Solenoid Valve Mix-Up

[MikePhua]

On a 2007-year-model Takeuchi TB135 excavator, the two crucial solenoid valves—one for lock-out function and another for two-speed travel—turned out to share identical internal electrical characteristics, differing only by connector type and part number. Technicians confirmed this by measuring electrical resistance and confirming similar readings, indicating shared coil windings despite differing harness plugs.
Terminology Explained

• Solenoid valve refers to an electrically operated valve that directs hydraulic flow when energized.
  
• Lock-out mode prevents unintended machine operation—often used to disable certain hydraulic functions for safety or transport.
  
• Two-speed travel allows the excavator to switch between low and high travel speeds for fine control or quick movement.
  
• Coil resistance testing helps determine if two solenoids are internally identical, despite differing connectors.
  

A Diagnostic Story
In remote Alaska, an operator replaced both solenoid valves after malfunctioning hydraulics. Unexpectedly, hydraulic functions only activated when the “jack-rabbit” high-speed switch was engaged—something clearly wrong. A more experienced technician pointed out that the wiring had likely been swapped: though the replacement valves were interchangeable internally, the differently keyed connectors demanded correct harness placement. A little detective work later revealed the wiring swap—and operation returned to correct behavior.
Specifications Snapshot

• Machine: Takeuchi TB135 mini-excavator
  
• Year: 2007, Serial No. 135-19406
  
• Hydraulic solenoid valves: lock-out and two-speed travel
  
• Coil resistance: identical between valves
  
• Symptom: hydraulics only operational in two-speed high mode
  

Solutions and Advice

• Verify wiring harness alignment carefully when replacing solenoids—don’t rely solely on plug shape to infer correct placement.
  
• Swap coils only if absolutely certain—any mix-up can result in erratic hydraulic behavior or safety issues.
  
• Use a multimeter to test coil resistance before installation—matching values confirm internal similarity, but wiring orientation still matters.
  
• Check for diagnostic clicks or magnetization when supplying 12-V power to each coil to verify operation prior to installation.
  
• Keep schematic reference handy—a repair or workshop manual specific to TB135 models simplifies troubleshooting (these manuals often include valve wiring diagrams and part numbers) .
  

Broader Context and Company Background
The Takeuchi TB135 belongs to the renowned TB100 series of compact excavators, produced in variants spanning the early 2000s. The TB135 typically weighs around 3.43 tonnes (approximately 3,430 kg), has a track width of 350 mm, a transport length close to 4.93 m, and a digging depth of roughly 3.42 m . Takeuchi Manufacturing, the Japanese company behind it, pioneered the compact excavator in 1971 and remains a leading innovator in high-quality, durable construction machinery .
Because Takeuchi supports older models, engine parts, hydraulics, and solenoid assemblies remain available through dealer networks, making servicing possible even for machines well over a decade old .
Closing Thought
This case of the switched solenoid wiring serves as a reminder that even when replacement parts are technically identical, harness wiring details can make all the difference. Simple resistance testing paired with thoughtful wiring alignment can save downtime—and confusion on the job site. Always double-check connectors, follow schematic guidance, and you’ll help keep your mini excavator operating safely and smoothly.