Komatsu PC750-7 Fault Code E977: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E977?

Fault Code E977 indicates a malfunction in the engine control system's communication circuit, specifically related to the CAN (Controller Area Network) bus communication error between the Engine Controller (ECM) and the Monitor Controller.

This code triggers when the ECM detects abnormal or interrupted data transmission on the CAN bus network, which is the critical communication backbone connecting all electronic control modules on the PC750-7. The CAN bus enables real-time data exchange between the engine controller, hydraulic system, and operator display. When this communication fails, the excavator's integrated control systems cannot function properly, potentially leading to reduced performance or complete shutdown to prevent damage.

For the PC750-7's Komatsu SAA6D140E-5 engine, maintaining proper CAN communication is essential for coordinating fuel injection timing, turbocharger control, and hydraulic pump output.

Common Symptoms

• Monitor display shows error messages or goes completely blank while engine continues running
• Multiple warning lights illuminate simultaneously on the instrument cluster
• Engine derating or entering limp mode with reduced power output
• Intermittent loss of gauge readings (fuel level, coolant temperature, hydraulic oil temperature)
• Hydraulic system performance issues due to loss of load-sensing communication

Potential Causes

Wiring harness damage is the most common culprit in used PC750-7 excavators, particularly where harnesses route near the engine mount points or swing bearing, causing abrasion over time.

Corroded or loose CAN bus connectors at the ECM (located behind the operator cab) or Monitor Controller connections, especially in machines operated in wet or coastal environments.

Failed termination resistors on the CAN network, which are critical for signal integrity in the 120-ohm terminated bus system.

ECM or Monitor Controller internal failure, though less common, can occur in high-hour machines (8,000+ hours).

Voltage supply issues to either controller, typically caused by corroded ground connections or failing main relay.

How to Troubleshoot and Fix Code E977

Step 1: Visual Inspection Begin by inspecting all wiring harnesses between the ECM and Monitor Controller. On used excavators, check for harness wear at common rub points: near the engine mounting brackets, along the right-hand frame rail, and where cables enter the cab. Look for pinched, cut, or abraded wires exposing copper.

Step 2: Connector Examination Disconnect and inspect the CAN bus connectors at both the ECM (typically a 48-pin connector) and Monitor Controller. Clean contacts with electrical contact cleaner and check for bent pins, corrosion (green/white deposits), or moisture intrusion. Apply dielectric grease before reconnecting.

Step 3: CAN Bus Resistance Check Using a digital multimeter, measure resistance across the CAN-High and CAN-Low terminals with all controllers connected and ignition off. You should read approximately 60 ohms (two 120-ohm termination resistors in parallel). A reading of 120 ohms indicates one missing terminator; infinite resistance suggests an open circuit.

Step 4: Voltage Testing With ignition on, verify battery voltage (24V nominal) is present at both controllers' power supply pins. Check ground continuity with less than 0.5 ohms resistance to chassis ground.

Step 5: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to read real-time CAN bus traffic and identify which controller is failing to communicate. Monitor for intermittent dropouts during engine operation.

Critical for Used Equipment: Before replacing expensive controllers, thoroughly test all harnesses and connectors. In machines with 5,000+ operating hours, harness replacement often resolves E977 without controller replacement, saving thousands in parts costs.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC750-7 service manual and consider professional diagnosis for complex electrical issues. Improper repairs may cause additional damage or safety hazards.