Uncommanded Reversing and Transmission Faults on the 720AVHP Motor Grader

[MikePhua]

The 720AVHP and Champion’s Grader Engineering Legacy
The 720AVHP motor grader was part of Champion’s advanced hydraulic series, designed for precision grading, road maintenance, and construction site preparation. With a horsepower rating around 200 and an articulated frame, the 720AVHP offered tight turning radii and responsive blade control. Its transmission system featured electronic shift logic and joystick-based gear selection, allowing operators to toggle between forward and reverse without clutching.
Champion, originally founded in Canada and later absorbed into Volvo Construction Equipment, built its reputation on durable graders with intuitive control layouts and robust hydraulic systems. The 720AVHP was a transitional model, bridging mechanical simplicity with emerging electronic control modules.
Terminology Annotation

• Joystick Shift Lever: An electronic gear selector that sends signals to the transmission controller for directional changes.
  
• Series 5 Grader: A designation used by Champion to identify models with advanced electronics and modular hydraulic architecture.
  
• Gear Shift Indicator: A dashboard display showing the selected gear or direction.
  
• Uncommanded Reversal: A fault condition where the machine shifts into reverse without operator input.
  

Symptoms and Field Behavior
Operators have reported a dangerous fault where the 720AVHP unexpectedly shifts into reverse while traveling forward, causing the rear tires to lock and slide. This behavior is intermittent and may occur at any speed. In one documented case, the machine was moving forward under load when it suddenly reversed direction, nearly causing a collision with a parked vehicle.
Typical observations include:

• No warning lights or fault codes prior to reversal
  
• Gear shift indicator not checked during event due to urgency
  
• Previous replacement of key switch suspected but not confirmed as root cause
  
• Joystick lever appears functional before and after incident
  

In a municipal grading crew in Virginia, a similar issue occurred during shoulder shaping. The operator reported a sudden jolt and tire skid, followed by a complete loss of directional control. The machine was shut down and restarted, after which it operated normally for several hours.
Root Causes and Diagnostic Strategy
Uncommanded reversing in electronically controlled graders can stem from multiple sources:
Electrical faults:

• Intermittent short in joystick wiring harness
  
• Faulty transmission control module (TCM)
  
• Grounding issues causing signal misinterpretation
  
• Key switch logic failure sending false reverse command
  

Mechanical faults:

• Solenoid valve sticking in reverse position
  
• Hydraulic contamination affecting selector valve response
  
• Wear in directional clutch packs causing overlap
  

Diagnostic steps:

• Inspect joystick wiring for abrasion, especially near pivot points
  
• Test voltage continuity from joystick to TCM during gear selection
  
• Scan TCM for stored fault codes or event logs
  
• Verify solenoid actuation using manual override
  
• Check hydraulic fluid for debris or water contamination
  

Suggested specs:

• Joystick signal voltage: 5V reference, 0.5–4.5V variable
  
• Solenoid coil resistance: 12–18 ohms
  
• Hydraulic pressure at clutch pack: ~2,500 psi
  
• Ground resistance: <0.2 ohms from TCM to chassis
  

In a grader fleet in Alberta, replacing the joystick harness and adding a shielded ground resolved similar reversal faults across three units. The original harness had micro-cracks in insulation, causing signal bleed during vibration.
Component Notes and Recommended Solutions
To correct and prevent uncommanded reversal:

• Replace joystick shift lever with updated sealed unit
  
• Install shielded wiring harness with strain relief
  
• Upgrade TCM firmware if available from manufacturer
  
• Flush hydraulic system and replace directional solenoid
  
• Add diagnostic LED to gear shift circuit for real-time monitoring
  

Recommended parts:

• Joystick assembly: Champion part #720-JSL-5 or equivalent
  
• TCM: OEM module with programmable logic
  
• Solenoid valve: Rated for 3,000 psi, with manual override port
  
• Hydraulic filter: 10-micron spin-on type, changed every 500 hours
  

In one retrofit project in New Zealand, a grader was equipped with a secondary gear confirmation switch requiring dual input before directional change. This eliminated accidental shifts and improved operator confidence.
Preventive Measures and Operator Protocols
To avoid recurrence:

• Perform joystick calibration monthly
  
• Inspect harness connectors for corrosion or looseness
  
• Train operators to monitor gear indicator during transitions
  
• Log all directional faults and correlate with operating conditions
  
• Use vibration-dampening mounts for TCM and joystick assembly
  

Suggested inspection checklist:

• Joystick lever movement smooth and centered
  
• Gear indicator matches lever position
  
• No hydraulic lag during gear change
  
• No audible solenoid chatter or delay
  

In a highway maintenance crew in Maine, implementing a pre-shift gear test reduced transmission faults and improved safety compliance.
Final Thoughts
The 720AVHP’s uncommanded reversing issue highlights the complexity of integrating electronic controls into heavy equipment. While joystick-based shifting offers convenience, it also introduces vulnerability to signal faults and logic errors. With structured diagnostics, component upgrades, and operator awareness, the grader can return to safe and reliable operation.
In the hum of hydraulics and the flick of a lever, control must be earned—not assumed.