What is Komatsu PC138USLC-2 Fault Code E108?

Fault Code E108 on the Komatsu PC138USLC-2 indicates a hydraulic pump solenoid valve circuit malfunction, specifically related to the pump control system's electrical signal failing to reach proper specifications. This code triggers when the ECM (Engine Control Module) detects abnormal voltage or resistance in the solenoid circuit that regulates hydraulic pump displacement and flow.

This fault is critical because the PC138USLC-2 relies on precise electronic control of its variable displacement hydraulic pump to optimize fuel efficiency and hydraulic performance. When E108 appears, the excavator typically enters a derate mode where hydraulic power output is reduced to protect the pump control system. For used excavators, this code often surfaces due to age-related deterioration of electrical components, particularly in the harsh operating environments these machines endure.

Common Symptoms

• Reduced hydraulic power across all functions (boom, arm, bucket, and swing movements feel sluggish)
• Amber warning light illuminated on the instrument cluster with fault code displayed on the monitor panel
• Intermittent loss of hydraulic response, especially during cold starts or after extended idle periods
• ECM derate mode activation, limiting engine RPM to protect the hydraulic system
• Erratic pump noise or unusual whining sounds from the main hydraulic pump area

Potential Causes

The most common technical causes for E108 on used PC138USLC-2 excavators include:

• Failed pump control solenoid valve due to internal coil breakage or contamination from degraded hydraulic oil
• Damaged wiring harness near the pump mounting area where vibration and heat cause insulation breakdown (known wear point on this model)
• Corroded or loose electrical connectors at the pump solenoid, particularly the 3-pin connector susceptible to moisture ingress
• ECM internal fault or corrupted programming affecting solenoid driver circuits
• Low supply voltage from failing alternator or weak batteries causing insufficient solenoid actuation
• Hydraulic contamination causing mechanical binding of the solenoid spool, increasing electrical resistance

How to Troubleshoot and Fix Code E108

Step 1: Perform Visual Inspection Begin by inspecting the pump control solenoid harness running along the right side of the engine. Check for chafing against the engine block or hydraulic lines—a notorious issue on used PC138USLC-2 machines. Examine the solenoid connector for corrosion, moisture, or bent pins. Clean connections with electrical contact cleaner and apply dielectric grease.

Step 2: Test Solenoid Resistance and Voltage Using a digital multimeter, disconnect the solenoid connector and measure resistance across the solenoid terminals. Specification should be 12-18 ohms at room temperature. Next, check supply voltage with key-on, engine-off—should read 12V minimum. With engine running, verify the ECM command signal cycles between 0-12V during operation.

Step 3: Verify Hydraulic System Condition Test hydraulic oil quality for contamination that could mechanically bind the solenoid. On used excavators, metal particles from worn pump components often cause intermittent E108 codes. Replace the suction strainer and hydraulic filter if oil shows metallic debris.

Step 4: ECM Diagnostics and Component Replacement Connect Komatsu KOMTRAX or aftermarket diagnostic software to check for intermittent electrical faults stored in ECM memory. If resistance and voltage tests pass but code persists, replace the pump control solenoid valve (Komatsu part typically $350-600). For persistent issues after solenoid replacement, ECM reprogramming or replacement may be necessary.

Disclaimer: This guide provides general troubleshooting steps. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnostic assistance for complex electrical issues, especially when working with used equipment where multiple interrelated failures may exist.