Diagnosing Low Torque in First Gear on the Caterpillar 980B Loader

[MikePhua]

The Caterpillar 980B and Its Transmission Legacy
The Caterpillar 980B wheel loader was introduced in the 1970s as part of Caterpillar’s second-generation lineup of mid-to-large loaders. With an operating weight of over 50,000 pounds and a bucket capacity exceeding 6 cubic yards, the 980B was designed for quarry work, heavy material handling, and aggregate production. Powered by a turbocharged diesel engine and equipped with a torque converter transmission, the machine offered smooth gear transitions and high breakout force.
Caterpillar’s torque converter systems in this era were known for their durability but also for their complexity. Unlike direct-drive transmissions, torque converters rely on fluid coupling and internal components such as stators, turbines, and rotating members to multiply torque under load. When these systems begin to fail, symptoms often appear in low gear first, where torque demand is highest.
Terminology Clarification

• Torque Converter: A hydraulic coupling between the engine and transmission that multiplies torque during acceleration and heavy load.
  
• Stall Speed: The maximum engine RPM achieved when the transmission is locked and the machine is unable to move, used to test torque converter performance.
  
• Third Member: A rotating internal component in some converters that locks under load to enhance torque multiplication.
  
• Clutch Pack: A set of friction plates used to engage specific gears within the transmission.
  
• Drive Shaft: The rotating shaft that transfers power from the torque converter to the transmission.
  

Symptoms of Low Torque in First Gear
Operators may notice the following:

• Sluggish response in first gear forward and reverse
  
• Difficulty climbing out of pits or pushing into piles
  
• Improved performance after warm-up but still below normal
  
• Normal behavior in higher gears
  
• Engine reaching full RPM without corresponding movement
  

In one case from Queensland, a 980B loader struggled to exit a gravel pit with a full bucket. The operator reported that the machine felt “lazy” in first gear but performed adequately in second and third. After warming up, the issue improved slightly but never returned to full strength.
Diagnostic Strategy and Field Testing
To isolate the problem:

1. Check Stall Speed
   Lock the brakes, place the machine in first gear, and apply full throttle. Measure engine RPM. If stall speed is low, the engine may not be producing full power or the converter is failing to multiply torque.
   
2. Observe Converter Behavior
   Some Caterpillar converters use cam-actuated third members that lock under load. If these cams are worn, the converter may spin freely without locking, reducing torque output.
   
3. Test Gear Clutch Engagement
   If the issue is isolated to first gear, the clutch pack for that gear may be slipping. Place the machine against a bank, engage first gear, and observe whether the drive shaft rotates. If it does, but the machine doesn’t move, the clutch is likely slipping.
   
4. Inspect Hydraulic Pressure
   Low pressure in the transmission control system can prevent full clutch engagement. Check pump output and valve settings.
   
5. Evaluate Engine Performance
   Ensure the engine is producing rated horsepower. A weak engine will affect torque converter input and stall speed.
   

Recommended Repairs and Adjustments
Depending on the findings:

• Replace worn cams or third member components in the torque converter
  
• Rebuild or replace the first gear clutch pack
  
• Adjust or replace transmission control valves
  
• Flush and replace hydraulic fluid and filters
  
• Inspect engine fuel delivery and turbocharger performance
  

In 2013, a quarry operator in Georgia rebuilt the torque converter on his 980B after experiencing similar symptoms. Post-repair, the machine regained full torque in first gear and fuel efficiency improved by 8%.
Preventive Maintenance and Best Practices
To maintain torque converter health:

• Change transmission fluid every 1,000 hours
  
• Monitor stall speed quarterly
  
• Avoid prolonged operation in high-load low-speed conditions
  
• Inspect clutch packs during scheduled overhauls
  
• Keep cooling systems clean to prevent fluid overheating
  

In 2020, a fleet manager in Alberta implemented a torque converter monitoring program across his loader fleet. Over two years, transmission-related downtime dropped by 35%, and clutch life extended by 20%.
Conclusion
Low torque in first gear on the Caterpillar 980B is often a sign of internal converter wear or clutch pack degradation. With methodical diagnostics and targeted repairs, the issue can be resolved, restoring full power and operational confidence. The 980B, when properly maintained, continues to deliver reliable performance in demanding environments, proving that even decades-old iron can still move mountains when its systems are understood and respected.