Komatsu PC300-8M0 Fault Code 879AKA: Complete Diagnostic Guide

What is Komatsu PC300-8M0 Fault Code 879AKA?

Fault Code 879AKA indicates a communication error or abnormal signal from the machine monitoring system (MMS) to the engine controller (ECM). This code specifically relates to the CAN bus communication network that allows the machine monitoring system and engine control module to exchange critical operational data.

In the Komatsu PC300-8M0, this communication pathway is essential for coordinating engine performance with hydraulic demands, monitoring system health, and displaying real-time information to the operator. When this CAN bus communication fails or becomes intermittent, the excavator's integrated control systems cannot function properly, potentially leading to reduced performance or complete operational shutdown.

Common Symptoms

When fault code 879AKA is active, operators typically experience:

• Monitor display errors or complete loss of gauge readings on the cab display panel
• Engine derate mode where power output is automatically reduced as a protective measure
• Warning lights illuminated on the dashboard, particularly the engine or system malfunction indicators
• Intermittent loss of machine functions including erratic hydraulic response or throttle control
• Inability to access diagnostic information through the standard monitor interface

Potential Causes

The most common technical causes for code 879AKA in used PC300-8M0 excavators include:

• Corroded or damaged CAN bus connectors between the ECM and MMS, particularly at harness junction points
• Wiring harness damage from rubbing against the frame near the battery box or along the right-hand chassis rail (known wear point on this model)
• Faulty ECM or MMS module due to moisture intrusion or component aging
• Poor ground connections at the main chassis ground points or controller mounting brackets
• Voltage irregularities from failing batteries or alternator issues affecting communication signal integrity
• Previous repair attempts using non-OEM connectors or improper wire splicing techniques

How to Troubleshoot and Fix Code 879AKA

Step 1: Visual Inspection Begin by inspecting all wiring harnesses between the cab-mounted monitor and the engine-mounted ECM. Pay special attention to connector pins for corrosion (green/white deposits) and check for physical damage where harnesses pass through the bulkhead. On used excavators, examine areas where cables contact metal edges or moving components.

Step 2: Electrical Testing Using a digital multimeter, verify CAN-High and CAN-Low voltage at both the ECM and MMS connectors. With ignition on and engine off, you should read approximately 2.5V on each line. Check for 120-ohm termination resistance across the CAN bus lines. Use Komatsu KOMTRAX diagnostic software or a compatible scan tool to monitor real-time communication status.

Step 3: Component and Ground Verification Test all ground connections for resistance below 1 ohm. Clean and re-torque ground terminals at the battery, frame, and controller mounting points. If wiring and grounds test properly, the issue likely resides in either the ECM or MMS module—these should be tested by substitution or sent for professional evaluation.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and consider professional assistance for complex electrical diagnostics or controller replacement.