What is Kobelco SK200-8 Fault Code P0686?

Fault Code P0686 indicates a malfunction in the Main Relay Control Circuit, specifically detecting an abnormal voltage or open/short circuit in the ECM power relay control system. This code is part of the SAE J1939 diagnostic standard and signals that the Engine Control Module (ECM) has detected an issue with the relay responsible for supplying power to critical engine control components.

In the Kobelco SK200-8 excavator, this code directly affects the machine's ability to maintain proper electrical power distribution to fuel injection systems, sensors, and actuators. The main relay acts as a gatekeeper, controlling power flow from the battery to the ECM and related systems. When this circuit fails, the excavator may experience intermittent power loss, difficult starting conditions, or complete shutdown to prevent component damage. For used excavators operating in demanding environments, this fault often stems from age-related electrical degradation rather than catastrophic failure.

Common Symptoms

• Check Engine Light (CEL) illuminated on the instrument panel with stored diagnostic trouble code
• Difficult or inconsistent engine starting, including extended cranking times or no-start conditions
• Sudden engine shutdown during operation, particularly during high electrical load conditions
• Intermittent power loss to display screens, gauges, or electronic control functions
• ECM fails to energize after key-on, with no communication to diagnostic tools

Potential Causes

The P0686 code in used Kobelco SK200-8 machines typically originates from several age-related electrical issues:

Corroded or damaged relay contacts within the main power relay are extremely common in excavators with 5,000+ operating hours. Wiring harness chafing near the engine firewall or along the right-side frame rail creates intermittent grounds or open circuits. Connector corrosion at the ECM power supply connector (C101) allows moisture intrusion, especially in machines operating in wet or coastal environments.

Faulty main relay itself due to internal coil failure or contact welding from repeated cycling. ECM internal driver circuit failure can send incorrect voltage signals to the relay. Battery voltage irregularities below 11.5V or above 16V may trigger false relay control errors. Aftermarket electrical modifications or improperly spliced auxiliary circuits sometimes backfeed voltage into the relay control wire.

How to Troubleshoot and Fix Code P0686

Step 1: Verify Battery and Charging System Using a digital multimeter, check battery voltage with engine off (should read 12.4-12.8V) and running (13.8-14.4V). Low voltage from a weak battery or failing alternator can cause false relay control codes. Inspect battery cable connections for corrosion and ensure tight terminations at both battery posts and chassis ground points.

Step 2: Inspect Main Power Relay Locate the main relay in the fuse/relay panel (typically under the operator cab near the left console). Remove the relay and visually inspect terminals for burn marks, corrosion, or discoloration. Test relay resistance: coil should read 70-90 ohms, and contact resistance should be less than 0.5 ohms when energized. For used excavators, replace any relay showing signs of heat damage or contact pitting—a $45-$85 part that prevents costly downtime.

Step 3: Check Wiring Harness and Connectors Trace the relay control wire (usually a small-gauge wire from ECM pin to relay terminal) checking for abrasion points where harnesses contact frame members. Pay special attention to the ECM connector—disconnect and inspect for green corrosion, bent pins, or moisture. Apply dielectric grease and ensure positive connector lock engagement. Use your multimeter to test continuity (should read less than 1 ohm) and check for shorts to ground (should read infinite resistance).

Step 4: Test ECM Output Signal With relay removed and key in ON position, measure voltage at the relay control terminal—should read approximately 12V when ECM commands relay closure. If voltage is absent or incorrect, suspect ECM internal driver failure requiring module replacement or professional reprogramming using Kobelco diagnostic software.

Disclaimer: This guide provides general troubleshooting information. Always consult the Kobelco service manual for your specific machine serial number and consider professional diagnosis for complex electrical issues, especially when ECM replacement may be necessary.