Why Does the Bobcat S770 Bob-Tach Blow Its Fuse When Locking

[MikePhua]

The Bobcat S770 and Its Attachment System
The Bobcat S770 is a high-performance skid steer loader introduced in the early 2010s as part of Bobcat’s M-Series. With a rated operating capacity of 3,350 lbs and a turbocharged diesel engine producing over 90 hp, the S770 was designed for demanding tasks in construction, agriculture, and snow removal. Bobcat, founded in 1947, has sold millions of loaders globally, and the S770 remains one of its most popular mid-frame models.
One of its standout features is the Bob-Tach system—a hydraulic quick coupler that allows operators to switch attachments without leaving the cab. The system uses two hydraulic cylinders to lock and unlock attachments, controlled by a switch inside the cab. However, electrical faults in this system can lead to blown fuses and inoperability.
Terminology Notes

• Bob-Tach: Bobcat’s proprietary hydraulic quick coupler system for securing attachments.
  
• Lock Function: The action of extending the coupler cylinders to secure the attachment.
  
• Release Function: The action of retracting the cylinders to disengage the attachment.
  
• Fuse Rating: The amperage threshold at which a fuse will break the circuit to prevent damage.
  

Symptoms and Initial Observations
In one documented case, a 2013 Bobcat S770 consistently blew its 25A fuse when the operator pressed the “lock” button on the Bob-Tach switch. The “release” function worked normally, and voltage dropped from 14V to 7.5V during the lock attempt—indicating a short or overload.
This behavior suggests a fault in the lock circuit, possibly due to:

• Internal short in the lock solenoid
  
• Pinched or frayed wiring near the coupler
  
• Faulty switch or relay
  
• Hydraulic cylinder drawing excessive current due to mechanical resistance
  

Diagnostic Strategy and Field Testing

• Check Solenoid Resistance: Use a multimeter to measure ohms across the lock solenoid terminals. A reading below 5 ohms may indicate a short.
  
• Inspect Wiring Harness: Look for abrasion, pinching, or corrosion near the coupler and under the cab.
  
• Test Voltage Drop: Monitor voltage at the solenoid during activation. A sharp drop suggests high current draw or poor ground.
  
• Swap Solenoids: If the release solenoid works, swap it with the lock solenoid to isolate the fault.
  
• Check Cylinder Movement: Manually inspect the coupler cylinders for binding or misalignment that could cause overload.
  

Operator Anecdotes and Practical Advice
One technician in Alberta found that a wire near the coupler had rubbed against the frame, exposing copper and shorting during lock activation. Another operator in Ohio discovered that water ingress had corroded the solenoid terminals, causing intermittent shorts.
A mechanic in Georgia shared that a bent locking pin caused the cylinder to stall, drawing excess current and blowing the fuse. After replacing the pin and lubricating the coupler, the issue disappeared.
Preventive Measures and Recommendations

• Use Dielectric Grease on Terminals: Prevent corrosion and improve contact.
  
• Inspect Coupler Weekly: Check for debris, rust, and mechanical resistance.
  
• Replace Fuses with Correct Rating: Never upsize the fuse to mask a fault.
  
• Label Wiring During Repairs: Avoid confusion when tracing circuits.
  
• Train Operators to Recognize Symptoms: Early detection prevents downtime.
  

Conclusion
Fuse blowouts during Bob-Tach locking on the Bobcat S770 are typically caused by electrical shorts, solenoid failure, or mechanical resistance in the coupler. By methodically testing voltage, inspecting wiring, and verifying cylinder movement, operators can pinpoint the fault and restore functionality. The S770’s quick coupler system is a powerful tool—but only when its electrical and hydraulic components are working in harmony.