Why the AC Compressor Clutch Fails to Engage on a Kubota SVL90-2

[MikePhua]

The Kubota SVL90-2 and Its Cooling System Design
The Kubota SVL90-2 compact track loader, introduced in the early 2010s, was part of Kubota’s push into the high-performance CTL market. With an operating weight of over 11,000 lbs and a rated operating capacity of 2,800 lbs, it quickly became a favorite among contractors for its hydraulic power, cab comfort, and reliability. Kubota, founded in 1890 in Osaka, Japan, has sold tens of thousands of SVL-series machines globally, with the SVL90-2 being one of the most widely adopted in North America.
The SVL90-2 features a fully enclosed cab with HVAC systems designed for year-round operation. Its air conditioning system includes a belt-driven compressor, electric clutch, condenser fan, evaporator core, and a control circuit governed by relays and pressure sensors. When functioning properly, the system provides consistent cooling even in high ambient temperatures and dusty environments.
Terminology Clarification

• AC Compressor Clutch: An electromagnetic device that engages the compressor pulley when cooling is needed.
  
• Freon (Refrigerant): The fluid used in AC systems to absorb and release heat.
  
• Relay Pins 30 and 87: Terminals on a standard automotive relay used to control power flow.
  
• Low-Pressure Switch: A sensor that disables the clutch if refrigerant levels are too low.
  
• Evaporator Core: The component inside the cab that absorbs heat from the air.
  

Common Symptoms and Initial Checks
When the AC clutch fails to engage, the most common symptoms include:

• No cold air from vents despite AC switch activation
  
• Audible click missing when clutch should engage
  
• Compressor pulley spinning freely without load
  
• Warning lights or fault codes on the display panel
  

Before diving into electrical diagnostics, technicians should check:

• Whether the engine is running (clutch won’t engage with engine off)
  
• If the condenser fan is packed with debris, which can blow fuses
  
• If the evaporator core is clogged, reducing airflow and triggering system shutdown
  
• Fuse integrity in the HVAC circuit
  

One technician in Texas noted that the SVL90-2 evaporators tend to clog quickly in dusty environments, especially during land clearing. Cleaning the core restored airflow and allowed the clutch to engage normally.
Bypassing the Relay for Diagnostic Purposes
To test the clutch circuit manually:

• Locate the AC relay in the fuse panel
  
• With the engine running, jump pins 30 and 87 using a fused jumper wire
  
• Listen for the clutch to click and observe whether the compressor engages
  
• If it does, the wiring from relay to clutch is intact
  
• If cold air flows, refrigerant charge is likely sufficient
  
• If no engagement occurs, suspect a failed clutch coil or open circuit
  

This test should only be used momentarily to confirm system integrity. Running the AC system with a bypassed relay can lead to compressor damage if refrigerant is low.
Low Refrigerant and Safety Interlocks
Modern AC systems include low-pressure switches to prevent clutch engagement when refrigerant is low or absent. This protects the compressor from running dry, which can cause internal damage and draw a vacuum into the system.
If the clutch engages during a relay bypass but not during normal operation, and no cold air is produced, the system may be empty. In this case:

• Check for leaks using UV dye or electronic sniffers
  
• Recharge with the correct refrigerant type (typically R-134a)
  
• Monitor pressure readings during operation
  
• Replace damaged O-rings or fittings as needed
  

A technician in Ontario shared that a customer’s SVL90-2 had a slow leak at the compressor manifold. After replacing the seals and recharging, the clutch began engaging normally.
Control Circuit and Switch Diagnostics
If refrigerant levels are good and the clutch engages during bypass but not via the dashboard controls, the issue likely lies in the control circuit. Possible faults include:

• Failed AC switch in the cab
  
• No ground at the relay coil
  
• No power signal from the switch to the relay
  
• Corroded connectors or broken wires
  

Using a multimeter, technicians can verify voltage at the relay coil when the switch is activated. If no voltage is present, trace the circuit back to the cab switch and inspect for continuity.
Preventive Measures and Long-Term Solutions
To avoid future AC clutch failures:

• Clean condenser and evaporator cores regularly
  
• Inspect and replace cabin air filters
  
• Check refrigerant levels seasonally
  
• Protect wiring harnesses from rodent damage and abrasion
  
• Use dielectric grease on connectors to prevent corrosion
  

A contractor in Georgia installed a mesh screen over his condenser fan intake to reduce debris buildup. Since then, his SVL90-2’s AC system has run reliably through multiple summers.
Conclusion
The AC compressor clutch on the Kubota SVL90-2 may fail to engage due to low refrigerant, electrical faults, or clogged airflow components. By methodically checking fuses, relays, refrigerant charge, and control circuits, technicians can pinpoint the issue and restore cooling performance. With proper maintenance and occasional diagnostics, the SVL90-2’s HVAC system can remain dependable even in the harshest jobsite conditions.