Komatsu PC70-8 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu PC70-8 Fault Code S-2?

Fault Code S-2 on the Komatsu PC70-8 indicates a malfunction in the swing motor system, specifically related to the swing motor brake solenoid circuit or pressure sensor abnormality. This code is triggered when the Electronic Control Module (ECM) detects irregular voltage signals, open circuits, or pressure inconsistencies within the swing brake control system.

The swing motor brake system is critical for safe excavator operation, controlling the deceleration and stopping of the upper structure rotation. When S-2 appears, the machine's swing function may be compromised, directly affecting productivity and operator safety. This fault is particularly common in used PC70-8 models due to wear on electrical connectors and hydraulic components.

Common Symptoms

When Code S-2 is active, operators typically experience:

• Warning light illumination on the monitor panel with "S-2" displayed
• Erratic swing motor operation including jerky movements or failure to brake smoothly
• Reduced swing speed or complete loss of swing function in severe cases
• Abnormal hydraulic noises during swing deceleration (grinding or chattering sounds)
• Intermittent fault activation that may clear and reappear during operation

Potential Causes

The most common technical causes for Code S-2 on used PC70-8 excavators include:

• Faulty swing motor brake solenoid valve due to internal coil failure or mechanical sticking
• Damaged wiring harness at known rub points near the swing motor mounting area
• Corroded or loose electrical connectors on the solenoid valve circuit (common in machines with 3,000+ hours)
• Failed pressure sensor in the swing brake hydraulic circuit
• Low hydraulic oil level or contaminated hydraulic fluid affecting pressure readings
• ECM voltage supply issues or poor ground connections at the swing motor assembly

How to Troubleshoot and Fix Code S-2

Step 1: Initial Inspection Begin by checking hydraulic oil level and condition. Inspect the swing motor area for visible oil leaks, damaged wiring, or loose connectors. On used excavators, pay special attention to harness routing where cables contact the frame.

Step 2: Electrical Testing Using a digital multimeter, test the swing brake solenoid valve resistance (typically 20-30 ohms). Check for 12V supply voltage at the solenoid connector with ignition on. Inspect all ground connections for corrosion—a common issue on older PC70-8 models.

Step 3: Pressure Sensor Verification Test the swing brake pressure sensor output voltage (usually 0.5-4.5V range). Compare readings against Komatsu specifications using diagnostic software if available. Replace sensor if readings are erratic or out of range.

Step 4: Component Replacement If testing confirms component failure, replace the solenoid valve or pressure sensor. Before installation, clean all electrical connectors with contact cleaner and apply dielectric grease to prevent future corrosion.

Step 5: System Reset Clear the fault code using the monitor panel or diagnostic tool. Perform a complete swing function test under no-load and loaded conditions to verify repair.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and seek professional assistance for complex repairs or safety-critical systems.