Komatsu D155AX-3 Transmission Failure

[MikePhua]

Machine Background and Specifications
The Komatsu D155AX-3 (Super) is a heavyweight crawler dozer built between roughly 1995 and 1999. It weighs about 39.5 tonnes (≈ 88,900 lb) in operating condition. Its engine is the Komatsu S6D140E2, with approximately 306 hp (at around 1900 rpm) and comes fitted with a power shift / hydrostatic steering (PS/HSS) transmission. It has track shoes about 610 mm wide, and dimensions like over-the-tracks width around 8.85 feet.
Because of its size and power, the D155AX-3 is used in heavy earthmoving, mining support, large grading, and construction tasks where blade capacity, traction, and pushing force are required. Its components are correspondingly heavy-duty: transmission (power train), final drives, clutch packs, steering pumps, and joysticks or controllers for directional input.
Problem Description
A D155AX-3 dozer was reported to have a transmission problem: it would not move in forward or reverse at low idle; under high idle or full throttle, it might move slowly, but performance was weak, intermittent, and in many shift ranges (F1, F2, F3, R1, R2, R3) there was no or minimal movement. Also, there were error codes shown on the HMT (Hydraulic Monitoring / Transmission Controller) monitor, a flashing caution light, but no buzzer or full alarm. Steering seemed only to function when the transmission was in neutral. Strainers/power train strainers were cleaned but made little difference.
Terminology & Key Components

• Power Shift / Hydrostatic Steering (PS/HSS): Type of transmission / drive scheme used in D155AX-3; involves multiple forward/reverse gears plus hydraulic steering control.
  
• Idle Speed / Low Idle / High Idle: Engine rpm settings; low idle is minimal RPM for idling, high idle is increased RPM for load performance.
  
• HMT Controller / Monitor: Electronic controller for transmission and hydraulic systems; records error (fault) codes.
  
• Strainer / Transmission Strainer: Filter device to catch debris before fluid enters transmission or power train.
  

Likely Causes
Based on the symptoms and responses from technicians, the following causes are most plausible:

• Faulty Joystick / Controller Input: If the joystick or its electronic controller is not signaling correctly, the transmission controller may not shift into drive or reverse properly. Several users pointed out known joystick failures.
  
• Low Pilot Pressure or Low Hydraulic Volume: The dozer’s control circuits rely on hydraulic pilot pressure. If pilot pressure is too weak (due to pump wear, leaks, or blocked pilot filters/hoses), valves won’t operate properly, preventing shifts.
  
• Transmission Strainer / Filter Clogging: Dirty fluid or clogged strainers can starve internal transmission circuits, cause pressure drop, and impair gear engagement under low idle. Even though strainers were cleaned, residual debris or internal filters might remain blocked.
  
• Controller Faults / Wiring / Electrical Problems: Loose connectors, moisture/dirt in wiring, mistaken sensor readings can generate error/fault codes and inhibit correct operation. A few technicians noted many “fault codes” that ended up being connector/wiring issues not mechanical or hydraulic failures.
  

Diagnosis Approach
To pinpoint the issue, a structured diagnostic plan is needed:

1. Read and Interpret Error Codes: Use the HMT monitor error codes, cross-reference to service manual (look up relevant codes for no forward/reverse, steering in neutral only, etc.).
   
2. Check Joystick / Direction Control Mechanisms: Test joystick potentiometers or electronic sensors (for forward, reverse, neutral signals). Check calibration, continuity, and correct voltage/signals.
   
3. Measure Pilot Pressure and Volume: Use a pressure gauge to determine whether pilot pressure falls within spec under low idle and high idle. Also check for leaks in pilot circuits.
   
4. Inspect All Connectors, Wiring Harnesses, and Sensors: Particularly in harsh environments, connectors corrode; moisture or dust can cause intermittent or permanent signal loss.
   
5. Assess Transmission / Power Train Filters & Strainers: Clean/replace strainers; check internal filters if possible. Ensure fluids are clean and correct type.
   
6. Check Hydraulic Fluid Level and Quality: Low fluid or contaminated fluid can degrade performance. Ensure correct viscosity, no air entrainment.
   
7. Operational Tests in Different Shift Ranges: Try shifting in various gears and loads; note whether any gear operates properly; observe whether the dozer can move under heavy throttle or only lightly loaded.
   

Solutions & Repairs
Depending on what diagnostic tests reveal, possible corrective actions include:

• Replace or repair joystick/controller: sensors, levers, electronic parts.
  
• Repair wiring, clean/replace damaged connectors; protect with sealants or replacement connectors.
  
• Restore pilot circuit integrity: repair leaks, replace worn or damaged hoses, clean pilot filters.
  
• Replace internal transmission filters / rebuild transmission if internal wear (valves, packs) is causing low hydraulic pressure or loss of shifting capability.
  
• Flush transmission fluid, change to correct fluid (viscosity, quality), ensure no air is in system.
  
• If strainers/filters are repeatedly clogging, consider upgrading filtration or installing additional screens.
  

Real-World Case & Lessons
One case in a remote construction site involved a D155AX-3 whose operator assumed poor movement was due to worn undercarriage. After dumping considerable hours and replacing track parts, the movement problem remained. Eventually, a technician discovered that a joystick’s forward/reverse circuit had failed. The joystick had water ingress causing erratic signal; after replacement, full transmission function returned. Cost was modest compared to the previous part replacements.
In another scenario, a mining site maintenance team monitored pilot pressure over time and found that with filters partially clogged, pilot pressure dropped by about 20-30% below spec under low idle. Once filters were replaced and hoses checked, performance improved dramatically, with forward/reverse shifting smooth even at low idle.
Technical Data that Informs the Repair

• Engine power: about 306 hp used in relation to needing sufficient hydraulic output.
  
• Track width: 610 mm per side; operational weight ~ 39.5 t (≈ 88,900 lb), meaning the transmission must move significant mass; weak hydraulic / incorrect controls will fail under load.
  

Recommendations & Preventive Measures

• Maintain hydraulic system in top condition: regular changing of fluid, filters, inspecting strainers.
  
• Keep joystick and control electronics sealed, clean, free from moisture. If the unit is exposed, adding boots or covers may help.
  
• Conduct scheduled calibrations of controls and sensors as listed in the operation & maintenance manual.
  
• Regularly inspect and clean connectors, especially under cab and near transmission/wiring harnesses.
  
• Operators should be trained to recognize early signs (slow movement, loss of power in forward/reverse, erratic behavior) and shut down for diagnosis rather than pushing harder (which may cause damage).
  

Conclusion
On a Komatsu D155AX-3 dozer with no forward/reverse or very weak movement, the problem is likely not purely mechanical but involves controls (joystick / sensors), pilot pressure deficiency, electrical or filtration issues. With correct troubleshooting—reading error codes, checking wiring and sensors, measuring pressures, cleaning/replacing strainers and filters—the issue often can be resolved without full transmission rebuild. Given the machine’s weight and power, ensuring the hydraulic and control systems are intact is key to full mobility.