Kobelco SK135SR-2 Fault Code B173: Complete Diagnostic Guide

What is Kobelco SK135SR-2 Fault Code B173?

Fault Code B173 on the Kobelco SK135SR-2 excavator indicates a malfunction in the air conditioning compressor relay circuit. This code is triggered when the machine's Electronic Control Module (ECM) detects an abnormal voltage signal or open/short circuit condition in the wiring or relay controlling the A/C compressor clutch engagement.

While this code doesn't affect the excavator's primary hydraulic or engine systems, it's critical for operator comfort and cab climate control. In extreme temperatures, a non-functional A/C system can lead to operator fatigue, reduced productivity, and potential heat-related safety concerns. The B173 code specifically monitors the electrical pathway between the ECM, the climate control unit, and the compressor relay, ensuring proper communication and power delivery.

Common Symptoms

When Code B173 is active on your SK135SR-2, you may experience:

• A/C compressor fails to engage despite climate controls being set to cooling mode
• Warning indicator on the instrument cluster or diagnostic lamp illumination
• Intermittent A/C operation that works sporadically before completely failing
• No response when adjusting temperature controls, with compressor clutch remaining silent
• ECM may log the fault without immediately alerting the operator, discoverable only during diagnostic scans

Potential Causes

The most common technical reasons for B173 on used SK135SR-2 excavators include:

• Faulty A/C compressor relay (relay coil resistance out of specification or contacts burned/pitted)
• Damaged wiring harness between the ECM and relay—particularly at known rub points near the right-side engine mounts and battery box
• Corroded or loose connector pins at the relay socket or climate control module (common in machines with 5,000+ hours)
• Failed A/C compressor clutch coil, creating excessive amperage draw that trips ECM protection
• ECM internal driver circuit failure (less common but possible in older units)
• Blown fuse in the A/C system circuit (typically 10-15A rating)

How to Troubleshoot and Fix Code B173

Step 1: Visual Inspection Begin by locating the A/C compressor relay in the main fuse/relay box (typically under the right-side service panel). Inspect all connector terminals for corrosion, pushed-back pins, or moisture intrusion. On used excavators, pay special attention to harness wear points where wires contact metal edges or vibrate against components.

Step 2: Relay Testing Remove the A/C relay and test it with a multimeter. Measure coil resistance (should typically read 70-90 ohms). Apply 12V directly to the relay coil terminals while checking for continuity across the switched contacts. Replace the relay if resistance is out of range or contacts don't close properly.

Step 3: Circuit Voltage Testing With the relay removed and ignition ON, check for battery voltage (12V) at the relay socket's power supply terminal. Using Kobelco's diagnostic software (or compatible J1939 scan tool), command the A/C system ON and verify the ECM sends a ground signal to the relay control wire (should drop to <1V).

Step 4: Compressor Clutch Verification Measure the A/C compressor clutch coil resistance directly at the compressor (typically 3-5 ohms). Check for shorts to ground. Excessive resistance or direct ground indicates clutch coil replacement is necessary.

Step 5: Harness Continuity Check Trace the entire wiring from ECM connector to relay socket. Check for opens, shorts to ground, or shorts to power. Used machines often develop harness failures where cables flex near moving components.

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Disclaimer: This guide provides general troubleshooting steps for educational purposes. Always consult the official Kobelco service manual for your specific machine serial number. If you're unfamiliar with electrical diagnostics or lack proper tools, seek assistance from a certified Kobelco technician to prevent further damage.