Troubleshooting Hydraulic Function Failure in the Komatsu D155AX-5 Dozer

[MikePhua]

Overview of the Komatsu D155AX-5
The Komatsu D155AX-5 is a heavy-duty crawler dozer built for large-scale earthmoving, mining, and land-clearing operations. With a powerful Komatsu SAA6D140E-5 engine delivering roughly 354 HP and a robust hydromechanical transmission, it's a workhorse designed for performance and reliability. However, like any complex hydraulic machine, it can encounter specific functional failures that disrupt productivity, especially in its blade and ripper systems.
One of the more vexing issues operators have reported is a loss of function in the blade or ripper controls—systems that rely on the machine’s electrohydraulic circuits. These failures can manifest intermittently or suddenly and often require a systematic diagnostic approach.
Symptoms and Initial Observations
A typical case begins with the operator noticing that one or more hydraulic functions—such as blade lift, tilt, or ripper raise—stop responding. In the Komatsu D155AX-5, these systems are electronically controlled via joysticks that send signals to solenoids, which in turn actuate hydraulic pilot valves. When a signal is lost or a valve sticks, the function can fail.
Common symptoms reported:

• Blade refuses to raise or lower
  
• Ripper becomes unresponsive to joystick input
  
• Blade tilt works, but lift does not
  
• Functions cut in and out intermittently
  

Often, the rest of the machine—including drive, steering, and engine—continues to operate normally, isolating the problem to a specific subsystem within the hydraulic or electrical network.
Understanding the Control System
The Komatsu D155AX-5 uses a pilot-controlled electrohydraulic system, where low-pressure pilot oil (typically around 400 psi) is routed by electrically actuated solenoid valves to control high-pressure main hydraulic functions (which operate at 3,000–4,000 psi).
Key components include:

• Pilot control valve group – Receives electric signals from the joystick
  
• Solenoid valves – Electromagnetically actuated to redirect pilot flow
  
• Hydraulic pilot pump – Supplies pilot pressure independently from main pump
  
• Hydraulic control ECM – Interprets operator input and manages signal routing
  
• Fuse and relay panel – Powers ECM and solenoid banks
  

Failure in any of these components can disrupt function, particularly if electrical faults such as loose connectors, corroded terminals, or failed relays are involved.
Step-by-Step Diagnostic Procedure

1. Visual Inspection
   • Check for broken, pinched, or disconnected wires at the joystick, ECM, and solenoids
     
   • Inspect pilot control valve block for visible hydraulic leaks
     
   • Verify hydraulic fluid levels and look for signs of contamination (e.g., milky oil = water ingress)
     
2. Check Fuses and Relays
   • Locate fuse panel inside the cab or electrical compartment
     
   • Replace suspect relays with known good spares
     
   • Ensure key fuses for hydraulic and joystick systems are intact
     
3. Test Joystick Input and Power Supply
   • Use a multimeter to verify voltage at the joystick output wires during function attempts
     
   • Check for ground continuity and power feed to the hydraulic ECM
     
4. Solenoid Testing
   • Disconnect the solenoid electrical plug and check resistance (typically 10–20 ohms depending on model)
     
   • Apply 12V power directly to solenoid pins to test manual function—listen for a click
     
   • If solenoid clicks but nothing moves, the problem may be in the valve or pilot circuit
     
5. Hydraulic Flow Testing
   • If electrics check out, attach a pressure gauge to the pilot circuit
     
   • A pressure below 300 psi suggests pilot pump failure or clogged filter
     
   • Use manual override levers on valve block if available to verify mechanical valve movement
     
6. ECM Fault Code Retrieval
   • Komatsu machines can store fault codes in the dashboard module
     
   • Use service tools or diagnostic display (if equipped) to retrieve stored fault codes
     

Root Causes Identified in Real-World Cases

• Loose connector at pilot solenoid block – The vibration from the engine can cause plugs to work loose
  
• Internal solenoid coil failure – Especially in high-hour machines operating in wet conditions
  
• Corroded ECM connector pins – A single oxidized pin can prevent voltage from reaching a solenoid
  
• Failed pilot pump – Rare but possible, especially if the pump’s internal filter is clogged or neglected
  
• Wiring harness chafing – Harness running under the cab can wear through, especially at frame contact points
  

One particularly tricky case involved a broken wire inside the insulation, which passed a voltage test with no load but failed under current. Only a load-simulating tool revealed the voltage drop when the joystick was engaged.
Operator Insights and Maintenance Practices
Operators who’ve dealt with similar issues often emphasize the importance of:

• Routine inspection of wiring harness clips and weather seals
  
• Using dielectric grease on all major connectors to prevent corrosion
  
• Installing additional vibration damping for sensitive plug connections
  
• Keeping the cab dry and sealing leaks that might affect fuse panels
  
• Logging intermittent faults immediately to help track patterns
  

One contractor described a case where a brand-new solenoid failed within a week—only to discover the replacement was never torqued to spec, allowing it to vibrate loose. In another instance, a root cause was traced back to a rodent chewing the insulation under the cab, disrupting signal continuity.
Comparative Note: CAT vs. Komatsu Control Systems
While Caterpillar and Komatsu both use advanced electrohydraulic systems in their dozers, Komatsu tends to favor modular valve banks and isolated ECMs, whereas CAT integrates more deeply into the main controller. This can make Komatsu systems easier to isolate function-by-function but harder to access physically. Mechanics often find Komatsu hydraulic schematics more detailed but physically tight in routing.
Conclusion: Systematic Troubleshooting Prevents Downtime
When faced with hydraulic function loss in machines like the Komatsu D155AX-5, a structured diagnostic process saves time, money, and unnecessary part swapping. The key is to understand the interaction between electrical signals and hydraulic actions—then isolate failure through methodical elimination.
In the end, even complex machines speak a language—electrical resistance, fluid pressure, voltage drops—and listening closely with the right tools turns frustrating failures into solvable puzzles. As one veteran technician aptly said, “A machine won’t lie to you—but you have to ask the right questions.”