Why Won’t the Kobelco 135SR Excavator Start

[MikePhua]

The Kobelco 135SR and Its Engineering Background
The Kobelco 135SR is a short-radius hydraulic excavator designed for urban construction, utility trenching, and confined job sites. Introduced as part of Kobelco’s SR series, it features a compact tail swing, advanced hydraulic control, and a fuel-efficient diesel engine. Kobelco, founded in Japan in 1930, has built a reputation for smooth hydraulic performance and operator-friendly design. The 135SR is powered by a four-cylinder diesel engine, often a Mitsubishi or Isuzu variant depending on market, and includes electronic monitoring systems to manage engine and hydraulic functions.
Despite its reliability, the 135SR can experience starting issues—especially as machines age or undergo inconsistent maintenance. Diagnosing a no-start condition requires a methodical approach, beginning with the basics and moving into electrical and fuel systems.
Initial Checks and Common Oversights
Before diving into complex diagnostics, operators should verify:

• Battery voltage and condition
  
• Clean and tight terminal connections
  
• Functional starter relay and fuse integrity
  
• Proper fuel level and filter condition
  
• Neutral safety switch engagement
  
• Key switch operation and dashboard response
  

Even with 24 volts showing on a multimeter, a weak battery under load can fail to deliver sufficient cranking amperage. A load test is essential. One technician in Ontario discovered that a battery showing full voltage would drop below 9 volts under starter load—causing the ECM to shut down ignition sequencing.
Electrical System Vulnerabilities
The 135SR’s electrical system includes:

• ECM (Engine Control Module)
  
• Monitor panel with diagnostic display
  
• Starter relay and solenoid
  
• Fuse box and wiring harness
  
• Grounding straps and frame connections
  

A faded or sun-damaged monitor panel can obscure fault codes, making diagnosis harder. If the screen is unreadable, technicians should check for stored codes using a handheld scanner or by testing voltage at key sensors. Ground faults are especially common—corroded or loose ground straps can interrupt signal flow, preventing the starter from engaging.
In one case, a machine in Georgia failed to start due to a broken ground wire beneath the cab. The operator had replaced the starter and batteries without success. Once the ground was repaired, the machine fired immediately.
Fuel System and Combustion Readiness
Diesel engines require:

• Clean fuel delivered at proper pressure
  
• Air-free lines and primed injectors
  
• Functional lift pump and injection pump
  
• Operational glow plugs or intake heaters in cold weather
  

Air intrusion is a frequent culprit. If filters were recently changed or lines disconnected, air pockets can prevent fuel from reaching the injectors. Bleeding the system manually or using the electric lift pump (if equipped) can restore flow.
Cold weather compounds the issue. In freezing conditions, fuel gelling or thickened oil can slow cranking speed. Installing a block heater or switching to synthetic oil improves cold starts. A crew in Minnesota added a timer-controlled block heater to their 135SR fleet, reducing failed starts by 80% during winter months.
Starter Motor and Relay Behavior
If the starter clicks but doesn’t engage:

• Check solenoid voltage during key turn
  
• Inspect starter motor for brush wear or internal corrosion
  
• Test relay function by bypassing with a jumper wire
  
• Confirm that the neutral safety switch is aligned and responsive
  

Some operators report success by grounding the starter relay manually to test engagement. If the starter spins but the engine doesn’t fire, the issue likely lies in fuel delivery or ECM coordination.
Sensor Feedback and ECM Logic
Modern excavators rely on sensor input to authorize startup. Faulty readings from:

• Oil pressure sensor
  
• Coolant temperature sensor
  
• Throttle position sensor
  
• Crankshaft or camshaft position sensors
  

can prevent ignition. If the ECM detects implausible values—such as zero oil pressure—it may block startup to protect the engine. Replacing or bypassing the sensor temporarily can confirm diagnosis.
A technician in Texas found that a faulty coolant sensor was reporting extreme temperatures, causing the ECM to abort startup. Replacing the sensor restored normal function.
Recommendations for Persistent No-Start Conditions
If basic checks fail:

• Use a wiring diagram to trace voltage from battery to starter
  
• Inspect all relays and fuses visually and with a multimeter
  
• Test fuel pressure at the rail or injector inlet
  
• Scan for fault codes using compatible diagnostic tools
  
• Check ECM power and ground continuity
  
• Consult service bulletins for known issues with the 135SR model
  

Documenting each step helps avoid redundant work and speeds up troubleshooting. Keeping a log of voltage readings, component swaps, and test results can reveal patterns.
Conclusion
A Kobelco 135SR that won’t start is rarely a mystery—it’s a puzzle with clues hidden in voltage drops, sensor signals, and fuel flow. With patience, proper tools, and a systematic approach, even the most stubborn no-start condition can be resolved. In the field, downtime is costly—but knowledge is power. And in the cab of a 135SR, power begins with a clean start.