Komatsu PC850SE-8R Fault Code S-2: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code S-2?

Fault Code S-2 on the Komatsu PC850SE-8R indicates a malfunction in the swing motor system, specifically related to the swing motor brake solenoid valve circuit or pressure sensor abnormality. This code is generated when the Electronic Control Module (ECM) detects irregular voltage signals, open circuits, or pressure discrepancies within the swing brake control system.

The swing motor brake system is critical for the PC850SE-8R's operational safety and precision. This hydraulic excavator relies on precise swing control for stability during heavy lifting and positioning operations. When the brake solenoid valve fails to engage or disengage properly, it compromises both machine performance and operator safety. The ECM continuously monitors this circuit to prevent uncontrolled swing movement or brake failure during operation.

Common Symptoms

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

• Swing function warning light illuminated on the monitor panel with audible alarm
• Erratic swing brake operation – brake may not release smoothly or fails to hold position
• Reduced swing speed or complete swing function lockout in severe cases
• Intermittent jerking or chattering during swing rotation, especially when starting or stopping
• Error messages on the multi-monitor display indicating hydraulic system faults

Potential Causes

The most common technical causes for Code S-2 on used PC850SE-8R excavators include:

• Swing brake solenoid valve failure due to internal coil degradation or contaminated hydraulic fluid causing valve sticking
• Damaged wiring harness at known rub points near the swing motor mounting area where cables flex during rotation
• Corroded or loose electrical connectors at the solenoid valve or ECM interface, common in machines with high operating hours
• Faulty swing pressure sensor providing incorrect feedback to the control system
• Low hydraulic pressure in the swing circuit due to worn pump components or internal leakage
• ECM software glitches or corrupted calibration data in older machines

How to Troubleshoot and Fix Code S-2

Step 1: Initial Diagnostic Scan Connect Komatsu diagnostic software (KOMTRAX Plus or equivalent) to retrieve detailed fault data. Document active and stored codes, freeze frame data, and operating conditions when the fault occurred. Check hydraulic oil level and condition – contaminated fluid often causes valve issues.

Step 2: Electrical Circuit Testing Using a digital multimeter, measure solenoid valve resistance (typically 20-40 ohms when cold). Inspect the wiring harness from the swing motor to the main harness, paying special attention to the rotating joint area where cables experience constant flexing. Check for voltage supply (typically 12-24V DC) at the solenoid connector with ignition on. On used excavators, examine all connector pins for corrosion, bent terminals, or moisture intrusion.

Step 3: Component and Pressure Verification Test the swing pressure sensor output voltage against manufacturer specifications (usually 0.5-4.5V range). Physically inspect the brake solenoid valve for hydraulic leaks, debris, or mechanical damage. Measure actual hydraulic pressure in the swing brake circuit using a calibrated pressure gauge. If all electrical tests pass, the valve itself may have internal contamination requiring replacement.

Step 4: Repair and Verification Replace faulty components using genuine Komatsu parts when possible. For used machines, always replace O-rings and seals during solenoid valve installation. Clear fault codes, perform swing function tests under no-load conditions, then verify proper operation under working loads. Re-scan for codes after 30 minutes of operation.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and software version. Complex hydraulic and electrical repairs should be performed by certified technicians with proper training and equipment.