Komatsu PC55MR-2 Fault Code S-4: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code S-4?

Fault Code S-4 on the Komatsu PC55MR-2 indicates a malfunction in the main relief valve solenoid circuit or pressure sensor system within the hydraulic control network. This code specifically relates to abnormal electrical signals detected by the Electronic Control Module (ECM) when monitoring the hydraulic system's pressure regulation components.

The S-4 code triggers when the ECM detects voltage irregularities, open circuits, or short circuits in the relief valve solenoid wiring or when the pressure sensor provides readings outside the manufacturer's specified parameters (typically 0.5-4.5V range). This is critical for the PC55MR-2 because this compact excavator relies on precise hydraulic pressure modulation for smooth operation of the boom, arm, and swing functions. Ignoring this code can lead to erratic hydraulic performance, increased fuel consumption, and potential damage to the hydraulic pump or related components.

Common Symptoms

When Code S-4 is active on your Komatsu PC55MR-2, operators typically experience:

• Hydraulic system warning light illuminated on the instrument cluster (often amber or red)
• Sluggish or inconsistent response from boom, arm, or bucket controls
• Intermittent loss of hydraulic power during operation, particularly under heavy loads
• Unusual whining or cavitation noises from the hydraulic pump area
• Auto-idle function may fail to engage properly, causing abnormal engine RPM behavior

Potential Causes

The S-4 code in used PC55MR-2 excavators commonly results from:

• Corroded or damaged connectors at the relief valve solenoid (especially in machines with 3,000+ operating hours)
• Failed main relief valve solenoid due to internal coil breakdown or mechanical jamming
• Worn or frayed wiring harness near the hydraulic pump mounting area—a known rub point on this model
• Faulty hydraulic pressure sensor providing erratic voltage signals to the ECM
• Contaminated hydraulic oil causing valve sticking and abnormal pressure readings
• ECM software glitches or internal ECM relay failures (less common but documented in older units)

How to Troubleshoot and Fix Code S-4

Step 1: Visual Inspection Begin by inspecting the relief valve solenoid connector located near the hydraulic pump assembly. Look for green corrosion, pushed-back pins, or moisture intrusion. Check the wiring harness routing for signs of rubbing against the pump body or frame—a frequent issue in used machines. Clean all connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a digital multimeter, disconnect the relief valve solenoid connector and measure resistance across the solenoid terminals. Specification should be 10-15 ohms (consult your service manual for exact values). Then check for supply voltage at the harness connector with the ignition on—you should see battery voltage (12V). If voltage is absent, trace the harness back to the ECM for opens or shorts to ground.

Step 3: Pressure Sensor Verification Test the hydraulic pressure sensor by backprobing the signal wire (typically green or yellow) with the engine running at idle. Voltage should read approximately 0.8-1.2V at idle and increase proportionally with engine RPM. Erratic or fixed readings indicate sensor failure. For used excavators, always inspect the sensor's O-ring seal for leaks that allow moisture into the electrical connection.

Step 4: Component Replacement and Calibration If testing confirms a faulty solenoid or sensor, replace with genuine Komatsu parts or verified OEM-equivalent components. After replacement, clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scanner. Perform a hydraulic system pressure test to verify proper relief valve operation (standard pressure: 245-265 bar). In older machines, consider flushing and replacing hydraulic oil if contamination is suspected.

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Disclaimer: This guide provides general diagnostic procedures for the Komatsu PC55MR-2 S-4 fault code. Hydraulic and electrical systems involve high pressures and voltages. Always consult the official Komatsu service manual for your specific machine serial number and consider hiring a certified heavy equipment technician for complex repairs. Improper diagnostics can lead to equipment damage or personal injury.