ASV SR80 Hydraulic Failure and Pressure Loss Investigation

[MikePhua]

The SR80 and ASV’s Compact Track Loader Evolution
The ASV SR80, introduced in the mid-2000s, was part of ASV’s push into high-performance compact track loaders designed for demanding terrain and precision control. With a 3,500 lb rated operating capacity and a Cummins 2.8L turbo diesel engine producing 80 horsepower, the SR80 was engineered for forestry, grading, and utility work. ASV, founded in 1983 and later acquired by Terex and then Yanmar, built its reputation on suspended undercarriages and high-flow hydraulic systems tailored for rugged environments.
The SR80 features a tandem hydraulic pump system, a pilot-operated joystick control scheme, and a charge pump that supplies pressure to the drive and loader circuits. When hydraulic pressure drops or fails entirely, the machine becomes inoperable, and pinpointing the cause requires a methodical approach.
Terminology annotation:
- Tandem pump: A dual-section hydraulic pump that supplies separate circuits, such as drive and loader functions.
- Charge pump: A smaller hydraulic pump that maintains pressure in the pilot and control circuits, enabling valve actuation.
Initial Failure and Mechanical Breakdown
One documented case involved a complete hydraulic shutdown. Upon inspection, the tandem pump showed zero output pressure, despite the charge pump reading a healthy 450 psi. The root cause was traced to broken bolts between the engine mounting plate and the tandem pump. This allowed the pump shaft to disengage, resulting in no rotation and no pressure generation.
Repair steps included:

• Replacing the pump shaft, bearing, and seal using Rexroth components
  
• Reinstalling the tandem pump and verifying alignment
  
• Refilling the system with recommended hydraulic fluid
  
• Testing standby pressure, which returned to 218 psi
  

Suggestions:

• Use thread-locking compound on mounting bolts to prevent vibration-induced failure
  
• Inspect shaft splines for wear during reassembly
  
• Torque bolts to spec and recheck after 10 hours of operation
  

Terminology annotation:
- Standby pressure: The baseline pressure maintained in a hydraulic circuit when valves are closed and no flow is demanded.
- Spline: A series of ridges on a shaft that engage with matching grooves in a coupling to transmit torque.
Secondary Failure and Pressure Drop
Despite initial success, the machine failed to operate the next morning. Charge pressure had dropped to 100 psi, well below the required 450 psi for pilot control. Tandem pump pressure remained at 218 psi, indicating that the loader circuit was intact but the drive system was disabled.
Troubleshooting steps included:

• Capping drive motors to isolate pressure loss
  
• Inspecting the charge relief valve (475 psi spec) for sticking
  
• Disassembling the charge pump and noting irregular wear
  
• Verifying pressure at test port A in the engine bay
  

Recommendations:

• Flow test the charge pump at a hydraulic shop (15.4 GPM @ 2600 rpm, 475 psi)
  
• Replace worn internal components and seals
  
• Check seat and restraint bar switches for electrical faults affecting hydraulic enablement
  

Terminology annotation:
- Charge relief valve: A pressure-limiting valve that protects the charge circuit from overpressure.
- Pilot control: A low-pressure hydraulic signal used to actuate main valves and control machine functions.
Electrical Interlocks and Safety System Influence
The SR80 includes seat and seatbelt switches as part of its safety system. If these switches are bypassed or malfunctioning, the hydraulic system may not engage properly. In some cases, disabling these controls can lead to unpredictable behavior, especially if the system expects a signal to enable pilot pressure.
Checklist:

• Verify continuity of seat switch and restraint bar sensor
  
• Check for voltage at solenoid valves during startup
  
• Inspect wiring harness for damage or corrosion
  
• Reset safety interlocks and test with OEM configuration
  

Suggestions:

• Use dielectric grease on connectors to prevent moisture intrusion
  
• Replace damaged harness sections with shielded wire
  
• Avoid bypassing safety systems unless diagnostic testing confirms necessity
  

Terminology annotation:
- Dielectric grease: A non-conductive lubricant that protects electrical connections from corrosion and moisture.
- Solenoid valve: An electrically actuated valve that controls hydraulic flow based on input signals.
Fluid Contamination and Cold Weather Behavior
Another contributing factor to hydraulic failure is fluid contamination, especially water ingress. In cold climates, water in the hydraulic tank can freeze or emulsify, reducing flow and damaging components. The SR80’s joystick controls are hydraulic-over-hydraulic, meaning fluid constantly circulates through the joystick body and can stagnate if contaminated.
Preventative measures:

• Drain and flush hydraulic lines one at a time to remove stagnant fluid
  
• Use water-separating additives in diesel fuel to prevent fuel system blockage
  
• Install a moisture indicator on the hydraulic reservoir
  
• Replace filters regularly and monitor fluid clarity
  

Terminology annotation:
- Emulsification: The mixing of water and oil into a milky suspension that reduces lubrication and flow efficiency.
- Hydraulic-over-hydraulic: A control system where hydraulic pressure is used to actuate other hydraulic valves, rather than electrical signals.
Conclusion
The ASV SR80’s hydraulic system is a complex interplay of mechanical, electrical, and fluid dynamics. When pressure drops or functions fail, the root cause may lie in broken shafts, worn pumps, faulty safety switches, or contaminated fluid. By methodically inspecting each subsystem—starting with mechanical drive, then charge pressure, then electrical interlocks—technicians can restore full operation and prevent future failures. In compact track loaders, precision and pressure go hand in hand, and the SR80 demands both to perform at its peak.