PC35MR-2 Sudden Travel Change

[MikePhua]

Overview of the Komatsu PC35MR-2
The Komatsu PC35MR-2 is a compact excavator in the 3.5-ton class, widely used for utility work, landscaping, small construction projects, and farm jobs. As a “zero tail swing” machine, its upper structure stays within the width of the tracks when rotated, which makes it ideal for tight spaces such as residential backyards and narrow streets. Since its introduction in the early 2000s, the PC-MR series has sold tens of thousands of units globally, particularly in North America, Europe, and Japan, thanks to its combination of reliability, fuel efficiency, and good hydraulic performance for its size.
The PC35MR-2 uses a hydrostatic travel system: hydraulic motors drive the tracks, and the operator controls direction and speed through hand levers or pedals that send pilot-pressure signals to a main control valve bank. While these systems are generally robust, small problems in the pilot controls, valves, or linkages can cause dramatic changes in how the machine travels—such as one track refusing to move forward while still moving strongly in reverse.
This article examines a real-world scenario of a PC35MR-2 that suddenly developed abnormal travel behavior after a repair, and uses it as a case study for systematic diagnosis and prevention.
Symptom Description Sudden Change in Travel Behavior
An operator bought a used PC35MR-2 of unknown year and hours and immediately put it to work. On the first job, the machine seemed to perform normally in digging and travel functions, except for one issue: one of the travel levers tended to stick in the operating position instead of springing back to neutral. That kind of sticking is often caused by dirt, rust, or dried grease around the control linkage or spool.
Back at the workshop, the operator decided to fix the sticky lever properly. He removed the floor mat, the control levers, the rocker assembly, and the valve body directly underneath, following the procedures from a shop manual. After carefully cleaning and re-assembling the components, the expectation was a smoother travel control. Instead, a completely new and much worse problem appeared:

• The right track would run at full speed in reverse (100%),
  
• The right track would not move at all in forward (0%),
  
• The left track would only move at about 10% of normal speed in either direction when operated alone,
  
• When both travel controls were operated together, the left track suddenly functioned normally, while the right track still would not move forward,
  
• All other machine functions—boom, arm, bucket, swing—worked as expected.
  

There was one more clue: with the right travel lever held in the forward position, briefly bumping the auxiliary hydraulic control would nudge the right track forward by roughly half an inch. That tiny forward jerk suggested that forward oil flow toward the right travel motor still existed but was being heavily restricted or starved on the pilot side.
Understanding the Travel Control System
To understand why these symptoms matter, it helps to outline the key components involved in travel on a compact excavator:

1. Travel Levers and Rocker Assembly
   These mechanical parts transmit the operator’s input to small hydraulic pilot valves. When the lever is moved, it pushes a spool that shifts a low-pressure pilot oil flow.
   
2. Pilot Valves and Pilot Pressure
   Most modern excavators use a pilot system that runs at approximately 400–500 psi (28–35 bar). This relatively low pressure is enough to control the main valve spools, which in turn route high-pressure oil (often 3000–5000 psi, or 200–350 bar) to the travel motors and other functions. A typical technician might use 6000-psi gauges to check the main system but lower-range gauges to read pilot pressure accurately.
   
3. Main Control Valve Bank
   This is the “hydraulic brain” of the machine. Separate sections control boom, arm, bucket, swing, and each travel circuit. Pilot pressure from the control levers shifts spools inside this bank to send high-pressure flow to the selected function.
   
4. Travel Motors and Final Drives
   These convert hydraulic energy into track motion and torque. Internal issues here usually show up as loss of power in both directions or noise/heat, not as a one-direction-only issue immediately after a control repair.
   

Given that only specific directions and combinations were affected, and that the problem appeared right after working on the floor controls and pilot valves, the most likely area of concern was the pilot control circuit and valve bank—not the travel motors themselves.
Common Causes of Sudden Travel Changes After Repairs
When travel behavior changes drastically after a control repair, three broad categories should be considered:

1. Reassembly Errors or Mis-orientation of Components
   • Wrong orientation of spools or springs inside the pilot valve block
     
   • Incorrect connection of pilot lines (forward and reverse swapped, left and right crossed, etc.)
     
