What is Komatsu PC120LC-6 Fault Code E106?

Fault Code E106 on the Komatsu PC120LC-6 indicates a malfunction in the swing motor brake solenoid circuit. This code is triggered when the Electronic Control Module (ECM) detects an abnormal electrical condition—either an open circuit, short circuit, or incorrect voltage signal—in the wiring or solenoid valve that controls the swing motor's braking function.

The swing brake solenoid is critical for safe operation, as it controls the engagement and disengagement of the swing motor brake. When functioning properly, it allows smooth rotation and secure positioning of the upper structure. On the PC120LC-6, this hydraulic-controlled brake system relies on consistent electrical signals from the ECM. Any disruption compromises operator safety and machine productivity, making E106 a priority code to address immediately.

Common Symptoms

When E106 is active, operators typically experience:

• Swing function becomes inoperative or intermittently locks, preventing upper structure rotation
• Warning indicator light illuminates on the monitor panel, often accompanied by an audible alarm
• Swing motor brake fails to release, causing jerky or restricted movement during rotation attempts
• Hydraulic system derate may occur, limiting other functions as a safety precaution
• Unusual grinding or clicking noises from the swing motor area when attempting rotation

Potential Causes

The most common technical causes for E106 on used PC120LC-6 excavators include:

• Damaged or corroded wiring harness at the swing motor mounting bracket—a known rub point where insulation wears through after extended service hours
• Failed swing brake solenoid valve due to internal coil failure, contamination, or mechanical wear
• Corroded electrical connectors at the solenoid or ECM terminals, particularly in machines operating in wet or corrosive environments
• ECM internal fault or degraded output driver circuit (less common but possible on high-hour machines)
• Loose or broken ground connections near the swing motor assembly
• Hydraulic contamination causing solenoid valve sticking, which increases electrical current draw and triggers fault detection

How to Troubleshoot and Fix Code E106

Step 1: Visual Inspection Inspect the wiring harness running from the ECM to the swing brake solenoid for physical damage, especially where it passes near rotating components or sharp edges. Check all connectors for corrosion, bent pins, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a multimeter, measure resistance across the solenoid coil terminals (typically 8-15 ohms for Komatsu solenoids). Check for continuity in the harness from ECM to solenoid. With ignition on but engine off, verify supply voltage at the solenoid connector (should read battery voltage when commanded). Compare readings against Komatsu service manual specifications.

Step 3: Component Testing Remove the swing brake solenoid valve and inspect for contamination or mechanical damage. For used excavators, contamination from degraded hydraulic oil is common. Test solenoid operation by applying 12V DC directly—it should click audibly and allow airflow through the valve body. Check hydraulic filter condition and oil quality.

Step 4: ECM Diagnostics Connect Komatsu diagnostic software (KOMTRAX or compatible interface) to verify ECM output signals and perform active solenoid tests. Monitor real-time data for voltage drops or irregular command patterns. Clear codes after repairs and perform operational tests.

For used excavators, always inspect harness routing and secure with new tie wraps, as vibration-induced chafing is accelerated in older machines.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific machine serial number and seek qualified technician assistance for complex electrical diagnostics or repairs.