Komatsu PC120LC-6 Fault Code S-5: Complete Diagnostic Guide

What is Komatsu PC120LC-6 Fault Code S-5?

Fault Code S-5 indicates a malfunction in the swing motor lock solenoid valve circuit. This code is triggered when the Electronic Control Module (ECM) detects abnormal voltage or current flow in the electrical circuit controlling the swing motor's locking mechanism.

The swing motor lock system prevents unwanted rotation of the upper structure during specific operating conditions. When this circuit fails, the ECM cannot properly control the swing motor lock function, which directly impacts machine safety and operational control. For the PC120LC-6 model, this hydraulic pilot-operated system is critical for preventing unexpected swinging movements, especially on slopes or during precision work.

Common Symptoms

When Code S-5 is active, operators typically experience:

• Swing lock warning indicator illuminated on the instrument panel
• Inability to engage or disengage the swing motor lock properly
• Swing motor operates continuously without the ability to mechanically lock
• Intermittent loss of swing lock function during operation
• No response when activating the swing lock control switch

Potential Causes

The most common technical causes for Code S-5 in used PC120LC-6 excavators include:

• Swing lock solenoid valve failure due to internal coil degradation or mechanical sticking
• Damaged wiring harness near the swing motor mounting area where cable flexing occurs
• Corroded or loose connectors at the solenoid valve or ECM interface points
• ECM output circuit malfunction affecting the solenoid driver transistor
• Short circuit to ground in the solenoid valve wiring (common at harness routing points under the cab)
• Low voltage supply caused by corroded battery terminals or failing alternator (aged machines)

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection Inspect the swing lock solenoid valve located near the swing motor assembly. Check for physical damage, oil contamination on connectors, and verify all wiring harnesses for abrasion, especially where they route across moving joints. On used excavators, examine connector pins for corrosion or fretting damage.

Step 2: Electrical Testing Using a digital multimeter, measure the solenoid coil resistance (should be 20-30 ohms typically). Check supply voltage at the solenoid connector with ignition on (should read 12-24V depending on system). Verify ground continuity from the solenoid body to chassis (less than 1 ohm).

Step 3: Circuit Diagnosis Disconnect the solenoid valve connector and use Komatsu diagnostic software (KOMTRAX or compatible scan tool) to command the solenoid on/off. Monitor ECM output voltage at the harness connector. If voltage is present but solenoid doesn't operate, replace the solenoid valve. If no voltage appears, inspect wiring between ECM and solenoid for opens or shorts.

Step 4: Harness and Connector Service For used machines, clean all connector contacts with electrical contact cleaner and apply dielectric grease. Check for harness rub-through at the swing bearing area—a known wear point on this model. Repair or replace damaged sections using proper gauge wire and weatherproof connectors.

Disclaimer: This guide provides general troubleshooting procedures. Always consult the official Komatsu service manual for your specific machine serial number. Complex electrical diagnostics should be performed by certified technicians with proper diagnostic equipment.