What is Komatsu PC120-6 Fault Code E108?

Komatsu PC120-6 Fault Code E108 indicates a malfunction in the hydraulic pump control solenoid circuit or proportional pressure control valve system. This code specifically relates to the electronic control system's inability to properly regulate hydraulic pump output pressure through the electromagnetic proportional valve.

The Electronic Control Module (ECM) monitors the current and voltage supplied to the pump control solenoid valve, which modulates hydraulic pressure based on operational demands. When the ECM detects abnormal electrical values—such as open circuits, short circuits, or resistance outside specified parameters (typically 10-30 ohms)—it triggers E108. This fault is critical because the PC120-6 relies on precise hydraulic pressure control for bucket, arm, and boom operations. Compromised solenoid function leads to inefficient hydraulic response and reduced productivity.

Common Symptoms

• Sluggish hydraulic movements across all functions, particularly noticeable during combined operations (e.g., simultaneous boom lift and bucket curl)
• Engine automatically derating to lower RPMs or entering a protective mode to prevent damage
• Warning lamp illumination on the instrument panel, often accompanied by intermittent buzzer alerts
• Erratic hydraulic response, where controls feel inconsistent or unresponsive during startup
• Increased fuel consumption due to the pump running at maximum displacement regardless of load requirements

Potential Causes

On used PC120-6 excavators, E108 typically stems from age-related electrical degradation:

• Corroded or damaged connector pins at the solenoid valve harness (known wear point near the pump mounting bracket)
• Failed proportional solenoid coil due to internal winding breakdown after 8,000+ operating hours
• Chafed wiring harness where the cable bundle rubs against the hydraulic tank edge—common on machines with replaced pumps
• ECM internal faults or corrupted software, particularly in early production units (serial numbers below 52000)
• Contaminated hydraulic oil causing solenoid valve spool sticking and abnormal current draw
• Poor ground connection at the chassis grounding point behind the operator's cab

How to Troubleshoot and Fix Code E108

Step 1: Visual Inspection
Inspect the solenoid valve connector located on the main hydraulic pump. Check for oil contamination, bent pins, or green corrosion. On used machines, examine the wiring harness routing for abrasion points, especially where it passes near the swing motor.

Step 2: Electrical Testing
Using a digital multimeter, disconnect the solenoid connector and measure resistance across the coil terminals. Specification is 18-24 ohms at 20°C. Values outside this range indicate coil failure. Also check for 12V supply voltage with the ignition on.

Step 3: Connector and Ground Verification
Clean all connector contacts with electrical contact cleaner and apply dielectric grease. Verify chassis ground integrity by measuring less than 0.5 ohms resistance between the ECM ground terminal and battery negative.

Step 4: Advanced Diagnosis
Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to monitor real-time solenoid command vs. actual current draw. Deviations exceeding 15% suggest valve mechanical binding.

For used excavators, always inspect for previous repair attempts, aftermarket harness splices, or universal connectors that increase resistance. Replace the entire harness segment rather than splicing if insulation damage is found.

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Disclaimer: This guide provides general diagnostic information. Always consult Komatsu service manuals and qualified technicians for your specific machine configuration and operating conditions.