Accessing and Servicing Electrical Relays on the Takeuchi TB135

[MikePhua]

Takeuchi’s Compact Excavator Design Philosophy
Takeuchi Manufacturing, founded in Japan in 1963, pioneered the compact excavator market with the introduction of the world’s first mini-excavator in 1971. The TB135, part of the TB100 series, was designed for tight-space excavation, utility trenching, and light demolition. With an operating weight of approximately 3,500 kg and a dig depth of over 3 meters, the TB135 became a popular choice among contractors and rental fleets for its hydraulic finesse and mechanical reliability.
However, like many compact machines, the TB135’s electrical system is tucked into tight compartments, making access to relays and fuses a challenge—especially when troubleshooting power loss to components like the fuel stop solenoid or horn.
Relay Compartment Location and Access Challenges
The TB135 houses its electrical relays within a metal enclosure located beneath the left-hand control lever assembly. This compartment is shielded by hydraulic lines that run directly across the access plate, making direct entry difficult without partial disassembly.
Operators attempting to diagnose electrical faults—such as no power to the fuel solenoid or horn—often find that the fuse itself is intact and energized, pointing to a failed relay or corroded contact. Unfortunately, the relay bank is not easily reachable from above or below without removing the hydraulic lines connected to the left joystick controller.
Cab Tilt and Hydraulic Line Removal
Tilting the cab does not provide sufficient clearance to reach the relay compartment. The only viable method is to disconnect the hydraulic lines obstructing the access plate. While this may seem daunting, the process is manageable with proper preparation:
Recommended steps:

• Label each hydraulic line before removal using numbered tags or colored tape
  
• Depressurize the hydraulic system by cycling controls with the engine off
  
• Use absorbent pads and drip trays to catch residual fluid
  
• Loosen fittings with line wrenches to avoid rounding flare nuts
  
• Remove the access plate and inspect the relay bank for corrosion or loose terminals
  

Once the relays are exposed, each should be tested with a multimeter or swapped with a known-good unit. Dielectric grease can be applied to terminals to prevent future oxidation.
Electrical Fault Isolation and Relay Testing
When diagnosing electrical faults in the TB135:

• Confirm voltage at the fuse terminal using a test light or multimeter
  
• Trace wiring from the fuse block to the relay and then to the component
  
• Check relay coil resistance (typically 70–100 ohms)
  
• Verify continuity across the switch terminals when energized
  
• Inspect ground paths for rust or loose bolts
  

Relays controlling the fuel solenoid and horn may share a common ground or ignition circuit. A failed relay can interrupt both functions simultaneously, even if the fuse remains live.
Anecdote from Oregon
An operator in Oregon faced a no-start condition due to lack of power at the fuel solenoid. After verifying fuse integrity, he suspected a relay failure. Accessing the relay bank required removing all hydraulic lines from the left controller—a tedious but necessary step. Once inside, he found a corroded relay socket and replaced the unit. The machine started immediately, and the horn function was restored. He noted that while the process wasn’t enjoyable, it was far less complex than expected.
Recommendations for Future Maintenance
To simplify future electrical service:

• Relocate high-frequency relays to a more accessible location using extension harnesses
  
• Install a hinged access panel with quick-release fasteners
  
• Use waterproof relay sockets and sealed connectors
  
• Maintain a wiring diagram annotated with relay functions and fuse assignments
  
• Periodically inspect and clean electrical compartments during scheduled service
  

For fleet operators, standardizing relay types and stocking spares can reduce downtime and improve troubleshooting efficiency.
Conclusion
Accessing the relay bank on a Takeuchi TB135 requires patience and mechanical care, but it’s a solvable challenge. By removing hydraulic lines and exposing the compartment beneath the left controller, operators can diagnose and repair electrical faults that affect critical systems like the fuel solenoid and horn. With thoughtful labeling, clean work practices, and preventive upgrades, the TB135’s electrical system can be maintained reliably—even in the tightest corners of its compact frame. In the world of mini-excavators, persistence and preparation go hand in hand.