Komatsu PC120-6 Fault Code E104: Complete Diagnostic Guide

What is Komatsu PC120-6 Fault Code E104?

Fault Code E104 indicates a malfunction in the pump control solenoid valve circuit within the hydraulic system. This code specifically signals that the Engine Control Module (ECM) has detected an abnormal voltage or current flow in the electrical circuit controlling the main hydraulic pump's output regulation.

The pump control solenoid is critical for the PC120-6's performance because it regulates hydraulic flow based on operator demand and engine load. When this circuit fails, the excavator cannot properly modulate pump displacement, resulting in reduced efficiency and potential over-pressurization or inadequate hydraulic power delivery.

Common Symptoms

When Code E104 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Warning light illumination on the instrument cluster (typically the action lamp or diagnostic indicator)
• Erratic hydraulic response where attachment speed doesn't match joystick input
• Engine RPM fluctuation under load due to improper pump-engine communication
• Machine entering derate mode, limiting maximum engine speed to protect components

Potential Causes

The most common technical failures triggering E104 on used PC120-6 excavators include:

• Solenoid coil failure due to internal short circuit or open winding (resistance typically should measure 10-15 ohms)
• Wiring harness damage near the hydraulic pump mounting area where vibration causes conductor breakage
• Corroded connectors at the solenoid plug, especially on machines operated in wet or coastal environments
• ECM internal driver circuit failure, though less common than field-side issues
• Voltage supply problems from the main power harness or blown fuses in the electrical junction box

How to Troubleshoot and Fix Code E104

Step 1: Visual Inspection Locate the pump control solenoid mounted on the main hydraulic pump (left side of engine). Inspect the wiring harness for abrasion points where it contacts the pump body or frame. Check the connector for corrosion, moisture intrusion, or pushed-back pins—common issues on used excavators with 5,000+ operating hours.

Step 2: Electrical Testing Using a digital multimeter, disconnect the solenoid connector and measure coil resistance between the two terminals. Specification should be 10-15 ohms. If open (infinite resistance) or shorted (near zero), replace the solenoid valve assembly (Komatsu part #702-21-55901).

Step 3: Circuit Verification With ignition ON and solenoid disconnected, check for battery voltage (approximately 24V on PC120-6) at the power supply wire. Verify ground continuity on the return circuit to the ECM. Voltage drop should not exceed 0.5V on ground circuits.

Step 4: ECM Output Test If solenoid and wiring test correctly, use Komatsu diagnostic software (KOMTRAX or dealer-level tools) to command solenoid activation while monitoring current draw. Normal operation draws 1.5-2.0 amps. No current flow indicates ECM driver failure requiring controller replacement.

Critical Note for Used Machines: Before replacing components, thoroughly clean all electrical connections with contact cleaner and apply dielectric grease. Many E104 codes on older excavators resolve through connector maintenance alone, saving hundreds in unnecessary parts replacement.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number range and consider professional diagnostic assistance for complex electrical issues.