Locating Fill Points and Fluid Types for the Komatsu D55S-3 Track Loader

[MikePhua]

The D55S-3 and Komatsu’s Mid-Size Loader Heritage
The Komatsu D55S-3 is a mid-size track loader powered by the 4D120 diesel engine, a naturally aspirated four-cylinder workhorse known for its torque and reliability. Produced during the late 1970s and early 1980s, the D55S-3 was part of Komatsu’s push to expand its crawler loader lineup for general construction, quarrying, and forestry applications. With an operating weight of approximately 30,000 lbs and a bucket capacity of around 2.5 cubic yards, the machine offered a balance of power and maneuverability.
Komatsu, founded in 1921 in Japan, had by this era become a global competitor to Caterpillar, Allis-Chalmers, and International Harvester. The D55S-3 was widely sold across North America and Asia, with thousands of units still in operation today—many in private fleets, farms, and municipal yards.
Understanding the Fluid Systems and Fill Locations
The D55S-3 features multiple fluid systems, each with its own fill point and service requirements. These include:

• Engine oil system
  
• Transmission and torque converter
  
• Hydraulic system
  
• Final drives (left and right)
  
• Steering clutches
  
• Cooling system
  

While some fill points are clearly marked, others—especially for the steering clutch and torque converter—can be difficult to locate without a service manual or prior experience.
Engine Oil Fill and Service
The 4D120 engine requires approximately 14 liters (3.7 gallons) of SAE 15W-40 diesel-rated engine oil. The fill cap is located on top of the valve cover, and the dipstick is typically mounted on the right side of the block. Oil should be changed every 250 hours, with the filter replaced at each interval.
Signs of contamination include:

• Milky oil (water intrusion)
  
• Excessive soot (combustion blow-by)
  
• Metallic sheen (bearing wear)
  

Use a magnetic drain plug to monitor for ferrous particles during oil changes.
Transmission and Torque Converter Fluid
The transmission and torque converter share a common housing but may have separate fill and inspection ports. The fill point is usually located on the top of the transmission case, accessible from the operator platform. The system uses approximately 40 liters (10.5 gallons) of Type C hydraulic transmission fluid or equivalent.
To check fluid level:

• Locate the dipstick or sight glass near the rear of the transmission
  
• Run the engine at idle for 5 minutes to circulate fluid
  
• Check level with the machine on level ground
  

If the machine hesitates during gear changes or stalls under load, low fluid or contamination may be the cause.
Final Drive Fill Points and Oil Condition
Each final drive has two fill plugs—one for the planetary gear case and one for the steering clutch housing. The planetary drive typically uses SAE 90 gear oil, while the clutch housing may require hydraulic fluid or a lighter gear oil depending on configuration.
To inspect oil condition:

• Remove the lower plug and observe flow
  
• Dark, gritty oil indicates wear or contamination
  
• Milky oil suggests water ingress
  
• Burnt odor may signal overheating
  

Final drive oil should be changed every 500 hours or annually. Use high-viscosity gear oil with EP additives for heavy-duty applications.
Steering Clutch Fill and Maintenance
The steering clutch system is often overlooked due to its hidden location. Fluid is added through a small plug on the clutch housing, typically accessed from under the seat or side panel. If the clutch slips or fails to disengage, low fluid or worn friction discs may be the cause.
Recommended fluid:

• Type C hydraulic fluid or SAE 30 non-detergent oil
  
• Capacity varies by model, typically 4–6 liters per side
  

Bleed the system after filling to remove air pockets and ensure full engagement.
Hydraulic System and Reservoir Access
The hydraulic reservoir is usually mounted behind the operator station or under the hood. It supplies fluid to the lift arms, bucket tilt, and auxiliary functions. Use ISO 46 hydraulic oil or equivalent, with a total system capacity of approximately 60 liters (16 gallons).
Check fluid level with the dipstick or sight glass while the machine is off and attachments are lowered. Replace filters every 250 hours and flush the system every 1,000 hours.
Cooling System and Radiator Fill
The radiator fill cap is located at the top of the cooling system, often under a hinged panel. Use a 50/50 mix of ethylene glycol coolant and distilled water. Capacity is around 20 liters (5.3 gallons).
Inspect for:

• Scale buildup
  
• Hose cracking
  
• Fan belt tension
  

Flush the cooling system every 1,000 hours or two years, whichever comes first.
Field Experience and Practical Advice
A machine owner in Pennsylvania reported difficulty locating the fill points for the torque converter and steering clutch. After tracing the hydraulic lines and inspecting the housing, he discovered that the steering clutch fill was hidden behind a removable panel near the seat base. He added fluid and regained full steering response within minutes.
Another operator in Alberta noted that his final drive oil appeared clean but lacked viscosity. After draining and replacing with SAE 90 gear oil, the machine’s climbing power improved noticeably.
Conclusion
Maintaining fluid levels and understanding fill locations on the Komatsu D55S-3 is essential for reliable operation and long-term durability. While some ports are obvious, others require careful inspection and familiarity with the machine’s layout. By using the correct fluids, monitoring oil condition, and servicing each system at regular intervals, operators can keep these classic loaders running strong—proving that even decades-old iron still has plenty of life left when properly cared for.