• Avoid abrupt gear changes under load
  
• Maintain proper fluid levels and change transmission oil every 500 hours
  
• Inspect spline interfaces during annual service
  
• Use torque converters correctly—avoid excessive stall time
  
• Train operators to recognize early signs of vibration or delayed engagement
  

Adding a magnetic drain plug can help detect early metal wear by capturing fine particles before they circulate.
Manufacturer Design Evolution
Later models like the JD670C and JD672G introduced improved transmission designs with better spline engagement, upgraded couplers, and electronic monitoring. Deere’s modular transmission architecture allows faster rebuilds and easier access to internal components.
The company also introduced telematics systems that monitor transmission temperature, gear engagement patterns, and fluid condition, helping fleet managers anticipate failures before they occur.
Recommendations for Fleet Managers

• Keep spare input shafts and couplers in inventory for legacy machines
  
• Document all transmission repairs and fluid changes
  
• Perform vibration analysis during scheduled maintenance
  
• Retrofit older machines with upgraded spline interfaces if available
  
• Consider transmission rebuilds after 8,000–10,000 hours of service
  

Conclusion
The JD670B remains a capable and respected motor grader, but like all aging machines, it requires vigilant maintenance and informed diagnostics. Transmission input shaft failure is a serious but solvable issue, especially when identified early and repaired with precision. With proper care, the 670B can continue shaping roads and ditches for years to come, proving that even legacy iron has a place in modern infrastructure work.