What is Kobelco SK200-8 Fault Code A255?

Kobelco SK200-8 Fault Code A255 indicates a malfunction in the pump control solenoid valve circuit, specifically related to abnormal electrical current or voltage in the main hydraulic pump control system. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an out-of-range signal from the pump flow control solenoid, which directly regulates hydraulic oil flow based on operational demand.

This code is critical for the SK200-8 because the proportional solenoid valve controls the swash plate angle of the main hydraulic pump. When this circuit fails, the excavator loses precise hydraulic control, affecting all machine functions from boom speed to swing torque. In used excavators, this fault often stems from electrical degradation rather than mechanical pump failure, making accurate diagnosis essential before costly component replacement.

Common Symptoms

• Erratic hydraulic performance with inconsistent boom, arm, or bucket speeds during operation
• Loss of hydraulic power or sluggish response across all functions, particularly under load
• Warning light illumination on the instrument cluster, often accompanied by error code display
• Intermittent hydraulic surging or unusual pressure fluctuations during combined operations
• Engine derate or limited RPM in severe cases where the ECM enters protective mode

Potential Causes

The most common technical causes for Code A255 on used SK200-8 excavators include:

• Solenoid valve coil failure due to internal short circuit or open winding (typical resistance should be 3-5 ohms)
• Corroded or damaged wiring harness near the pump mounting area where vibration causes insulation breakdown
• Loose or oxidized connector pins at the pump solenoid or ECM interface (common on machines with 5,000+ hours)
• ECM internal circuit fault affecting the solenoid driver output stage
• Hydraulic oil contamination causing mechanical sticking of the solenoid spool, increasing electrical draw
• Voltage supply issues from damaged main power harness or poor chassis ground connections

How to Troubleshoot and Fix Code A255

Step 1: Visual Inspection and Connector Check Begin by accessing the main hydraulic pump area and inspecting the solenoid valve connector. Check for oil contamination, bent pins, or corrosion. On used excavators, examine the entire wiring harness from pump to ECM for rub points against the frame or heat damage near the turntable area.

Step 2: Electrical Resistance Testing Disconnect the solenoid connector and use a digital multimeter to measure coil resistance across the solenoid terminals. Specification should be 3-5 ohms at 20°C. Values outside this range indicate solenoid replacement is needed. Also check for continuity to ground (should be infinite resistance).

Step 3: Voltage Supply Verification With ignition on and connector attached, back-probe the harness to verify the ECM is supplying proper voltage (typically 12V PWM signal). Use Kobelco TCMS diagnostic software or compatible scan tool to monitor live solenoid command vs. actual current draw.

Step 4: Harness and Ground Testing Trace the wiring harness for voltage drop issues. Measure resistance from solenoid ground to battery negative—should be less than 0.5 ohms. Clean all ground connections at the pump mounting bracket.

Step 5: Component Replacement or ECM Diagnosis If all electrical checks pass but the code persists, the ECM driver circuit may be faulty. Before ECM replacement, verify hydraulic oil cleanliness and check for mechanical binding in the solenoid spool using a replacement known-good solenoid.

---

Disclaimer: This guide provides general diagnostic information. Always consult the Kobelco SK200-8 service manual and consider professional technician assistance for complex electrical or hydraulic system repairs.