What is Komatsu PC100-6 Fault Code S-8?

Fault Code S-8 on the Komatsu PC100-6 excavator indicates a malfunction in the swing motor pilot solenoid valve circuit. This diagnostic trouble code is triggered when the electronic control module (ECM) detects abnormal electrical signals from the swing motor's pilot solenoid valve system, which controls the upper structure rotation function.

The pilot solenoid valve regulates hydraulic pressure to the swing motor, enabling smooth rotational movement of the excavator's upper carriage. When the ECM identifies voltage irregularities, open circuits, or short circuits within this system, it logs S-8 and may limit machine functionality. For the PC100-6 model, this circuit operates on a 12-24V DC system, and proper electrical continuity is essential for maintaining precise swing control during operation.

Common Symptoms

• Swing function becomes erratic or completely inoperative – the upper structure may not rotate or responds intermittently to joystick input
• ECM warning light illuminates on the instrument panel, often accompanied by diagnostic mode activation
• Reduced swing speed or sluggish response when attempting rotational movements under load
• Unusual clicking or buzzing sounds from the solenoid valve assembly near the swing motor
• Machine enters derate mode, limiting overall hydraulic performance to protect system integrity

Potential Causes

The most common technical failures triggering S-8 on used PC100-6 excavators include:

• Solenoid valve coil failure due to age-related insulation breakdown or moisture ingress (resistance typically measures 20-30 ohms when functional)
• Wiring harness damage at known rub points near the swing bearing and rotating joint, where constant movement causes wire chafing
• Corroded or loose electrical connectors at the solenoid valve plug, particularly on machines operating in wet or coastal environments
• ECM internal fault or poor ground connections at the controller mounting location
• Blown fuses or damaged relay in the swing control circuit within the main electrical panel

How to Troubleshoot and Fix Code S-8

Step 1: Visual Inspection Begin by inspecting the swing motor solenoid valve wiring harness for obvious damage. On used excavators, check the rotating joint area where cables transition from the lower frame to the upper structure – this is a critical wear point on PC100-6 models. Look for abraded insulation, broken wires, or oil-contaminated connectors.

Step 2: Electrical Testing Using a digital multimeter, check solenoid valve coil resistance. Disconnect the electrical connector and measure across the solenoid terminals – readings should be 20-30 ohms. Also verify supply voltage at the connector with ignition on (should read 12-24V DC). Check ground circuit continuity to the ECM.

Step 3: Component Testing If electrical values are correct, perform a functional test by manually energizing the solenoid with a 12V power source. A healthy valve produces an audible click. On used machines, disassemble and inspect the valve spool for carbon buildup or corrosion that may cause mechanical binding.

Step 4: Connector Service Clean all electrical connectors with contact cleaner and apply dielectric grease. Replace any corroded terminals – aftermarket connector kits are available for PC100-6 models. Ensure proper wire routing with new cable ties to prevent future chafing.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may void warranties or create safety hazards.