Komatsu PC55MR-2 Fault Code S-8: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code S-8?

Fault Code S-8 on the Komatsu PC55MR-2 indicates a malfunction in the swing motor parking brake solenoid circuit. This code is triggered when the Electronic Control Module (ECM) detects an abnormal electrical signal—either an open circuit, short circuit, or voltage irregularity—in the wiring or solenoid controlling the swing parking brake system.

The swing parking brake is critical for machine stability and safety. When this system fails, the excavator may not lock the upper structure properly during shutdown or transport, creating significant safety hazards. The PC55MR-2's compact design places this solenoid in an area prone to environmental exposure, making electrical faults particularly common in used machines.

Common Symptoms

When Code S-8 becomes active, operators typically experience:

• Swing function warning light illuminated on the instrument panel or monitor display
• Inability to release the swing parking brake, preventing upper structure rotation even when hydraulic pressure is present
• Erratic swing brake engagement/disengagement, causing jerky or uncontrolled swing movements
• Audible clicking or buzzing from the swing motor area when attempting to activate swing function
• Machine may enter derate mode limiting overall hydraulic performance as a safety precaution

Potential Causes

The most common technical reasons for S-8 fault activation on used PC55MR-2 excavators include:

• Solenoid coil failure due to internal winding breakdown from heat cycling and age
• Harness chafing at the swing motor mounting bracket, a known wear point on this model where cables rub during rotation
• Corroded connector pins at the solenoid plug—moisture intrusion is common in the swing base area
• Damaged wiring between the main control valve harness and swing motor solenoid
• Weak or inconsistent voltage supply from aging battery or alternator issues affecting solenoid activation
• ECM internal fault (rare, but possible in high-hour machines)

How to Troubleshoot and Fix Code S-8

Step 1: Visual Inspection Begin with a thorough physical examination. Disconnect the solenoid connector at the swing motor and inspect for green corrosion, bent pins, or moisture. Check the entire harness routing from the control valve to the swing motor for abraded insulation, particularly where cables contact metal edges or move during swing operation.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid coil resistance. For the PC55MR-2 swing brake solenoid, expect 20-30 ohms resistance between terminals. Next, check for continuity in the supply wire from the harness connector back toward the control valve. Resistance should be less than 1 ohm. Test for short to ground by measuring resistance between each solenoid wire and machine chassis—it should read infinite (open circuit).

Step 3: Voltage Supply Verification With the ignition on and swing function activated, check for battery voltage (12V) at the solenoid connector using your multimeter. If voltage is absent or weak (below 10.5V), trace back through the circuit checking fuses, relays, and wiring integrity. On used excavators, pay special attention to ground connections—clean and tighten all ground terminals in the swing circuit.

Step 4: Component Replacement If testing confirms solenoid failure, replace with a genuine Komatsu 702-16-07010 or verified aftermarket equivalent. When reinstalling, apply dielectric grease to connector pins to prevent future corrosion. Reroute any harness sections showing wear patterns, using protective loom or spiral wrap at friction points.

Step 5: Code Clearing and Verification After repairs, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scanner). Operate the swing function through multiple full rotations while monitoring for code recurrence. Test parking brake engagement and release several times to confirm proper operation.

---

Disclaimer: This guide provides general diagnostic procedures for informational purposes. Always consult the official Komatsu service manual for your specific serial number and consider professional technical assistance for complex electrical diagnostics or safety-critical repairs.