What is Kobelco SK350LC-8 Fault Code B173?

Fault Code B173 on the Kobelco SK350LC-8 excavator indicates a malfunction in the air conditioning compressor relay circuit. This diagnostic trouble code (DTC) is triggered when the machine's Electronic Control Module (ECM) detects an abnormal voltage or current flow in the electrical circuit controlling the A/C compressor clutch relay.

The code specifically monitors the relay's ability to properly energize and de-energize the air conditioning compressor. While this may seem like a comfort-related issue, it's critical for operator productivity and cab environment control, especially in extreme working conditions. On used SK350LC-8 models, this fault can also indicate broader electrical system degradation that may affect other cab control circuits if left unaddressed.

Common Symptoms

When Code B173 is active, operators typically experience:

• Air conditioning system completely inoperative – compressor will not engage regardless of controls
• Warning light or message on the monitor panel indicating an A/C system fault
• Intermittent A/C operation that cycles on and off unexpectedly during machine operation
• Electrical burning smell near the cab control panel or relay box (in severe cases with shorted circuits)
• Normal excavator functions remain unaffected, but operator comfort is significantly compromised

Potential Causes

The most common technical causes for B173 on used SK350LC-8 excavators include:

• Failed A/C compressor relay due to contact wear or coil burnout from repeated cycling
• Damaged wiring harness between the ECM and relay, particularly at flex points near the cab mounting
• Corroded or loose electrical connectors in the relay circuit from moisture intrusion or age
• Shorted A/C compressor clutch coil drawing excessive current and triggering protective shutdown
• ECM internal relay driver circuit failure (less common but possible in high-hour machines)
• Blown fuse in the A/C system power supply circuit
• Poor ground connections at the cab chassis ground points causing voltage irregularities

How to Troubleshoot and Fix Code B173

Step 1: Initial Inspection and Code Verification Using Kobelco-compatible diagnostic software (such as KOMTRAX Link or equivalent scan tool), verify the fault code and check freeze frame data for voltage readings when the fault occurred. Clear the code and attempt to recreate the fault by activating the A/C system while monitoring live data.

Step 2: Relay and Fuse Testing Locate the A/C compressor relay in the cab fuse/relay panel (consult the SK350LC-8 electrical schematic). Remove the relay and inspect for burn marks or corrosion. Using a digital multimeter, test relay coil resistance (typically 70-90 ohms) and verify continuity through the switching contacts. Check the associated fuse for proper amperage rating and continuity.

Step 3: Circuit Voltage and Wiring Inspection With the relay removed, check for battery voltage (12V) at the power supply terminal with ignition on. Measure voltage at the ECM control wire (should show 0V when A/C is off, battery voltage when commanded on). Crucial for used excavators: Physically inspect the entire harness from the relay box to the compressor, focusing on areas where the harness passes through the cab mounting or engine compartment bulkhead—these are known wear points for the SK350LC-8.

Step 4: Compressor Clutch Testing Disconnect the A/C compressor clutch connector and measure coil resistance (typically 3-5 ohms). Check for shorts to ground using a multimeter in ohms mode between the coil terminals and compressor housing.

Step 5: Connector and Ground Verification Clean all connectors with electrical contact cleaner and inspect pins for corrosion or backing-out. Verify ground connections at the cab mounting points show less than 0.5 ohms resistance to battery negative.

Professional Note: If all electrical tests pass but the code persists, ECM internal relay driver diagnosis requires factory-level diagnostic equipment. For used excavators with unknown service history, replacement of the relay and thorough harness inspection resolves B173 in approximately 80% of cases before considering ECM replacement.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Kobelco service manual for your specific machine serial number and seek qualified professional assistance for complex electrical diagnostics or repairs.