What is Kobelco SK135SRLC-2 Fault Code B173?

Fault Code B173 on the Kobelco SK135SRLC-2 excavator indicates a malfunction in the air conditioning (A/C) compressor relay circuit. This code is triggered when the machine's Electronic Control Module (ECM) detects an abnormal voltage or current flow in the wiring harness or relay controlling the air conditioning compressor clutch engagement.

The A/C system on this model relies on precise electrical signals from the cab climate control panel to the ECM, which then activates the compressor relay. When this circuit fails, the ECM logs B173 and typically disables A/C operation to prevent electrical damage. While not critical to excavator hydraulic or engine performance, this fault significantly impacts operator comfort, particularly in used machines operating in hot climates where cab environment directly affects productivity and safety.

Common Symptoms

• Air conditioning system completely non-functional despite controls being activated
• Warning light or error message displayed on the monitor panel indicating A/C system failure
• Compressor clutch fails to engage (no audible click when A/C is switched on)
• Intermittent A/C operation that cuts in and out randomly during machine operation
• No change in engine RPM when attempting to activate the A/C (normally causes slight RPM increase)

Potential Causes

The most frequent causes of Code B173 in used Kobelco SK135SRLC-2 excavators include:

• Failed A/C compressor relay due to age-related contact degradation or coil burnout
• Damaged wiring harness between the ECM and compressor relay, especially at routing points near the engine compartment where vibration and heat cause insulation breakdown
• Corroded or loose connector terminals at the relay socket or ECM interface (common in machines with high hours or exposure to moisture)
• Shorted or open circuit in the compressor clutch coil itself, causing abnormal current draw
• Faulty ECM output driver for the A/C relay circuit (less common but possible in high-hour machines)
• Blown fuse in the A/C compressor circuit protecting the relay power supply

How to Troubleshoot and Fix Code B173

Step 1: Initial Inspection and Code Verification Connect a diagnostic scanner compatible with Kobelco equipment to retrieve active and stored codes. Clear the code and attempt to activate the A/C system to see if B173 returns immediately. Locate the A/C compressor relay (typically in the main relay box under the operator's seat or in the engine compartment fuse panel).

Step 2: Relay and Fuse Testing Remove the A/C compressor relay and visually inspect for burnt contacts or damaged casing. Use a multimeter to test relay coil resistance (typically 70-90 ohms) and verify continuity between normally-open contacts when energized with 12V. Check the associated fuse for continuity and replace if blown. Inspect the relay socket for corrosion, bent pins, or moisture intrusion—common issues in used excavators.

Step 3: Wiring Harness and Voltage Checks With the relay removed and ignition on, measure voltage at the relay socket power supply terminal (should read battery voltage, approximately 12-14V). Check for ground continuity on the relay ground circuit. Inspect the wiring harness from the ECM to the relay for chafing, particularly where it routes near moving engine components or sharp metal edges—a known wear point on this model.

Step 4: Compressor Clutch Circuit Verification Test the A/C compressor clutch coil resistance directly at the compressor (typically 3-5 ohms). Check for voltage at the clutch connector when the A/C is commanded on. If no voltage appears despite a good relay, suspect ECM output driver failure or internal wiring faults. For used excavators, always examine connector housings for corrosion or terminal push-back, which creates intermittent connections.

Step 5: Component Replacement and System Reset Replace any faulty components identified (relay, fuse, or damaged harness sections). Ensure all connectors are cleaned with electrical contact cleaner and properly sealed. Clear fault codes with the diagnostic scanner and test A/C operation under load. Monitor for code recurrence over several operating cycles.

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Disclaimer: This guide provides general troubleshooting information for Kobelco SK135SRLC-2 Fault Code B173. Always consult the official Kobelco service manual for your specific machine serial number and software version. If you're uncomfortable performing electrical diagnostics, contact a certified Kobelco technician to prevent damage to sensitive electronic components.