Komatsu PC750LC-7 Fault Code E977: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E977?

Fault Code E977 indicates a malfunction in the Engine Control Module (ECM) communication circuit, specifically related to the CAN (Controller Area Network) bus system that enables data exchange between the engine controller and other machine control systems.

This code triggers when the ECM detects abnormal voltage levels, signal interruptions, or complete communication failure on the CAN bus network. For the PC750LC-7, this is critical because the CAN system coordinates engine performance with hydraulic controls, transmission functions, and the machine's monitoring systems. When communication breaks down, the excavator cannot properly regulate fuel injection timing, hydraulic response, or emission controls, potentially causing severe operational limitations.

Common Symptoms

When Code E977 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel, often accompanied by a communication error message
• Engine derate mode activated, limiting power output to 50-70% of normal capacity
• Erratic hydraulic response or delayed implement movements due to coordination failures
• Intermittent gauge readings or complete loss of monitor display functions
• Difficulty starting or unexpected engine shutdowns during operation

Potential Causes

The most common technical causes for E977 on used PC750LC-7 excavators include:

• CAN bus wiring harness damage from rubbing against the engine block or frame rails (common wear point near the fuel injection pump)
• Corroded or loose connector pins at the ECM harness connection (C101 connector behind the operator cab)
• Failed termination resistors on the CAN network (120-ohm resistors at network endpoints)
• ECM internal circuit failure due to voltage spikes or moisture intrusion
• Damaged shielding on CAN wiring allowing electromagnetic interference from starter motor or alternator

How to Troubleshoot and Fix Code E977

Step 1: Visual Inspection Begin by inspecting all wiring harnesses between the ECM and machine controllers. On used excavators, check for harness chafing where cables pass through the engine bay firewall and near hydraulic lines. Look for green corrosion on connector pins, particularly at the ECM's main connector.

Step 2: Resistance Testing Using a digital multimeter, disconnect both ends of the CAN bus and measure resistance between the CAN-High and CAN-Low wires. You should read approximately 60 ohms (two 120-ohm termination resistors in parallel). Readings above 100 ohms or below 50 ohms indicate termination resistor failure or wiring shorts.

Step 3: Voltage Verification With the key on and engine off, measure voltage at the ECM connector. CAN-High should read approximately 2.5V, and CAN-Low should read 2.5V relative to ground. Significant deviation indicates circuit problems or ECM failure.

Step 4: Diagnostic Software Analysis Connect Komatsu KOMTRAX or compatible diagnostic software to retrieve detailed fault timestamps and communication error counts. This identifies whether the fault is intermittent (suggesting harness issues) or constant (indicating component failure).

For used excavators, always clean and apply dielectric grease to all connectors before replacing expensive components like the ECM.

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Disclaimer: This guide provides general diagnostic information. Always consult Komatsu service manuals and qualified technicians for machine-specific repairs, especially when working with electronic control systems.