Kobelco SK135SR-2 Fault Code B174: Diagnosis and Repair Guide

What is Kobelco SK135SR-2 Fault Code B174?

Fault Code B174 on the Kobelco SK135SR-2 excavator indicates a malfunction in the air conditioning (A/C) system control circuit, specifically related to the A/C compressor clutch relay or its associated wiring harness. This code is stored in the electrical control module (ECM) when the system detects an abnormal voltage signal or open/short circuit within the A/C compressor engagement pathway.

While not directly impacting core hydraulic or engine performance, this fault affects operator comfort and can indicate broader electrical system issues. On used excavators, B174 often reveals aging electrical components that may cascade into more serious control system failures if left unaddressed. The A/C compressor clutch relay controls electromagnetic engagement of the compressor, and any disruption prevents proper climate control operation.

Common Symptoms

When Code B174 is active on your SK135SR-2, operators typically experience:

• A/C system completely non-functional despite controls being activated
• Dash warning light or service indicator illuminated on the instrument cluster
• No audible click from the compressor clutch when A/C button is pressed
• Intermittent A/C operation that works sporadically before failing completely
• Battery drain issues in severe cases where relay remains stuck in closed position

Potential Causes

The most common technical causes for Code B174 on used SK135SR-2 excavators include:

• Failed A/C compressor clutch relay due to contact wear or coil burnout (common after 5,000+ hours)
• Corroded or damaged wiring harness connections at the relay socket or ECM connector
• Chafed wiring near the engine firewall where harness routing experiences vibration and heat exposure
• Blown fuse in the A/C circuit protecting the relay power supply
• Faulty ECM output driver controlling relay activation (less common but possible on high-hour machines)
• Poor ground connections at chassis grounding points near the cab electrical box

How to Troubleshoot and Fix Code B174

Step 1: Visual Inspection Begin with the relay box located under the operator's seat or behind the left-hand console panel. Remove the A/C compressor relay (consult your service manual for exact location—typically a 4-pin or 5-pin relay). Inspect the relay terminals and socket for corrosion, heat discoloration, or burnt contacts. On used excavators, socket corrosion is extremely common in humid operating environments.

Step 2: Electrical Testing Using a digital multimeter, check relay coil resistance (should be 70-90 ohms on most Kobelco relays). Test for 12V power supply at the relay socket with ignition on and A/C activated. Check continuity of the ground circuit from relay to chassis ground. Measure voltage at ECM control wire (typically 12V when A/C requested, 0V when off).

Step 3: Harness and Connector Inspection Trace the wiring harness from the relay to the ECM connector and compressor clutch. Focus on areas where the harness passes through bulkheads or near hot engine components—these are known rub points on SK135SR-2 models. Look for abraded insulation, green corrosion on terminals, or loose connector locks. Clean all connections with electrical contact cleaner and apply dielectric grease before reassembly.

Step 4: Component Replacement If relay tests faulty, replace with genuine Kobelco part or quality equivalent rated for 20A minimum. If wiring damage is found, repair using heat-shrink solder connections rather than crimp connectors for durability in high-vibration environments. Clear codes using Kobelco diagnostic software or compatible scan tool and verify repair.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Kobelco SK135SR-2 service manual for model-specific procedures and safety precautions. If unfamiliar with electrical diagnostics, consult a certified Kobelco dealer or qualified heavy equipment electrician to prevent ECM damage or electrical system issues.