Diagnosing a No-Start Condition on the Komatsu PC100-6 Excavator

[MikePhua]

Background on the Komatsu PC100-6
The Komatsu PC100-6 is a mid-sized hydraulic excavator introduced in the 1990s as part of Komatsu’s sixth-generation lineup. Known for its mechanical reliability and straightforward design, the PC100-6 was widely used in utility trenching, roadwork, and small-scale earthmoving. It featured a Komatsu S4D102E diesel engine, delivering around 75 horsepower, and a robust hydraulic system with pilot controls. Despite its durability, age and long-term storage can lead to electrical and starting issues, especially when the machine sits idle for extended periods.
Symptoms of the No-Start Issue
In one case, a PC100-6 that had been parked for over two years failed to start despite having new batteries installed. When the ignition key was turned to the crank position, all power dropped from the console, and voltage at the starter terminal fell to just 2.5 volts. This dramatic voltage drop suggested a high-resistance fault or a failed relay in the starting circuit.
The operator identified a rusted electrical component near the battery positive cable, with two small white wires and a large terminal feeding the starter. This part was later confirmed to be the battery disconnect relay, a common feature in Japanese-built excavators.
Understanding the Battery Disconnect Relay
The battery disconnect relay is a solenoid-operated switch that isolates the starter and main electrical circuits until the ignition key is turned. It prevents parasitic drain and adds a layer of safety by cutting power when the machine is off. On Komatsu machines, this relay typically engages when the key is turned, allowing full voltage to reach the starter and control panel.
When rust or corrosion infiltrates the relay, it can cause:

• High resistance across the contacts
  
• Incomplete engagement of the solenoid
  
• Voltage drop under load
  
• Loss of power to the starter and monitor panel
  

In this case, the relay allowed 25 volts in but only delivered 2.5 volts to the starter when cranking—clearly indicating internal failure.
Temporary Bypass and Safety Precautions
To confirm the diagnosis and attempt a temporary fix, the operator can:

• Disconnect the battery ground to prevent arcing
  
• Move the starter cable from the relay’s output terminal to the input terminal with full voltage
  
• Reconnect the battery ground and attempt to start the engine
  

This bypasses the relay and allows direct power to the starter. However, it should only be used for testing or emergency starts. For long-term reliability, the relay should be replaced with a factory unit, typically costing around $100.
Electrical Testing and Load Verification
Voltage readings alone can be misleading. A circuit may show full voltage with no load, but drop to near zero when a load is applied. To properly test:

• Use a voltmeter with an assistant turning the key
  
• Start at the battery and follow the circuit to the starter
  
• Check each connection under load, not just static voltage
  
• Clean and tighten all terminals, especially ground points
  

This method helps identify weak connections, corroded terminals, or damaged wires that may not be obvious during visual inspection.
Additional Considerations for Long-Stored Equipment
Machines that sit idle for years often develop multiple issues:

• Battery sulfation and internal resistance
  
• Corroded terminals and relay contacts
  
• Stuck fuel injectors or gummed fuel lines
  
• Moisture intrusion into electrical connectors
  

Before attempting a full start, it’s wise to:

• Charge and test batteries individually
  
• Inspect and clean all electrical connections
  
• Prime the fuel system and check for water contamination
  
• Verify oil and coolant levels
  

Conclusion
A no-start condition on a Komatsu PC100-6 is often caused by a failed battery disconnect relay, especially after long-term storage. By understanding the function of this relay and using proper load-testing techniques, operators can isolate the fault and restore starting capability. While temporary bypasses may help in the field, replacing corroded components is essential for safe and reliable operation. With methodical troubleshooting and attention to electrical integrity, even an old excavator can be brought back to life.