Sumitomo SH300-6 Fault Code 685: Complete Diagnostic Guide

What is Sumitomo SH300-6 Fault Code 685?

Fault Code 685 on the Sumitomo SH300-6 excavator indicates a malfunction in the hydraulic pilot pressure sensor circuit, specifically detecting an abnormal voltage signal or sensor failure in the pilot system monitoring circuit. This code triggers when the Electronic Control Module (ECM) receives a voltage reading outside the acceptable range (typically 0.5-4.5V) from the pilot pressure sensor, which monitors the hydraulic pilot system that controls attachment and swing functions.

This fault is critical because the pilot pressure sensor provides essential feedback to the ECM for proper hydraulic system operation. When compromised, the machine may enter derate mode to protect hydraulic components from operating without proper pressure monitoring, directly impacting productivity and operational safety on the SH300-6 platform.

Common Symptoms

• Hydraulic function warning light illuminated on the instrument cluster with reduced hydraulic response
• Sluggish or inconsistent operation of boom, arm, bucket, or swing functions
• Intermittent loss of pilot pressure causing delayed joystick response
• ECM derate mode activated, limiting engine RPM to approximately 1,500-1,800 RPM
• Error code storage in ECM memory even after temporary system recovery

Potential Causes

The most common causes of Code 685 on used SH300-6 excavators include:

• Pilot pressure sensor failure due to internal component degradation (common after 8,000+ operating hours)
• Damaged or corroded wiring harness at the sensor connector, particularly where the harness routes near the swing bearing (known rub point on this model)
• Contaminated electrical connectors from hydraulic fluid seepage or moisture intrusion in the sensor mounting area
• Faulty ECM voltage supply to the sensor (5V reference circuit failure)
• Shorted or open sensor signal wire between the sensor and ECM, especially in harness sections exposed to frame flex

How to Troubleshoot and Fix Code 685

Step 1: Visual Inspection Begin by locating the pilot pressure sensor (typically mounted on the main control valve near the operator's cab). Inspect the sensor body for physical damage, oil contamination, or loose mounting. Check the wiring harness for visible damage, particularly at flex points and where it passes through the swing bearing area—a common wear location on used SH300-6 machines.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure the 5V reference voltage from the ECM (key on, engine off). Verify you have 4.8-5.2V at the supply pin. Next, measure sensor resistance across the sensor terminals; specifications typically range from 200-400 ohms at room temperature. Check for continuity in the signal wire back to the ECM and ensure less than 5 ohms resistance.

Step 3: Signal Voltage Verification Reconnect the sensor and backprobe the signal wire with the key on. Normal idle pressure should produce 1.5-3.0V. Activate pilot functions and observe voltage changes—readings should fluctuate smoothly. Erratic voltage or readings stuck at 0V or 5V indicate sensor failure or wiring faults.

Step 4: Component Replacement and Verification If testing confirms sensor failure, replace with an OEM or equivalent pilot pressure sensor (Sumitomo part number recommended for compatibility). For used excavators, always replace the connector pigtail if any corrosion is present—this prevents recurring faults. After installation, clear codes using diagnostic software (Sumitomo MMCS or compatible scan tool) and perform a pilot system function test to verify proper operation.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the Sumitomo SH300-6 service manual for complete specifications and safety procedures. Complex electrical or hydraulic issues should be addressed by certified heavy equipment technicians with proper diagnostic tools.