What is Kobelco SK135SRLC-2 Fault Code B072?

Fault Code B072 on the Kobelco SK135SRLC-2 excavator indicates a malfunction in the auxiliary output circuit #2 of the machine's Electronic Control Unit (ECU). This code specifically relates to an abnormal electrical signal or open/short circuit condition in one of the ECU's programmable auxiliary output channels, which typically controls optional equipment like thumbs, breakers, or supplementary hydraulic attachments.

The SK135SRLC-2 uses a sophisticated electronic control system to manage auxiliary hydraulic functions. When the ECU detects voltage irregularities, excessive current draw, or complete circuit failure on output #2, it logs B072 and may disable that specific auxiliary function to prevent component damage. This is critical because unresolved electrical faults can cascade into more expensive ECU failures or damage to the auxiliary valve solenoids.

Common Symptoms

• Auxiliary function #2 becomes completely inoperative (no response when activated from cab controls)
• Service wrench icon or warning light illuminates on the instrument panel
• Intermittent operation of the auxiliary attachment, working sporadically before failing completely
• No error message initially, but progressive loss of control responsiveness during operation
• ECU may log multiple B-series codes if electrical issue affects adjacent circuits

Potential Causes

The most common causes for B072 on used SK135SRLC-2 excavators include:

• Damaged wiring harness between the cab controller and auxiliary valve solenoid (especially at routing points near the boom pivot where repetitive flexing occurs)
• Corroded or moisture-infiltrated connector pins at the auxiliary solenoid plug (common in machines operating in wet/marine environments)
• Failed auxiliary solenoid coil in the control valve block (resistance out of specification, typically should read 20-30 ohms)
• Degraded ECU output driver circuit from previous electrical overload or voltage spike
• Aftermarket attachment wiring improperly integrated causing short circuits or ground faults
• Blown fuse or faulty relay in the auxiliary power distribution circuit

How to Troubleshoot and Fix Code B072

Step 1: Visual Inspection and Connector Check Begin with a thorough inspection of the wiring harness running from the cab control panel to the auxiliary valve bank. On used excavators, pay special attention to wear points where the harness passes through the boom base and around hydraulic lines. Remove and inspect the connector at the auxiliary valve solenoid #2—look for green corrosion, bent pins, or moisture inside the connector boot.

Step 2: Electrical Testing with Multimeter Using a quality digital multimeter, perform resistance checks on the auxiliary solenoid coil (should typically read 20-35 ohms; consult your service manual for exact specifications). Check for continuity in the wiring harness from the ECU connector to the solenoid. Measure voltage at the solenoid connector with ignition on and auxiliary function activated—you should see battery voltage (12-14V) when engaged.

Step 3: Advanced Diagnostics Connect Kobelco-compatible diagnostic software (such as genuine Kobelco diagnostic tools or compatible aftermarket scanners) to read live data from the ECU. Monitor the auxiliary output #2 command status while activating the control—if the ECU shows "commanded ON" but no voltage appears at the solenoid, the issue lies in wiring or connections. If the ECU doesn't register the command, investigate the cab control switch and its circuit.

Step 4: Component Replacement and Verification For used machinery, replace the solenoid coil if resistance readings are out of spec or if physical damage is evident. Repair or replace damaged harness sections using proper gauge wire with weather-sealed connectors. After repairs, clear the fault code using diagnostic software and cycle the auxiliary function 10-15 times to ensure consistent operation before returning to service.

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Disclaimer: This guide provides general troubleshooting information for Kobelco SK135SRLC-2 excavators. Always consult the manufacturer's official service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may void warranties or create safety hazards.