Diagnosing Hydraulic Drive Issues in the Drott 35D

[MikePhua]

The Drott 35 Series and Its Industrial Legacy
The Drott 35 series, particularly the 35D model, represents a class of mid-sized crawler excavators that gained popularity in the early 1980s. Manufactured by Drott Manufacturing Company, a Wisconsin-based firm known for its multipurpose earthmoving equipment, the 35D was designed for rugged applications in construction, forestry, and mining. Drott’s reputation was built on versatility—its machines often featured interchangeable attachments and robust hydraulic systems.
By 1983, the Drott 35D had evolved into a high-serial-number model with improved hydraulic architecture and a one-piece control valve block, distinguishing it from earlier A-series units that used stacked valve sections. Though Drott was eventually absorbed into Case Corporation, its machines remain in service across North America, especially in legacy fleets and rural operations where mechanical reliability is prized over digital sophistication.
Hydraulic Drive Symptoms and Initial Observations
A common issue reported in aging Drott 35D units involves the right track drive motor failing to engage in forward motion, while still functioning—albeit with a delay—in reverse. Operators often note the absence of engine load when attempting forward drive, suggesting a lack of hydraulic pressure or flow to the motor.
This symptom can be misleading. While the foot pedal may appear to operate normally, the underlying issue often lies within the control valve assembly or associated relief valves. The “thunk” sound during reverse engagement may indicate mechanical backlash or delayed hydraulic actuation.
Terminology Annotation

• Control Valve Block: A centralized hydraulic component that directs fluid to various actuators based on operator input.
  
• Relief Valve: A safety device that limits hydraulic pressure to prevent system damage.
  
• Swivel Joint: A rotating hydraulic coupling that allows fluid transfer between stationary and moving parts.
  
• Case Drain: A low-pressure return line that carries leakage oil from hydraulic motors back to the reservoir.
  
• Final Drive: The gear assembly that transmits torque from the hydraulic motor to the track sprockets.
  

Relief Valve Testing and Swap Procedure
In the 35D’s one-piece valve block, each track’s hydraulic circuit includes two relief valves—one above the spool and one at the end cap. These valves regulate pressure during forward and reverse travel. A practical diagnostic method involves swapping the relief valves to see if the problem shifts direction.
Steps include:

• Identify and fully remove both relief valves for the affected track
  
• Swap their positions without mixing internal components
  
• Measure the threaded adjustment screw to maintain approximate pressure settings
  
• Reinstall and test forward and reverse travel
  

If the issue reverses—i.e., forward now works and reverse fails—the faulty relief valve must be disassembled and inspected. Common faults include:

• Debris obstructing the valve seat
  
• Damaged springs
  
• Worn seals
  

The factory-set pressure for travel relief valves in the 35D is approximately 2500 psi. Deviations from this setting can cause sluggish or failed actuation.
Water Contamination and Varnish Build-Up
Another critical factor in hydraulic performance is fluid quality. Water ingress and varnish deposits are frequent problems in older machines. Water can enter through worn seals, condensation, or improper storage. Varnish forms from oxidized oil and can clog filters and valve seats.
Symptoms include:

• Delayed or failed actuation
  
• Sticky spools
  
• Reduced flow rates
  

Solutions:

• Drain and flush the hydraulic system thoroughly
  
• Replace filters and inspect for varnish residue
  
• Use water-detection test strips to confirm contamination
  
• Switch to high-quality anti-oxidation hydraulic fluid
  

In one case, a Drott 35D purchased secondhand showed persistent water discharge during fluid changes and heavy varnish in the filters. After multiple flushes and filter replacements, performance improved significantly, though some valve components required manual cleaning.
Advanced Diagnostics and Component Isolation
If relief valve swaps and fluid maintenance fail to resolve the issue, deeper diagnostics are needed. Technicians can isolate hydraulic flow using caps and plugs:

• Disconnect the tube-to-hose connections at the valve block
  
• Plug the lines and activate the travel lever
  
• Listen for engine load changes to confirm pressure generation
  

If pressure is present, reconnect and test the drive motor:

• Remove the motor cover and plug its inlet and outlet lines
  
• Activate the travel lever again
  
• If pressure is confirmed, the motor may be leaking internally into the case drain
  

Further steps include:

• Swapping drive motors between tracks to see if the issue follows
  
• Inspecting the short coupler between motor shaft and final drive input
  
• Checking for stripped splines or worn coupler teeth
  

Operators should exercise caution when handling couplers, as internal splines can become razor-sharp over time.
Mechanical Wear and Sprocket Inspection
Mechanical failures can also mimic hydraulic issues. A stripped spline between the axle and sprocket may cause the sprocket to remain stationary even when the axle rotates. To test:

• Start the engine and engage the travel spool
  
• Observe the large nut securing the sprocket shaft
  
• If the nut turns but the sprocket does not, the splines are likely stripped
  

This type of failure requires disassembly of the final drive and replacement of worn components. It’s a known issue in high-hour Drott machines, especially those used in abrasive environments like quarries or demolition sites.
Conclusion
The Drott 35D remains a testament to mechanical engineering from an era before digital diagnostics. Troubleshooting its hydraulic drive issues demands a blend of methodical testing, component knowledge, and field experience. Whether the fault lies in a relief valve, contaminated fluid, or worn splines, solutions are achievable with patience and precision. For operators and technicians maintaining legacy equipment, the Drott 35D offers both challenge and reward—proof that old iron still has lessons to teach.