Rebuilding the Four-Speed Transmission on a Case 680E Backhoe

[MikePhua]

A Durable Machine with a Unique Gearbox
The Case 680E backhoe loader, produced in the mid-1970s, was part of Case’s evolution toward more powerful and versatile construction equipment. With a mechanical four-speed transmission rather than the more common shuttle shift, the 680E offered direct gear engagement and robust torque delivery. This transmission was favored for its simplicity and reliability, especially in rural and agricultural applications where hydraulic shuttle systems were considered more complex and prone to wear.
Transmission Layout and Removal Strategy
The four-speed gearbox in the 680E is mounted beneath the operator platform and connected to the engine via a driveshaft and universal joints. Unlike shuttle shift systems, which often integrate torque converters and hydraulic clutches, this transmission uses a dry clutch and mechanical linkage.
To remove the transmission:

• Disconnect the driveshaft by rotating and unbolting the U-joints.
  
• Mark and note the position of all mounting bolts—some bolts are different lengths, and incorrect placement can strip blind holes.
  
• Use a transmission jack or lifting cradle to support the gearbox during removal.
  
• Lower the unit vertically through the belly pan opening.
  

One technician emphasized the importance of using a proper jack due to the transmission’s awkward weight distribution. Improvised lifting methods have led to cracked housings and damaged input shafts.
Terminology Clarification

• Dry Clutch: A friction-based clutch system that operates without hydraulic fluid, common in older mechanical transmissions.
  
• Blind Hole: A threaded hole that does not pass through the material, requiring precise bolt length to avoid bottoming out.
  
• U-Joint (Universal Joint): A flexible coupling that allows the driveshaft to transmit torque at varying angles.
  
• Transmission Jack: A specialized lifting device designed to support and maneuver heavy gearboxes during service.
  

Parts Availability and Compatibility
The transmission in the 680E shares design elements with light truck gearboxes from the same era, though it lacks reverse gearing in some configurations. While Case no longer manufactures parts for this model, several suppliers offer rebuild kits, bearings, seals, and even complete remanufactured units. Recommended sources include:

• Joseph Industries (known for legacy Case drivetrain support)
  
• Minpar (specializes in obsolete construction equipment parts)
  
• Local salvage yards with vintage Case inventory
  

Before ordering parts, confirm the transmission model and serial number. Some units were retrofitted with shuttle shift systems or modified for specific applications, and internal components may differ.
Common Wear Points and Rebuild Tips

• Input shaft bearings often fail due to misalignment or dry operation.
  
• Synchronizers wear unevenly, especially in machines used for frequent gear changes under load.
  
• Shift forks may bend or crack if the linkage is misadjusted.
  
• Oil seals degrade over time, leading to leaks and contamination.
  

During rebuild:

• Replace all seals and gaskets, even if they appear intact.
  
• Use high-quality gear oil rated for GL-4 applications to protect brass synchronizers.
  
• Torque all fasteners to spec and use thread locker where appropriate.
  
• Inspect the clutch disc and pressure plate for wear; replace if thickness is below spec.
  

Conclusion
Rebuilding the four-speed transmission on a Case 680E is a rewarding task for mechanically inclined owners and technicians. With proper planning, accurate part sourcing, and attention to bolt placement and alignment, the gearbox can be restored to reliable service. This model’s mechanical simplicity and rugged design continue to make it a viable workhorse decades after its release.