What is Kobelco SK140LC-8 Fault Code B104?

Fault Code B104 on the Kobelco SK140LC-8 excavator indicates a malfunction in the working light relay circuit, specifically detecting an abnormal voltage or current condition in the lighting system's control circuit. This code is generated by the machine's Electronic Control Module (ECM) when it detects that the working light relay is not responding properly to command signals or is drawing incorrect amperage.

This fault is part of Kobelco's electrical safety monitoring system, which continuously checks all lighting circuits to ensure operator visibility during critical operations. While not directly affecting hydraulic or engine performance, this code can compromise job site safety, particularly during low-light conditions. The SK140LC-8's integrated electrical architecture makes proper diagnosis essential, as recurring B104 codes may indicate broader electrical system degradation common in used excavators operating in harsh environments.

Common Symptoms

• Working lights fail to illuminate when activated via the cab switch, or lights flicker intermittently during operation
• Warning lamp illumination on the instrument cluster, typically accompanied by an audible alert chime
• Lights remain constantly on regardless of switch position, indicating a stuck relay condition
• Battery drain issues when the machine is shut down, suggesting the relay is not disengaging properly
• ECM may log multiple instances of the code, visible through diagnostic software scan history

Potential Causes

The most common technical causes for Code B104 on used SK140LC-8 excavators include:

• Faulty working light relay – Internal relay contacts become pitted or welded shut after thousands of cycles, particularly in machines with 4,000+ operating hours
• Corroded or loose wiring connectors at the relay socket or bulkhead connectors near the counterweight, where moisture and vibration cause connection failures
• Damaged wiring harness – Insulation breakdown along the boom routing path where cables flex repeatedly, creating short circuits to ground
• Failed light bulbs creating abnormal load – Blown halogen bulbs can cause unexpected resistance readings that trigger ECM fault detection
• ECM internal fault (rare) – Control module software glitches or internal circuit board damage from voltage spikes
• Poor ground connections at the frame mounting points, especially where paint or rust prevent proper electrical contact

How to Troubleshoot and Fix Code B104

Step 1: Visual Inspection and Preliminary Checks Begin by inspecting all working light bulbs – replace any blown units before proceeding. Check the fuse box for blown fuses in the lighting circuit (typically 15-20 amp). On used excavators, examine all visible wiring harnesses from the cab to the lights, looking for chafing, bare wires, or evidence of previous repairs with electrical tape.

Step 2: Test the Working Light Relay Locate the working light relay in the main electrical panel beneath the operator's seat. Using a digital multimeter (DMM), measure resistance across relay terminals with the key off – you should see infinite resistance (open circuit). Have an assistant activate the light switch while you listen for an audible "click" and measure continuity change. If the relay doesn't click or shows constant continuity, replace the relay (Kobelco part number varies by production date – verify with serial number).

Step 3: Check Voltage and Harness Integrity With the key on (engine off), use your multimeter to verify 12-14V DC at the relay input terminal when the light switch is activated. Trace the harness from the relay to each light assembly, checking for voltage drop exceeding 0.5V, which indicates resistance from corrosion or damaged wiring. Pay special attention to connector pins at the boom base and upper structure junction – spray with electrical contact cleaner and check for green corrosion. On used machines, connector housings often crack, allowing moisture intrusion.

Step 4: Ground Circuit Verification Measure resistance from the relay ground terminal to battery negative – it should read less than 1 ohm. Clean all ground connection points on the frame with a wire brush, removing paint and corrosion. Many B104 codes on older SK140LC-8s resolve simply by re-establishing proper grounds.

Step 5: Clear Code and Monitor After repairs, use Kobelco diagnostic software or a compatible J1939 scan tool to clear the fault code from ECM memory. Operate the lights through multiple on/off cycles and monitor for code recurrence. If B104 returns immediately, suspect ECM internal issues requiring dealer-level diagnostics.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Kobelco service manual for your specific serial number and production year. Electrical system diagnosis on used excavators may reveal additional issues requiring professional assessment. If unfamiliar with electrical diagnostics or working with 12V systems, consult a certified Kobelco technician to avoid equipment damage or personal injury.