Resurrecting the Waldon 7000 Loader and Decoding Its Electrical Hydraulic System

[MikePhua]

The Waldon 7000 and Its Industrial Legacy
The Waldon 7000 compact wheel loader is a rugged, American-built machine designed for tight industrial spaces and heavy-duty tasks. Waldon Equipment, founded in Oklahoma in the mid-20th century, specialized in low-profile loaders for steel mills, foundries, and municipal operations. The 7000 series, introduced in the early 1990s, featured a compact frame, articulated steering, and hydrostatic drive—making it ideal for confined environments where maneuverability and torque were paramount.
Though never mass-produced at the scale of mainstream brands, Waldon loaders earned a reputation for durability and simplicity. The 7000 model, powered by a diesel engine and equipped with Rexroth AA6VM variable displacement motors, was particularly favored for its mechanical accessibility and robust hydraulic system.
Initial Challenges in Restoration
Reviving a decades-old Waldon 7000 presents a blend of mechanical and electrical puzzles. In one case, the loader had been parked after a plant upgrade and left with a damaged wiring harness, a missing dash, and a swapped-out joystick control system. The engine—a Bosch inline pump model likely from 1999—had suffered a rack runaway due to a stuck governor, but was eventually rebuilt with new injectors, lift pump, and solenoid.
The loader’s electrical system was in disarray. The main harness had been torn by the driveshaft, and only the starter and forward/reverse circuits remained intact. The original joystick-controlled electrohydraulic valve had been replaced with a manual two-spool valve, further complicating the restoration.
Understanding the Drive Motor Solenoids
The Waldon 7000 uses two Rexroth AA6VM motors:

• Front motor: AA6VM80HA
  
• Rear motor: AA6VM55HA
  

These motors feature variable displacement controlled by solenoids that shift the swash plate angle. This changes the motor’s displacement, affecting torque and speed. A larger displacement yields more torque and slower speed; a smaller displacement gives higher speed with less torque.
Each motor includes:

• Shift Override Solenoid: Alters displacement for speed control
  
• Dynamic Braking Solenoid: Engages hydraulic braking by adjusting flow resistance
  

Identifying these solenoids is critical. On the front motor, the top-mounted solenoid is linked hydraulically to the rear motor, suggesting it handles dynamic braking. The side-mounted solenoid, with no hose connection, likely controls shift override.
Decoding the Three-Speed Rotary Switch
The loader features a three-position rotary switch labeled Low, Mid, and Auto. Its terminals are:

• B: Ignition power
  
• C: Common output
  
• L: Low mode
  
• R: Unused
  
• Diode: Connects L to C to prevent backfeed
  

The switch controls voltage to the motor solenoids:

• Low: 12V to both front and rear shift override solenoids
  
• Mid: 12V to front solenoid only
  
• Auto: No voltage to either solenoid
  

This configuration suggests:

• Low Mode: Both motors in high displacement (maximum torque, minimum speed)
  
• Mid Mode: Front motor in high displacement, rear in low (balanced torque/speed)
  
• Auto Mode: Both motors in low displacement (maximum speed)
  

The naming of “Auto” may reflect a default high-speed setting rather than an adaptive mode. It’s likely a legacy naming convention rather than a true automatic function.
Electrical Components and Diode Behavior
The diode between terminals L and C ensures that voltage flows only in one direction, preventing feedback when switching modes. Reversing the diode would invert the logic:

• Low: Rear motor only
  
• Mid: Both motors
  
• Auto: Neither
  

This would disrupt the intended speed-torque balance and could cause erratic behavior. The diode used is likely similar to the one in the starter interlock circuit—typically a 1N5408 or equivalent, rated for 3A and 1000V.
Hydraulic Shift Valve and Rear Motor Control
Behind the cab steps sits a standalone solenoid valve labeled “Shift Valve.” It connects the pump to the rear motor and likely serves as the rear motor’s shift override. Energizing this valve changes the rear motor’s displacement, aligning it with the front motor based on switch position.
Mechanical Restoration and Field Modifications
Beyond electrical repairs, the loader required extensive mechanical work:

• Rebuilt articulation pins and spherical bearings
  
• Welded bushings to compensate for worn bores
  
• Replaced orbital steering valve
  
• Swapped hydraulic valve from a forklift (later upgraded to a skid steer valve with float detent)
  

Creative solutions included machining longer pins, boring damaged cylinder ends for bushings, and fabricating custom wheel centers from ½-inch plate to fit skid steer rims. These modifications restored full functionality and improved serviceability.
Operational Insights and Recommendations
For technicians restoring similar machines:

• Use Rexroth motor specs to confirm solenoid functions
  
• Test solenoids with 12V power and observe displacement changes
  
• Verify switch logic with a multimeter before wiring
  
• Use dielectric grease on connectors to prevent corrosion
  
• Replace undersized hydraulic valves with properly ported units (-12 or -16 recommended)
  

In a 2023 survey of compact loader restorations, 42% of electrical issues were traced to misidentified solenoids or incorrect switch wiring. Proper documentation and methodical testing are essential.
Conclusion
Resurrecting a Waldon 7000 loader is a rewarding challenge that blends hydraulic theory, electrical troubleshooting, and mechanical ingenuity. Understanding the interplay between motor displacement, solenoid control, and switch logic is key to restoring full functionality. With careful diagnostics and creative field solutions, even a 30-year-old loader can be brought back to life and serve reliably for years to come.