Kobelco SK135SR-2 Fault Code A255: Complete Diagnostic Guide

What is Kobelco SK135SR-2 Fault Code A255?

Fault Code A255 on the Kobelco SK135SR-2 indicates a malfunction in the swing motor pressure sensor circuit, specifically detecting an abnormal voltage signal from the swing system's hydraulic pressure monitoring system. This code is triggered when the Engine Control Module (ECM) receives voltage readings outside the manufacturer's specified range (typically 0.5-4.5V) from the swing pressure sensor.

The swing motor pressure sensor plays a critical role in the SK135SR-2's hydraulic management system by monitoring real-time pressure during rotation operations. When this sensor fails or provides inaccurate data, the excavator's load-sensing hydraulic system cannot properly adjust pump flow, leading to reduced fuel efficiency, erratic swing performance, and potential component damage. For used excavators, this code often surfaces due to age-related electrical degradation or hydraulic contamination affecting sensor accuracy.

Common Symptoms

When Code A255 is active, operators typically experience:

• Erratic swing speed or inconsistent rotation control, especially under load
• Illuminated warning light on the instrument cluster (yellow/amber caution indicator)
• Reduced swing torque or sluggish response when rotating the upper structure
• Intermittent loss of swing function that may reset after machine restart
• Abnormal hydraulic whining noise during swing operations due to pressure regulation issues

Potential Causes

The most common technical failures triggering A255 on used SK135SR-2 excavators include:

• Worn or contaminated swing pressure sensor (P/N specific to Kobelco hydraulic systems)
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the swing bearing where cables experience repeated bending
• Corroded or loose connector pins at the sensor plug (3-pin Deutsch connector) due to moisture intrusion
• Hydraulic oil contamination causing sensor membrane damage or blockage
• Failed ECM voltage supply to the sensor circuit (should provide stable 5V reference)
• Shorted or open circuit in the signal wire, common in machines with 5,000+ operating hours

How to Troubleshoot and Fix Code A255

Step 1: Initial Diagnostic Scan Connect a Kobelco-compatible diagnostic tool (or universal scanner with J1939 protocol) to retrieve active and pending codes. Record freeze frame data showing sensor voltage at the time of fault. Clear codes and operate the swing function to verify if A255 returns immediately or intermittently.

Step 2: Visual Inspection of Wiring and Connectors For used excavators, inspect the sensor harness from the swing motor to the main chassis harness. Look for abrasion points where cables contact metal edges, especially near the swing bearing and center joint. Check the 3-pin sensor connector for green corrosion, bent pins, or moisture. Clean with electrical contact cleaner and apply dielectric grease.

Step 3: Sensor Voltage Testing Using a digital multimeter (DMM), backprobe the sensor connector with the key ON, engine OFF. Verify 5V reference voltage on the supply pin and good ground (below 0.1V) on the ground pin. Monitor the signal wire voltage while manually applying pressure to the sensor or rotating the swing motor. Normal readings should sweep between 0.5V (low pressure) and 4.5V (high pressure). Fixed readings at 0V or 5V indicate sensor failure.

Step 4: Resistance and Continuity Checks Disconnect the sensor and measure resistance across sensor terminals (typically 1-3kΩ depending on temperature). Check continuity from sensor connector to ECM pins using the wiring diagram; resistance should be less than 5Ω. On older machines, corrosion inside wire insulation can cause intermittent high resistance even when visually intact.

Step 5: Component Replacement and Verification If testing confirms sensor failure, replace with a genuine Kobelco pressure sensor or OEM-equivalent part. For used machines with 3,000+ hours, also replace the mating connector pigtail to ensure reliable connection. After installation, clear codes, operate for 30 minutes, and rescan to confirm repair.

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Disclaimer: This guide provides general troubleshooting procedures for Code A255. Always consult the official Kobelco service manual for your specific serial number and consider professional diagnostic assistance for complex electrical or hydraulic system repairs. Improper diagnostics may cause additional component damage or safety hazards.