Komatsu PC850SE-8R Fault Code S-15: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code S-15?

Fault Code S-15 indicates a malfunction in the swing motor system, specifically related to the swing motor pressure sensor circuit or abnormal pressure readings during excavator rotation operations. This code is triggered when the Electronic Control Module (ECM) detects voltage irregularities, sensor signal loss, or pressure values outside the manufacturer's specified parameters in the swing hydraulic system.

For the PC850SE-8R, this is critical because the swing motor system relies on precise pressure feedback to modulate hydraulic flow and prevent component damage. When S-15 activates, the ECM may limit swing speed or torque to protect the hydraulic pump and motor from operating under unsafe conditions, directly impacting productivity on job sites.

Common Symptoms

When Code S-15 is active, operators typically experience:

• Swing function warning light illuminated on the monitor panel with audible alarm
• Reduced swing speed or jerky, inconsistent rotation movement
• Complete loss of swing function in severe cases
• Hydraulic system derate mode activated, limiting overall machine performance
• Error message displayed on the multi-information display indicating swing system fault

Potential Causes

The most common technical causes for S-15 on used PC850SE-8R excavators include:

• Swing motor pressure sensor failure due to internal component degradation or contaminated hydraulic fluid
• Damaged or corroded wiring harness connections at the sensor connector (common wear point near the swing bearing)
• Hydraulic contamination causing erratic pressure readings or sensor port blockage
• ECM communication errors from voltage drops or poor ground connections
• Physical damage to sensor mounting or broken sensor housing from impact or vibration
• Worn swing motor seals causing actual pressure loss that triggers legitimate fault detection

How to Troubleshoot and Fix Code S-15

Step 1: Initial Inspection Connect Komatsu diagnostic software (KOMTRAX Plus or equivalent) to retrieve active and stored codes. Check live data for swing motor pressure readings. Physically inspect the pressure sensor mounted on the swing motor housing for oil leaks, physical damage, or loose mounting bolts.

Step 2: Electrical Testing Disconnect the sensor harness connector and inspect for corrosion, bent pins, or moisture intrusion—critical on used machines. Using a digital multimeter, measure sensor resistance (typically 50-150 ohms) and supply voltage (should read 5V reference from ECM). Check harness continuity and inspect for wire chafing along the swing bearing rotation path.

Step 3: Hydraulic System Verification Test actual swing motor pressure using a calibrated hydraulic pressure gauge at the test port. Compare readings against Komatsu specifications (consult service manual). If pressure is abnormal, investigate hydraulic pump output, relief valve settings, or internal motor wear. Replace contaminated hydraulic filters and sample fluid for contamination analysis.

Step 4: Component Replacement If sensor tests fail specifications, replace the swing motor pressure sensor with genuine Komatsu parts. Clear codes and perform operational testing through full swing rotation cycles under load.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and seek qualified technician assistance for complex hydraulic system repairs.