Komatsu PC750LC-7 Fault Code E979: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E979?

Fault Code E979 indicates a malfunction in the Engine Control Module (ECM) communication circuit or a critical internal ECM error. This code specifically relates to the controller area network (CAN) communication system that allows the engine ECM to interface with other machine control systems on the PC750LC-7.

This fault affects the SAA6D140E-3 engine's ability to properly communicate operational data between the engine controller and the machine's monitoring systems. When E979 triggers, the excavator's integrated control system cannot reliably exchange critical information about engine parameters, hydraulic demands, and safety protocols. For a machine of this size and complexity, maintaining proper ECM communication is essential for coordinated operation between the engine, hydraulics, and operator interface systems.

Common Symptoms

When Code E979 is active, operators typically experience:

• Warning lamp illumination on the monitor panel with possible error message display
• Intermittent or complete loss of engine parameter readings on the dashboard (RPM, temperature, pressure gauges)
• Engine derate mode activation, limiting power output to protect the machine
• Erratic hydraulic response due to disrupted communication between engine load sensing and hydraulic demands
• Inability to access diagnostic information through the standard monitor interface

Potential Causes

The most common technical causes for E979 on used PC750LC-7 excavators include:

• Corroded or damaged CAN bus connectors at the ECM harness connection points (common wear area behind the engine compartment)
• Wiring harness chafing where the main engine harness routes near the engine mounting brackets or hydraulic lines
• ECM internal failure due to moisture intrusion, vibration damage, or component aging in high-hour machines
• Voltage supply issues from failing alternator or battery connections affecting ECM power stability
• Aftermarket modifications or improper repairs disrupting CAN network termination resistance

How to Troubleshoot and Fix Code E979

Step 1: Visual Inspection Begin with a thorough inspection of the main engine harness and all ECM connectors. On used excavators, check specifically where the harness passes through the engine bay firewall and near hydraulic pump mounting areas—these are known rub points. Look for frayed wires, corroded pins, or moisture in connector boots.

Step 2: Electrical Testing Using a digital multimeter, verify proper voltage supply to the ECM (should be 24V nominal). Check CAN-High and CAN-Low circuit resistance between ECM pins—proper termination should show approximately 60 ohms between CAN lines with key off. Use Komatsu KOMTRAX or compatible diagnostic software to monitor real-time CAN bus traffic and identify communication dropouts.

Step 3: Component Testing Test ground circuits at the ECM mounting location for resistance (should be less than 1 ohm to chassis ground). If all wiring checks pass, the ECM itself may require replacement. For used machines, consider that ECMs can develop internal failures from years of vibration and thermal cycling.

Step 4: Harness and Connector Repair Before replacing expensive components, repair any damaged wiring and clean all connector terminals with electrical contact cleaner. Apply dielectric grease to prevent future corrosion—critical in machines operating in humid or dusty environments.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic services for complex electrical issues.