   • Missing or incorrectly installed O-rings leading to internal leaks or blocked ports
     
   • Mechanical interference in the rocker assembly stopping full spool stroke
     
   Even when photos are taken and parts are reinstalled “exactly as before,” small internal components like check valves, shims, pins, or spacer washers are easy to reverse. A single mis-located check ball can block pilot flow in one direction while leaving the other direction functional.
   
2. Damage or Contamination in the Valve Bank
   • Dirt or metal chips introduced during disassembly can lodge in small passages
     
   • A spool in the main control valve bank can stick partially open or closed
     
   • A pilot passage might be plugged, causing weak or delayed response
     
   These problems often show up as one direction being strong while the other is weak or dead, or as functions that work only when another function is activated (as in the case where bumping the AUX control helps the track move).
   
3. Cab Position and Hose Routing
   On some compact excavators, the cab or canopy must be tilted to access the pilot valves and main valve bank. When the cab is lowered again, it can pinch or kink a pilot hose if it is not routed correctly or if the hose is too short. This can choke pilot oil flow to a particular function or direction.
   In this particular case, the cab was left tilted up while testing, so pinched hoses from cab weight were less likely—but mis-routing or partially kinked hoses were still a possibility.
   

Pilot Pressure Testing Strategy
The operator did the right thing by ordering a set of hydraulic test gauges. To diagnose pilot issues on a machine like the PC35MR-2, an experienced technician would typically:

• Plumb a pressure gauge into each pilot line going to the right travel circuit—one for forward, one for reverse.
  
• Observe pilot pressure when the right travel lever is moved forward and backward.
  
• Compare the readings with manufacturer specifications (often around 400–500 psi for pilot, varying slightly by model).
  

If reverse shows full pilot pressure but forward shows little or none, the problem is clearly upstream, likely in:

• The travel control lever mechanism and spool;
  
• The pilot valve block;
  
• The pilot line itself (pinched, restricted, or blocked).
  

If both directions show correct pilot pressure but the track still behaves abnormally, the issue moves downstream toward:

• The main control valve section for right travel;
  
• The main hydraulic supply to that circuit;
  
• The travel motor and its internal components (less likely in this immediate post-repair scenario).
  

Interpreting the Symptom of “Bumping” the Auxiliary Function
The fact that bumping the auxiliary control with the right travel lever held forward would nudge the right track ahead about half an inch is an important clue. It suggests:

• The main hydraulic pump can deliver oil to the travel motor in the forward direction.
  
• The travel circuit forward path may be starved for pilot pressure or partially restricted, so the spool is not fully shifting. When the AUX circuit is activated momentarily, overall system pressure and flow change, allowing the spool to move slightly or oil to sneak past some restriction, just enough to move the track a small amount.
  

This behavior often indicates a marginal pilot signal or a sticky main spool rather than a severe mechanical failure of the travel motor.
Recommended Diagnostic Steps
When faced with similar travel problems after a control repair on a PC35MR-2 or another compact excavator, a practical step-by-step approach would be:

1. Verify Mechanical Linkage
   • Confirm both travel levers are actually moving their pilot spools through full stroke.
     
   • Check for bent linkages, missing pins, or misaligned rocker arms.
     
   • Make sure return springs are correctly installed and not binding.
     
2. Check Pilot Line Routing
   • Trace each pilot hose from the control valve block to the main control valve.
     
   • Look for sharp kinks, tight bends, or obvious pinches.
     
   • Confirm that forward and reverse pilot lines are connected to the correct ports for both left and right travel.
     
3. Measure Pilot Pressure
   • Install gauges on the pilot ports for right travel forward and reverse.
     
   • Record readings when the levers are moved, with the machine supported safely off the ground.
     
