What is Komatsu PC110-7 Fault Code S-4?

Fault Code S-4 on the Komatsu PC110-7 excavator indicates a problem with the Swing Motor Lock Solenoid Valve circuit. This code is triggered when the Electronic Control Unit (ECU) detects an abnormal electrical signal—either an open circuit, short circuit, or incorrect voltage—in the wiring or solenoid controlling the swing motor lock function.

The swing motor lock solenoid valve is critical for this machine's operational safety and efficiency. It controls whether the swing motor can rotate the upper structure freely or remains locked in position. When this system malfunctions, the excavator's ability to secure the swing function during transport or operation is compromised, potentially creating safety hazards and reducing productivity on job sites.

Common Symptoms

When Code S-4 is active on your PC110-7, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by an audible alarm
• Swing motor lock function failure—the upper structure may not lock properly when the control lever is engaged
• Intermittent or complete loss of swing lock control, making transport preparation difficult
• Erratic swing behavior where the lock engages unexpectedly or fails to release smoothly
• ECU storing the fault code even after key cycles, indicating a persistent electrical issue

Potential Causes

The most common technical causes for S-4 on used PC110-7 excavators include:

• Damaged or corroded wiring harness connections at the swing motor lock solenoid—particularly where harnesses route near moving components or metal edges
• Failed swing motor lock solenoid valve due to internal coil burnout or mechanical wear from extended use
• Connector corrosion at the solenoid plug, especially common in machines operated in wet or marine environments
• Chafed wiring along harness routing points between the ECU and solenoid valve, creating intermittent shorts
• Poor ground connections at the solenoid mounting bracket or ECU grounding points
• ECU relay or internal circuit failure (less common but possible in high-hour used machines)

How to Troubleshoot and Fix Code S-4

Step 1: Visual Inspection and Connector Check

Begin by locating the swing motor lock solenoid valve, typically mounted on the swing motor assembly. Inspect the wiring harness for obvious damage, abrasion, or pinch points along its entire length from the ECU to the solenoid. On used excavators, pay special attention to areas where the harness passes through bulkheads or near rotating components—these are common failure points. Disconnect the solenoid connector and inspect both male and female terminals for corrosion, bent pins, or moisture intrusion.

Step 2: Electrical Testing with Multimeter

Using a digital multimeter, test the solenoid coil resistance. With the connector disconnected, measure across the solenoid terminals; you should typically see 20-50 ohms (consult your service manual for exact specifications). If you read infinite resistance (open circuit) or near-zero ohms (short circuit), the solenoid has failed internally. Next, check for supply voltage at the harness connector with the ignition on—you should see battery voltage (approximately 12-14V DC) when the system attempts to activate the lock.

Step 3: Harness Continuity and Ground Testing

Test continuity from the ECU connector pins to the solenoid connector, checking for breaks in the wire. Also verify proper ground continuity from the solenoid body to chassis ground—resistance should be less than 1 ohm. For used machines, remove protective conduit sections and inspect for wire chafing against metal edges, a frequent cause of intermittent faults.

Step 4: Component Replacement and System Reset

If testing reveals a faulty solenoid, replace it with a genuine Komatsu replacement part or quality aftermarket equivalent. When replacing, apply dielectric grease to connector terminals to prevent future corrosion. If wiring damage is found, repair using heat-shrink soldered connections rather than crimp connectors for durability. After repairs, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and perform a functional test of the swing lock system.

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Disclaimer: This guide provides general troubleshooting information for the Komatsu PC110-7 fault code S-4. Always consult the manufacturer's official service manual for your specific machine serial number and production year. If you're uncomfortable performing electrical diagnostics, contact a certified Komatsu technician or qualified heavy equipment mechanic to prevent further damage or safety issues.