Komatsu D85A-18 Bulldozer Track Engagement Delay and Transmission Diagnostics

[MikePhua]

The Komatsu D85A-18 and Its Transmission Legacy
The Komatsu D85A-18 bulldozer is part of Komatsu’s long-standing D85 series, designed for heavy-duty earthmoving, mining, and land clearing. With an operating weight exceeding 28 tons and powered by a Komatsu S6D125 diesel engine producing over 260 horsepower, the D85A-18 is built for torque, traction, and reliability. Komatsu, founded in Japan in 1921, has consistently engineered its dozers with modular drivetrain systems and robust undercarriages suited for extreme conditions.
The D85A-18 features a powershift transmission with torque converter drive, allowing smooth directional changes and gear selection under load. Its transmission system is hydraulically actuated and electronically monitored, with clutch packs engaging the final drives to propel the tracks. A delay in track engagement after gear selection can signal underlying issues in hydraulic pressure, clutch actuation, or control logic.
Symptoms of Hesitation Before Track Engagement
Operators may notice the following behaviors:

• Delay of several seconds after shifting into forward or reverse
  
• Engine RPM remains stable but tracks do not respond immediately
  
• No fault codes or warning lights on the dash
  
• Transmission eventually engages and operates normally
  
• Hesitation worsens when machine is cold or under load
  

Terminology notes:

• Clutch pack: A set of friction discs and steel plates that engage to transmit torque within the transmission.
  
• Torque converter: A fluid coupling that multiplies engine torque and allows smooth power transfer to the transmission.
  

In Alberta, a contractor reported a 5–7 second delay in track movement after shifting into gear. The issue was more pronounced during cold mornings and gradually improved as the machine warmed up.
Primary Causes and Diagnostic Strategy
Several systems can contribute to delayed track engagement:

• Low hydraulic pressure in the transmission control circuit
  
• Internal leakage in clutch packs or valve body
  
• Sticky spool valves or worn seals in the control valve
  
• Weak or contaminated transmission fluid
  
• Electrical signal delay from gear selector or solenoid
  

Diagnostic steps:

• Check transmission hydraulic pressure at test ports (should exceed 250 psi during clutch engagement)
  
• Inspect transmission fluid level, color, and viscosity
  
• Test solenoid response time and voltage at shift selector
  
• Monitor clutch pack engagement timing with infrared thermometer
  
• Inspect valve body for contamination or wear
  

Recommended tools:

• Hydraulic pressure gauge with Komatsu adapter fittings
  
• Multimeter for solenoid voltage checks
  
• Infrared thermometer for clutch housing temperature
  
• Service manual with transmission schematics
  
• Clean catch pan for fluid sampling
  

In Chile, a mining crew diagnosed a sticky spool valve in the transmission control block. After cleaning and replacing the O-rings, the hesitation disappeared.
Hydraulic System Behavior and Fluid Integrity
The transmission relies on clean, high-pressure hydraulic fluid to actuate clutch packs. Common issues include:

• Fluid aeration from suction leaks or low reservoir level
  
• Varnish buildup in control valves from aged fluid
  
• Contaminated filters reducing flow rate
  
• Incorrect fluid type affecting viscosity and pressure curve
  
• Internal leakage in clutch piston seals
  

Maintenance tips:

• Replace transmission fluid every 1,000 hours or annually
  
• Use Komatsu-approved hydraulic transmission oil
  
• Replace filters with OEM-rated elements
  
• Inspect suction lines and breather for cracks or blockage
  
• Monitor fluid temperature during operation (should remain below 90°C)
  

In Tennessee, a grading crew switched to synthetic transmission fluid after repeated engagement delays. The improved cold-flow properties reduced hesitation during winter startup.
Clutch Pack and Valve Body Inspection
If pressure and fluid are within spec, the clutch packs and valve body may be at fault:

• Remove transmission access covers and inspect clutch piston movement
  
• Check for scoring or discoloration on clutch plates
  
• Measure clutch pack clearance with feeler gauge
  
• Inspect valve body spool movement and spring tension
  
• Replace worn seals and clean valve bores with approved solvent
  

Terminology notes:

• Spool valve: A cylindrical valve that slides within a bore to direct hydraulic flow.
  
• Clutch piston: A hydraulic actuator that compresses clutch packs to engage drive.
  

In South Africa, a vineyard crew rebuilt the clutch pack on their D85A-18 after discovering uneven wear and delayed piston response. The rebuild restored crisp engagement and reduced fuel consumption.
Electrical and Control System Considerations
Modern variants of the D85A-18 may include electronic shift selectors and solenoids. Electrical faults can mimic hydraulic delays:

• Test gear selector switch for continuity and debounce delay
  
• Inspect solenoid connectors for corrosion or loose pins
  
• Verify ECM signal timing with diagnostic scanner
  
• Replace damaged harness sections with marine-grade wire
  
• Add ground strap from transmission housing to frame for signal stability
  

In New Zealand, a contractor added a diagnostic LED to the shift solenoid circuit. When the light delayed after gear selection, it confirmed an intermittent signal fault.
Preventive Maintenance and Long-Term Reliability
To avoid future engagement delays:

• Inspect transmission pressure monthly
  
• Replace fluid and filters on schedule
  
• Clean valve body annually during service
  
• Log engagement timing and fluid temperature trends
  
• Train operators to report hesitation early
  

Suggested spares to stock:

• Transmission filter kits
  
• Clutch pack seal sets
  
• Valve body O-rings and springs
  
• Shift solenoids and connectors
  
• Hydraulic test fittings
  

In Poland, a snow-clearing team added a transmission warm-up protocol to their winter checklist. This reduced hesitation and improved blade response during early shifts.
Operator Stories and Field Wisdom
In Florida, a demolition crew used a D85A-18 for pushing debris. After noticing delayed engagement, they installed a pressure gauge in the cab to monitor clutch actuation. The visibility helped them catch low-pressure events before damage occurred.
In Ontario, a contractor rebuilt his transmission valve body using a kit sourced from a retired D85A-21. The parts matched, and the machine returned to full responsiveness.
Conclusion
Track engagement hesitation in the Komatsu D85A-18 is often a symptom of hydraulic pressure loss, valve wear, or signal delay. With methodical diagnostics, clean fluid, and attention to clutch pack integrity, the issue can be resolved without major overhaul. Whether grading clay or pushing rock, the transmission must respond with confidence—and every second of hesitation is a signal worth investigating.