Hydraulic Slowdown on the John Deere 35D When Operating a Mower

[MikePhua]

The John Deere 35D and Its Auxiliary Hydraulic System
The John Deere 35D compact excavator was introduced in the mid-2000s as part of Deere’s expansion into the mini-excavator market. With an operating weight of approximately 7,800 lbs and a dig depth of over 10 feet, the 35D was designed for utility contractors, landscapers, and municipal crews. It features a Yanmar diesel engine and a load-sensing hydraulic system capable of powering both digging functions and auxiliary attachments.
The auxiliary hydraulic circuit is routed through a proportional control valve and can deliver up to 15.9 gallons per minute at pressures exceeding 2,600 psi. This makes it suitable for tools like augers, thumbs, and rotary mowers. However, when high-demand attachments are used, such as flail or brush mowers, the system can become overwhelmed, leading to sluggish performance in other hydraulic functions.
Symptoms of Hydraulic Saturation
Operators may notice:

• Boom, arm, or swing functions slow down or hesitate when the mower is engaged
  
• Travel motors lose torque or stall during simultaneous mowing
  
• Engine RPM drops under load, even at full throttle
  
• Hydraulic fluid temperature rises rapidly during extended mowing
  
• Attachment speed fluctuates or stalls intermittently
  

In one case from a contractor in Georgia, a 35D equipped with a flail mower began losing swing speed after 15 minutes of operation. The issue was traced to hydraulic saturation and insufficient flow prioritization.
Understanding Hydraulic Load Distribution
The 35D uses a variable displacement pump that adjusts output based on demand. When the mower is activated, it draws a large portion of available flow, leaving less for other functions. The system lacks a dedicated priority valve for auxiliary flow, meaning all circuits compete for pressure and volume.
Key components involved include:

• Main hydraulic pump
  
• Proportional control valve for AUX flow
  
• Return line routing and backpressure
  
• Pilot pressure circuit for valve actuation
  
• Thermostatic bypass valve in the cooling loop
  

If the mower’s return flow is restricted or routed through a high-pressure path, it can cause backpressure that slows down the entire system.
Solutions and Optimization Strategies
To improve performance:

• Ensure the mower’s return line is routed directly to the tank or through a low-pressure return port
  
• Install a case drain line if the mower motor requires it to relieve internal pressure
  
• Use quick-connect couplers rated for high flow and low restriction
  
• Adjust the auxiliary flow rate via the control panel or manual valve (if available)
  
• Clean or replace hydraulic filters to reduce resistance
  
• Monitor fluid temperature and consider upgrading to synthetic hydraulic oil for better thermal stability
  

Some operators retrofit flow dividers or priority valves to ensure digging functions retain minimum flow even when the mower is active. Others install inline pressure gauges to monitor system behavior in real time.
Field Anecdote and Practical Insight
In Shanghai, a landscaping crew used a 35D with a rotary mower to clear roadside vegetation. Initially, the machine ran well, but after 20 minutes, the boom slowed and the mower began to pulse. After inspection, they discovered the return line was routed through a spool valve designed for bidirectional flow, causing backpressure. Rerouting the return directly to the tank and adding a case drain resolved the issue. The crew later added a temperature sensor to the hydraulic reservoir to track heat buildup.
Preventive Measures and Long-Term Reliability
To maintain optimal performance:

• Change hydraulic fluid every 1,000 hours or annually
  
• Replace filters every 500 hours or sooner in dusty environments
  
• Inspect auxiliary hoses and couplers monthly
  
• Avoid prolonged mowing at full throttle without breaks
  
• Monitor engine load and hydraulic temperature during operation
  

Some fleets install auxiliary coolers or fan-assisted radiators to reduce fluid temperature during high-demand tasks.
Conclusion
Hydraulic slowdown on the John Deere 35D when operating a mower is typically caused by flow saturation, backpressure, and thermal stress. With proper routing, flow adjustment, and preventive maintenance, the machine can handle demanding attachments without compromising core functions. Whether clearing brush or maintaining roadside vegetation, understanding the hydraulic system’s limits is key to maximizing efficiency and uptime.