John Deere 750 Charge Pressure Loss on One Track

[MikePhua]

The Evolution of the John Deere 750 Dozer
The John Deere 750 crawler dozer was introduced in the late 1970s as part of Deere’s push into the mid-size dozer market. Designed for versatility in grading, land clearing, and construction, the original straight 750 model featured a naturally aspirated diesel engine, mechanical transmission, and a closed-center hydraulic system. It was built for durability, with a focus on mechanical simplicity and field serviceability. By the early 1980s, Deere had sold thousands of units across North America, particularly in agricultural and municipal fleets.
John Deere, founded in 1837, had already become a household name in agricultural machinery. Its expansion into construction equipment began in earnest in the 1950s, and by the time the 750 was released, the company had established a strong dealer network and parts support system. However, as newer models like the 750B and 750C emerged with hydrostatic drives and electronic diagnostics, support for the original straight 750 began to wane, leaving many owners reliant on independent expertise.
Understanding Charge Pressure in Track Drives
Charge pressure in a dozer’s transmission system refers to the hydraulic pressure that maintains lubrication and assists in clutch pack engagement. In the John Deere 750, each track is powered by a separate clutch and brake assembly, fed by a common hydraulic system. When charge pressure drops on one side, the affected track may lose torque, slip under load, or fail to respond to directional input.
Key components involved include:

• Charge pressure relief valve: Regulates system pressure to prevent overloading.
  
• Transmission relief valve: Protects the transmission from excessive pressure spikes.
  
• Neutral charge valve: Maintains baseline pressure when the machine is in neutral.
  

A loss of charge pressure on one track, especially after prolonged operation, suggests a localized issue—either hydraulic leakage, valve malfunction, or internal clutch wear.
Symptom Progression and Field Behavior
In one documented case, a 1979 straight 750 operated normally for about an hour before the left track began losing power under load. When the operator shifted to neutral, charge pressure returned, and the track regained function temporarily. This cyclical behavior points to heat-induced hydraulic degradation or component fatigue.
Such symptoms are often misattributed to pump failure or electrical issues, but in older mechanical systems, the root cause is typically mechanical or hydraulic. The fact that pressure returns in neutral suggests that the system can build pressure when unloaded but fails to maintain it under torque demand.
Diagnostic Pathway and Technical Strategy
To isolate the issue, a structured diagnostic approach is essential:

1. Verify system pressure: Use a calibrated gauge to measure charge pressure at both track ports during operation and under load.
   
2. Inspect relief valves: Remove and bench-test the charge, transmission, and neutral relief valves. Look for spring fatigue, debris obstruction, or seat wear.
   
3. Check clutch pack integrity: Disassemble the affected clutch housing and inspect for worn friction discs, warped plates, or oil contamination.
   
4. Examine hydraulic lines: Look for pinched hoses, internal delamination, or loose fittings that may restrict flow.
   

In one case from rural Oklahoma, a contractor discovered that a deteriorated internal hose lining had collapsed under heat, intermittently blocking fluid flow to the left clutch. Replacing the hose restored full pressure and eliminated the issue.
Preventative Measures and Long-Term Solutions
To avoid similar failures, consider the following recommendations:

• Upgrade to synthetic hydraulic fluid: These fluids resist thermal breakdown and maintain viscosity under load.
  
• Install temperature sensors: Monitoring fluid temperature can help detect overheating before pressure loss occurs.
  
• Replace relief valves every 5,000 hours: Even if functioning, aged valves may have reduced responsiveness.
  
• Flush hydraulic system annually: Contaminants like metal shavings or degraded seals can accumulate and impair valve function.
  

Historical Anecdote and Industry Insight
During a 1984 flood response in Missouri, a fleet of John Deere 750s was deployed to build emergency levees. One unit began losing drive on the right track after extended operation in muddy terrain. Field mechanics traced the issue to a clogged suction screen in the hydraulic reservoir, which had been packed with silt. After cleaning and fluid replacement, the machine returned to service within hours.
This incident underscores the importance of environmental awareness in hydraulic maintenance. Machines operating in wet or dusty conditions require more frequent inspection and fluid changes.
Conclusion
Charge pressure loss in a John Deere 750 dozer is a nuanced issue that demands a methodical approach. While the machine’s age and mechanical simplicity offer advantages in field repairability, they also require deeper understanding of hydraulic dynamics and wear patterns. By combining structured diagnostics with preventative maintenance, operators can extend the life of these legacy machines and keep them performing reliably in demanding conditions. The 750 may be decades old, but with care and expertise, it remains a workhorse in the field.