What is Kobelco SK135SR-2 Fault Code B104?

Fault Code B104 on the Kobelco SK135SR-2 excavator indicates a malfunction in the machine's hydraulic pilot pressure sensor circuit. This code specifically signals that the Electronic Control Unit (ECU) has detected an abnormal voltage reading or signal interruption from the pilot pressure sensor, which monitors the operator's input commands through the pilot hydraulic system.

The pilot pressure sensor plays a critical role in the SK135SR-2's operational safety and efficiency. This sensor translates the operator's joystick and pedal movements into electronic signals that the ECU uses to regulate hydraulic flow and implement functions. When this sensor circuit fails, the machine cannot accurately interpret operator commands, potentially leading to erratic machine behavior or complete operational shutdown. For used excavators, this fault often emerges due to accumulated wear on electrical connectors and wiring harnesses exposed to years of vibration and environmental stress.

Common Symptoms

When Code B104 is active on your Kobelco SK135SR-2, you may experience:

• Warning light illumination on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic response or sluggish control movements despite normal joystick input
• Intermittent loss of specific functions (boom, arm, or swing) that may temporarily restore after machine restart
• Automatic derate mode where engine power is limited to protect the hydraulic system
• Complete shutdown of hydraulic functions while the engine continues running normally

Potential Causes

The most common technical causes for B104 on used SK135SR-2 excavators include:

• Failed pilot pressure sensor due to internal component degradation or moisture infiltration
• Damaged wiring harness at known rub points near the cab pivot area or along the boom routing path
• Corroded or loose electrical connectors at the sensor plug or ECU interface (common in machines with 5,000+ operating hours)
• Short circuit or open circuit in the sensor supply voltage line (typically 5V reference)
• ECU internal fault affecting the sensor input processing circuit (rare, but possible in older units)
• Water intrusion into the connector housing from damaged seals or improper storage

How to Troubleshoot and Fix Code B104

Step 1: Visual Inspection and Connector Check

Begin by locating the pilot pressure sensor, typically mounted on the main control valve assembly. Inspect the sensor connector for corrosion, bent pins, or moisture. On used excavators, this is the most common failure point. Disconnect the connector and examine both the sensor pins and harness terminals. Clean any corrosion with electrical contact cleaner and ensure the connector seals are intact.

Step 2: Electrical Testing with Multimeter

Using a digital multimeter (DMM), check the sensor's reference voltage. With the ignition on and connector attached, backprobe the signal wire (consult your service manual for wire colors—typically a red or orange wire). You should read approximately 5 volts DC. Next, disconnect the sensor and measure resistance across the sensor terminals; a functional sensor typically reads between 500-3,000 ohms depending on pilot pressure. An open circuit (infinite resistance) or zero resistance indicates sensor failure.

Step 3: Harness Continuity and Ground Testing

Trace the wiring harness from the sensor to the ECU, paying special attention to areas where the harness contacts metal surfaces or moving components. Check for continuity in all wires and verify a solid ground connection (less than 1 ohm resistance to chassis ground). For used machines, inspect the harness routing near the cab mounting area and superstructure rotation points where wire chafing commonly occurs on the SK135SR-2.

Step 4: Component Replacement and Verification

If sensor replacement is necessary, use OEM Kobelco parts or high-quality aftermarket equivalents rated for the same pressure range. After installation, clear the fault code using Kobelco diagnostic software (or disconnect the battery for 10 minutes). Operate the machine through all functions and monitor for code recurrence. If B104 returns immediately, the ECU may require inspection by a qualified technician.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Kobelco service manual for your specific machine serial number and seek assistance from certified heavy equipment technicians for complex electrical diagnostics or repairs.