Can the Allison TT2420-1 Transmission Replace the TT2221-1 in a Case W20B Loader

[MikePhua]

Case W20B Loader and Transmission Background
The Case W20B wheel loader was introduced in the late 1970s and remained in production through the 1980s, serving as a mid-size, versatile machine for construction, agriculture, and municipal work. Powered by the Case 401BD diesel engine, the W20B featured a robust frame, articulated steering, and a torque converter transmission. Many units were equipped with Allison transmissions—specifically the TT2221-1—known for their durability and smooth shifting under load.
Allison Transmission, founded in 1915 and headquartered in Indianapolis, has long been a leader in automatic transmissions for commercial and off-highway applications. The TT series was designed for industrial loaders, graders, and other heavy equipment, offering hydraulic torque multiplication and planetary gearsets.
Terminology Note

• Torque Converter: A fluid coupling that multiplies engine torque and allows smooth engagement without a clutch.
  
• Output Shaft: The shaft that delivers torque from the transmission to the differential.
  
• Parking Brake Assembly: A mechanical brake mounted on the transmission output shaft to hold the machine stationary.
  
• Turbine: The rotating component inside the torque converter that receives fluid energy from the impeller.
  

Transmission Comparison and Compatibility
The TT2221-1 is the original transmission in the Case W20B. A technician in Sweden acquired a TT2420-1 from a Case W24 loader and asked whether it could be used as a replacement. Both transmissions share similar housing dimensions and mounting patterns, but there are key differences:

• TT2420-1 has a larger turbine, which may affect stall speed and torque multiplication.
  
• The TT2420-1 lacks a parking brake assembly, though it has bolt holes for mounting one.
  
• Gear ratios and internal clutch packs may differ, potentially altering shift behavior and travel speed.
  

Despite these differences, the transmissions are largely interchangeable from a mechanical standpoint. The TT2420-1 can be mounted in place of the TT2221-1, provided the parking brake is transferred or retrofitted. The larger turbine may offer improved torque at low speeds, beneficial for heavy loading or steep grades.
Recommendations for Installation and Adaptation

• Verify bellhousing alignment and bolt pattern before installation.
  
• Transfer the parking brake assembly from the TT2221-1 to the TT2420-1 using existing bolt holes.
  
• Inspect the output shaft spline count and diameter to ensure compatibility with the driveshaft.
  
• Flush the transmission cooler and lines to prevent contamination.
  
• Use factory torque specs and transmission fluid recommended by Allison for TT-series units.
  

Field Insight and Practical Advice
In Arkansas, a service technician confirmed that the TT2420-1 should interchange with the TT2221-1, assuming the parking brake is addressed. In other cases, operators have successfully swapped TT-series transmissions across Case loaders with minimal modification. However, it’s important to monitor shift quality and torque response after installation, as turbine size and gear ratios can subtly affect performance.
Maintenance and Longevity Tips

• Change transmission fluid every 500 hours or annually.
  
• Inspect torque converter seals and input shaft bearings during installation.
  
• Use genuine Allison filters and fluid to maintain clutch pack integrity.
  
• Keep a log of transmission serial numbers and part numbers for future reference.
  

Conclusion
The Allison TT2420-1 transmission can be used to replace the TT2221-1 in a Case W20B loader with minor adaptation. While the turbine size and parking brake configuration differ, the core architecture is compatible. With careful installation and attention to detail, the swap can restore functionality and extend the life of a classic Case loader—especially in regions where original parts are scarce.