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

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

Fault Code S-2 on the Komatsu PC55MR-2 indicates a malfunction in the main pump solenoid valve circuit, specifically a short circuit or open circuit condition in the electrical system controlling the hydraulic pump's variable displacement mechanism. This code is generated when the ECM (Engine Control Module) detects abnormal voltage or resistance readings from the proportional solenoid valve that regulates hydraulic flow and pressure.

This is a critical fault for the PC55MR-2 because the main pump solenoid directly controls the machine's hydraulic power output. When this circuit fails, the excavator cannot properly regulate hydraulic pressure, severely limiting digging force, swing speed, and overall operational efficiency. The PC55MR-2's compact design makes this solenoid particularly vulnerable to environmental contamination and connector corrosion in used machines.

Common Symptoms

• Hydraulic system operates in low-power mode or limp mode, with significantly reduced digging force and lifting capacity
• Monitor panel displays S-2 code along with a hydraulic system warning light (typically orange or red)
• Erratic or unresponsive hydraulic functions, including inconsistent boom, arm, or bucket movements
• Engine runs normally but machine feels sluggish across all hydraulic operations
• Intermittent loss of hydraulic power that may temporarily resolve, then return during operation

Potential Causes

Electrical wiring issues are the primary cause in used PC55MR-2 excavators, particularly harness chafing where the solenoid wiring passes near the hydraulic pump mounting bracket—a known wear point on this model.

Solenoid valve internal failure occurs due to contaminated hydraulic oil causing coil burnout or stuck valve spools, common in machines with irregular service intervals.

Connector corrosion at the 4-pin solenoid connector, especially in machines operating in wet or marine environments, creates intermittent open circuits.

ECM-related issues including damaged control module circuits or corroded ground connections at the ECM mounting location near the cab floor.

Voltage supply problems from the main electrical harness, including blown fuses or damaged relay contacts in the 12V power distribution system.

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection and Connector Check Begin by locating the main pump proportional solenoid valve on the right side of the hydraulic pump assembly. Disconnect the 4-pin connector and inspect for green corrosion, bent pins, or moisture intrusion. Clean contacts with electrical contact cleaner and check the wiring harness for visible damage, particularly where it routes around sharp edges near the pump mounting.

Step 2: Electrical Testing with Multimeter Using a digital multimeter, measure solenoid coil resistance directly at the valve terminals—specification should be 18-22 ohms at 20°C. Next, check supply voltage at the harness connector with ignition on: you should read battery voltage (11-14V). Test for short to ground by measuring resistance between each solenoid wire and chassis ground (should read infinite/OL).

Step 3: Dynamic Circuit Testing With the connector attached and engine running, use a backprobe kit to measure voltage at the solenoid while cycling hydraulic controls. Voltage should modulate between 2-10V depending on demand. Fixed readings indicate ECM output failure; no reading suggests open circuit in the harness.

Step 4: Component Replacement or Repair For used excavators, if resistance and voltage tests fail, replace the solenoid valve assembly (Komatsu part 702-21-55901 or equivalent). When replacing, always install a new connector pigtail if any corrosion was present. Clear codes using Komatsu diagnostic software or by disconnecting the battery for 30 minutes, then perform a complete hydraulic system function test.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC55MR-2 service manual and consider professional assistance for complex electrical or hydraulic repairs. Improper diagnostics may cause additional damage to ECM or hydraulic components.