   • Compare forward vs reverse and left vs right travel pilot pressures.
     
   • If one direction is much lower, focus on the pilot control for that direction.
     
4. Inspect and Clean the Pilot Control Block
   • Remove the pilot valve block again in a clean environment.
     
   • Disassemble each section carefully, lay out parts in order, and compare with the exploded view in the shop manual.
     
   • Look for debris, damaged O-rings, mis-oriented check valves, or spools that stick when moved by hand.
     
   • Use lint-free cloth and appropriate cleaning solvent; blow passages with clean, dry air.
     
5. Evaluate the Main Control Valve Section
   • If pilot pressures are correct yet the symptom persists, inspect the travel section of the main control valve.
     
   • Check spool movement, wear surfaces, and any integrated relief or check valves.
     
   • Look for scoring, galling, or contamination on the spool and in its bore.
     
6. Only Then Suspect the Travel Motor
   • If both pilot and main valve sections check out, test actual flow and pressure at the travel motor ports.
     
   • Listen for unusual noise, check for overheating, and inspect case drain flow.
     
   • Internal leakage or mechanical failure in the motor will show as low speed and torque in both directions, not just one.
     

Preventing Control-Related Travel Problems
To reduce the risk of similar issues when working on compact excavator controls:

• Document Everything Before Disassembly
  Photos are good, but also note the exact depth and orientation of spools, caps, and plugs. Use paint markers on hose ends and ports.
  
• Keep the Work Area Clean
  Work over a clean bench, plug hoses and ports as soon as they are disconnected, and avoid reusing damaged O-rings.
  
• Use the Correct Gauges
  A 6000-psi gauge is common for main hydraulics, but pilot systems need lower-range gauges for accurate readings. Misreading pilot pressure can easily lead to wrong conclusions.
  
• Test With the Machine Safely Supported
  Block the machine off the ground when testing travel. Even a small compact excavator can cause serious injury if a track suddenly grabs and the machine lunges.
  
• Check Cab and Panel Re-installation
  When tilting cabs or removing side panels, verify all hoses and harnesses are clear before closing. A pinched pilot hose can create maddening, intermittent problems.
  

A Short Story From the Field
A small contractor once purchased a used 3-ton excavator to dig water lines on a rural property. On day one, the machine travelled fine but had a sticky right pedal. After an evening cleaning and reassembling the pedal and pilot valve, the contractor discovered the right track would move only backward. In frustration, he assumed the travel motor had failed and priced a replacement at nearly one-third of what he paid for the machine.
A local technician offered to take a look first. Within an hour, using simple pilot gauges, he showed that there was no pilot pressure when the right pedal was pushed forward. A deeper inspection revealed that a small check plug had been swapped with a similar-looking plug during the cleaning job, blocking one of the forward pilot passages. After correcting the plug positions and bleeding the pilot system, both tracks worked normally—and the machine went on to dig hundreds of hours of trench without further incident.
The lesson matches the PC35MR-2 case: when a machine changes behavior right after you touch the controls, the root cause is often in what you just touched—especially in the delicate pilot circuit—rather than deep inside the final drive or travel motor.
Key Takeaways for PC35MR-2 Owners

• Sudden, directional travel problems after a control repair are usually linked to pilot controls, valve blocks, or hose routing—not typically the travel motor itself.
  
• Pilot systems on compact excavators commonly operate around 400–500 psi, and verifying those pressures with gauges is one of the fastest ways to narrow down the fault.
  
• Symptoms such as “track moves when AUX is bumped” point toward marginal pilot signal or sticky spools rather than full mechanical failure.
  
• Careful reassembly, clean work habits, and systematic pressure testing can save thousands in unnecessary parts and a lot of downtime.
  

For owners of older or unknown-hour machines like the PC35MR-2, taking the time to learn the basics of pilot hydraulic systems and investing in a small set of gauges is often more valuable than any single replacement part—because it lets you see what the machine is trying to tell you before you start throwing expensive components at the problem.