Komatsu D135A-1 Dozer Performance Reliability and Restoration Insights

[MikePhua]

The D135A-1 and Komatsu’s Heavy Crawler Legacy
The Komatsu D135A-1 crawler dozer was introduced in the late 1970s as part of Komatsu’s push into the mid-heavy dozer segment. Designed to compete with models like the Caterpillar D6 and D7, the D135A-1 offered a balance of raw pushing power, mechanical simplicity, and field serviceability. With an operating weight of approximately 33,000 lbs and a blade capacity exceeding 7.5 cubic yards, it was widely used in mining, forestry, and large-scale earthmoving.
Komatsu, founded in 1921 in Japan, had by then become a global force in construction equipment. The D135A-1 was built during a period of rapid expansion, with units shipped across North America, Asia, and Africa. Its mechanical transmission, robust undercarriage, and naturally aspirated diesel engine made it a favorite in remote regions where electronic diagnostics were impractical.
Core Specifications and Mechanical Layout
Key specifications:

• Engine: Komatsu SA6D125, 6-cylinder diesel
  
• Displacement: ~11 liters
  
• Power output: ~200 hp at 2,100 rpm
  
• Transmission: 3-speed powershift with torque converter
  
• Blade: Semi-U or straight blade options
  
• Track gauge: ~2,000 mm
  
• Ground pressure: ~0.75 kg/cm²
  
• Fuel capacity: ~300 liters
  

The SA6D125 engine is known for its long stroke and high torque at low RPM, ideal for pushing heavy loads. The transmission uses a torque converter to smooth gear changes, and the final drives are planetary reduction units housed in sealed compartments.
In one mining operation in Alberta, a D135A-1 was used to push overburden across a 1,000-meter haul road. Despite its age, the machine operated for 12-hour shifts with minimal downtime, thanks to a rebuilt transmission and upgraded cooling system.
Common Issues and Field Diagnostics
As with many legacy machines, the D135A-1 can develop wear-related issues over time. Common problems include:

• Hydraulic drift in blade lift or tilt cylinders
  
• Transmission hesitation or gear slippage
  
• Undercarriage wear including track links and rollers
  
• Cooling system inefficiency due to clogged cores
  
• Electrical faults in starter and lighting circuits
  

In one fleet in Texas, a D135A-1 began losing blade lift pressure. The technician discovered internal leakage in the lift cylinder and replaced the seals using a Komatsu OEM kit. The machine returned to full function after a fluid flush and filter change.
Undercarriage Wear and Maintenance Strategy
The undercarriage is a high-wear zone, especially in abrasive soils. Key components include:

• Track chains and pads
  
• Carrier and track rollers
  
• Idlers and recoil springs
  
• Sprockets and final drives
  

Maintenance recommendations:

• Inspect track tension monthly
  
• Replace pads when lug height drops below 50%
  
• Grease recoil springs and pivot points
  
• Monitor final drive oil for contamination
  

In one forestry job in Oregon, a D135A-1 was retrofitted with bolt-on rock guards and reinforced track pads. This reduced wear and improved traction on steep slopes.
Transmission Behavior and Rebuild Considerations
The powershift transmission in the D135A-1 is mechanically controlled and uses clutch packs to engage gear ranges. Symptoms of wear include:

• Delayed engagement
  
• Slipping under load
  
• Overheating during long pushes
  
• Metallic debris in transmission oil
  

Rebuild strategy:

• Replace clutch pack friction and steel plates
  
• Inspect torque converter for vane wear
  
• Flush oil cooler and replace filters
  
• Use Komatsu-approved transmission fluid
  
• Test stall speed and pressure after rebuild
  

In one rebuild in Maine, a technician used a stall test to diagnose a weak torque converter. After replacement, the dozer regained full pushing power and reduced fuel consumption.
Hydraulic System and Blade Control
The blade is controlled via pilot-operated hydraulic valves. Common issues include:

• Cylinder seal leakage
  
• Valve spool wear causing drift
  
• Air intrusion from loose fittings
  
• Contaminated fluid reducing response
  

Recommended actions:

• Replace cylinder seals and bushings
  
• Clean valve body and inspect spool clearance
  
• Bleed air from system after filter change
  
• Use ISO 46 hydraulic oil with anti-wear additives
  

In one grading job in France, a D135A-1 was used to contour a slope for a dam foundation. The operator noted improved blade control after switching to synthetic hydraulic fluid and replacing the tilt cylinder seals.
Electrical System and Starting Reliability
Older D135A-1 units may suffer from electrical degradation:

• Corroded battery terminals
  
• Frayed wiring near starter solenoid
  
• Faulty ignition switch or safety interlock
  
• Weak alternator output
  

Solutions:

• Replace battery cables with marine-grade wire
  
• Use dielectric grease on connectors
  
• Upgrade to sealed starter relay
  
• Test alternator output (target: 13.8–14.2V at idle)
  

In one rebuild in Chile, a technician installed a modern LED lighting kit and rewired the harness using split loom and waterproof connectors. The machine became more reliable and easier to service.
Restoration and Collector Interest
The D135A-1 has gained interest among collectors and restoration enthusiasts. Its mechanical simplicity and rugged design make it a candidate for full restoration. Some units are displayed at equipment museums or used in demonstration events.
In 2023, a fully restored D135A-1 sold at auction in Japan for ¥4.2 million, complete with original blade, rebuilt engine, and repainted chassis. The buyer planned to use it in a heritage quarry operation.
Conclusion
The Komatsu D135A-1 dozer remains a symbol of mechanical durability and field serviceability. With proper maintenance, targeted rebuilds, and thoughtful upgrades, it continues to serve in demanding environments decades after its debut. Whether pushing rock, grading slopes, or clearing land, the D135A-1 proves that old iron still has muscle.