Sumitomo Excavator Fault Code P0685: Complete Diagnostic Guide

What is Sumitomo General Fault Code P0685?

Fault Code P0685 indicates an ECM/PCM Power Relay Control Circuit malfunction. This diagnostic trouble code (DTC) signals that the Engine Control Module (ECM) has detected an abnormal voltage or current condition in the power relay control circuit that supplies battery voltage to various engine and machine control systems.

In Sumitomo excavators, this code specifically relates to the main power relay that controls electrical supply to critical engine management components. The ECM continuously monitors this circuit to ensure proper voltage delivery. When voltage falls outside the manufacturer's specified range (typically 10-16 volts during operation), the system logs P0685. This fault is critical because the power relay circuit directly affects fuel injection timing, sensor operation, and hydraulic system controls—all essential for optimal excavator performance and fuel efficiency.

Common Symptoms

• Engine cranks but fails to start or experiences extended cranking time before ignition
• Intermittent engine stalling during operation, particularly during cold starts or after sitting idle
• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Loss of electronic throttle response or erratic engine RPM control
• Hydraulic system performance issues, including sluggish boom or bucket response due to inadequate fuel delivery

Potential Causes

The most frequent causes of P0685 in used Sumitomo excavators include:

• Failed ECM power relay due to contact wear, internal coil failure, or moisture intrusion
• Damaged wiring harness between the relay and ECM, especially at common rub points near the engine mount brackets or along the main chassis harness
• Corroded or loose connector pins at the relay socket or ECM connector (particularly common in machines operating in wet/muddy conditions)
• Blown fuse in the ECM power supply circuit
• Faulty ECM ground connection causing voltage reference issues
• Weak battery or poor battery cable connections creating voltage drop conditions during startup

How to Troubleshoot and Fix Code P0685

Step 1: Visual Inspection Begin with a thorough physical examination of the power relay (typically located in the main fuse/relay box under the operator cab or near the battery compartment). Check for obvious signs of heat damage, melted plastic, or corrosion. On used excavators, inspect all wiring harnesses running to the relay for abrasion points where the harness contacts metal brackets or sharp edges—this is a common failure point after 3,000+ operating hours.

Step 2: Electrical Testing Using a digital multimeter, test the relay itself. Remove the relay and check coil resistance (typically 70-90 ohms). Apply battery voltage to the coil terminals and verify audible clicking and continuity through the contact terminals. Next, check the relay socket for battery voltage (should read 12-14V with key on). Inspect all connector pins for corrosion—apply dielectric grease after cleaning.

Step 3: Circuit Verification With the relay installed, check voltage at the ECM power supply pin while cranking (consult your Sumitomo service manual for specific pin location). Voltage should remain above 10V during cranking. If voltage drops excessively, test battery cables and ground straps—loose or corroded grounds are extremely common on older machines. Use Sumitomo's proprietary diagnostic software if available to monitor real-time relay control signals.

Step 4: Component Replacement If testing confirms relay failure, replace with OEM or equivalent relay meeting Sumitomo specifications. For used excavators with over 5,000 hours, consider replacing the entire relay socket if terminals show any pitting or discoloration. After repairs, clear codes and perform a full operating cycle test under load.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Sumitomo service manual and qualified heavy equipment technicians for repairs. Improper electrical system diagnosis can result in ECM damage or safety hazards.