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The Komatsu PC220-5 is a classic hydraulic excavator from the late 1980s to early 1990s, powered by the Komatsu 6D95L diesel engine. Its electrical system is relatively simple compared to modern machines, relying on analog sensors, solenoids, and a basic monitor panel. Despite its age, the PC220-5 remains popular in many regions due to its mechanical reliability and ease of repair.

The machine’s starting and throttle control systems are tied into a network of relays, solenoids, and the alternator. A small wire connected to the alternator plays a critical role in energizing the safety relay and controlling engine shutdown. When this wire malfunctions—often due to a failed diode—it can cause a cascade of electrical issues.

Symptoms and Initial Observations

Operators reported the following symptoms:

  • Engine dropped to idle during operation and throttle became unresponsive
  • Ignition key failed to shut off the engine
  • Machine would not restart using the key, but could be started by bridging the starter
  • Error codes displayed: S2 and 08
  • Replacing the ignition switch did not resolve the issue

These symptoms pointed to a fault in the electrical control circuit, not the mechanical fuel system.

Interpreting Error Codes

  • S2: Indicates the monitor panel is in “S” mode with power setting 2. This is normal and not a fault.
  • 08: Refers to a fault in the TVC (Throttle Valve Control) solenoid located on the hydraulic pump. This solenoid adjusts pump output based on engine speed and load.

While the 08 code suggests a solenoid issue, it does not directly cause the no-start or no-shutdown behavior. The root cause lies deeper in the electrical system.

Alternator Diode Failure and Safety Relay Behavior

The alternator on the PC220-5 includes a small wire—often labeled “R”—that supplies voltage to the safety relay when the key is turned. If the diode inside the alternator fails, this wire may backfeed voltage or fail to energize the relay properly.

Testing revealed:

  • Battery voltage present on terminals R and B with ignition on
  • Disconnecting the R wire allowed the machine to start normally
  • Excessive play in the throttle linkage contributed to shutdown failure

This confirmed that the alternator’s internal diode was leaking, causing the safety relay to malfunction. The engine could not be shut off via key because the relay remained energized, and it could not be started because the starter solenoid was not receiving proper signal.

Repair Options and Alternator Replacement

Operators considered replacing the 35A alternator with a 30A unit due to availability. This is acceptable if no additional electrical loads (e.g., radios, heaters) are present. However, caution is advised:

  • Ensure terminal configuration matches the original
  • Verify mounting bracket compatibility
  • Avoid going below 30A output to maintain battery health

One technician noted that a 20.4V charge rate was insufficient and could damage batteries over time. The correct running voltage should be around 28.4V for a 24V system.

Throttle Linkage and Engine Shutdown

The inability to shut down the engine was traced to excessive play in the throttle linkage. On older Komatsu machines, the throttle cable must fully retract to cut fuel. If the linkage is loose or misaligned, the engine may continue running even when the key is turned off.

Adjusting the linkage and ensuring full travel restored proper shutdown behavior.

Final Thoughts

The PC220-5’s electrical faults were ultimately traced to a failed alternator diode and worn throttle linkage. These issues caused a confusing mix of symptoms—no start, no shutdown, and idle lock—but were resolved with targeted diagnostics. For owners of older Komatsu excavators, understanding the interplay between alternator feedback, safety relays, and mechanical linkages is essential. With basic tools and methodical testing, even complex faults can be resolved without dealer intervention.

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