Komatsu PC800SE-7 Fault Code E979: Complete Diagnostic Guide

What is Komatsu PC800SE-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.

In the Komatsu PC800SE-7, the ECM serves as the central processing unit for engine management, controlling fuel injection timing, turbocharger operation, and emissions systems. When E979 triggers, it signals that the ECM has detected an internal processing fault or has lost critical communication pathways with peripheral controllers. This is particularly serious because it can affect the machine's ability to regulate engine performance, monitor hydraulic systems, and maintain proper operational safety protocols.

Common Symptoms

When Code E979 is active, operators typically experience:

• Engine warning light illuminated on the dashboard with possible derate mode activation
• Intermittent or complete loss of engine power, with the machine entering limp mode
• Erratic gauge readings or complete instrument cluster failure
• Hydraulic system irregularities due to communication loss between controllers
• Engine may fail to start or shut down unexpectedly during operation

Potential Causes

The most common technical causes for E979 on used PC800SE-7 excavators include:

• ECM internal component failure due to age, heat stress, or moisture intrusion
• Corroded or damaged main wiring harness connectors at the ECM interface points
• CAN bus wiring damage from harness chafing against frame components (common near the engine mounting area)
• Voltage supply issues from failing alternator or battery connections causing ECM brownouts
• Water infiltration into the ECM housing through compromised seals
• Previous repair attempts with incorrect ECM software versions or failed flash updates

How to Troubleshoot and Fix Code E979

Step 1: Initial Inspection Begin by visually inspecting all ECM connectors for corrosion, bent pins, or moisture. On used excavators, check the main harness routing near the engine mounts where vibration-induced wear is common. Clean all connections with electrical contact cleaner.

Step 2: Voltage and Ground Testing Using a digital multimeter, verify the ECM receives proper voltage (typically 24V on this model) at the power supply pins. Check ground circuits for resistance below 0.5 ohms. Poor grounds are frequent culprits in older machines.

Step 3: CAN Bus Communication Check With Komatsu diagnostic software (KOMTRAX Plus) or compatible scan tool, monitor CAN bus activity. Check for communication errors between the ECM and monitor controller. Measure CAN High and CAN Low wire resistance (should read approximately 60 ohms between terminals with key off).

Step 4: ECM Evaluation If all external circuits test properly, the ECM itself may require replacement or professional reprogramming. Before replacing, verify no aftermarket modifications have corrupted the software. For used excavators, consider having the ECM bench-tested by a Komatsu dealer to confirm internal failure.

Step 5: Harness Integrity Perform a continuity test on all wires between the ECM and junction boxes. Pay special attention to harness sections that pass through bulkheads or near hot engine components where insulation degradation occurs over time.

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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 assistance for ECM-related repairs, as improper procedures can cause additional damage.