Komatsu PC750-7 Fault Code E979: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E979?

Fault Code E979 indicates a malfunction in the hydraulic pump control system, specifically related to abnormal pump discharge pressure or pump control solenoid valve circuit failure. This code is triggered when the Electronic Control Module (ECM) detects that the hydraulic pump's discharge pressure does not match the commanded output, or when there's an electrical fault in the pump control solenoid circuit.

For the PC750-7, this is a critical fault because the hydraulic pump control system directly manages the machine's main hydraulic functions. The pump control solenoid valve regulates pump displacement based on load demands, and any malfunction severely impacts operational efficiency, fuel consumption, and overall machine performance. This code can force the machine into a protective derate mode to prevent catastrophic pump or hydraulic system damage.

Common Symptoms

When Code E979 is active, operators typically experience:

• Hydraulic system derate with noticeably reduced boom, arm, or bucket speed
• Illuminated warning lamp on the monitor panel with accompanying fault code display
• Erratic hydraulic response or inconsistent implement movement under load
• Increased engine RPM without corresponding hydraulic power output
• Intermittent loss of hydraulic function during operation, particularly under heavy loads

Potential Causes

The most common technical causes for E979 on used PC750-7 excavators include:

• Pump control solenoid valve failure due to internal coil degradation or contaminated hydraulic oil causing valve sticking
• Wiring harness damage at known rub points near the main pump assembly, particularly where harnesses route along the swing bearing
• Pressure sensor malfunction (pump discharge pressure sensor) from age-related drift or connector corrosion
• ECM communication errors caused by corroded connectors or voltage supply issues
• Contaminated hydraulic oil causing proportional valve malfunction within the pump control system
• Worn pump regulator components in high-hour machines affecting pressure feedback signals

How to Troubleshoot and Fix Code E979

Step 1: Connect Diagnostic Software Use Komatsu KOMTRAX or compatible diagnostic software to read live data. Monitor pump discharge pressure values and solenoid command signals while operating hydraulics. Compare actual vs. commanded pressure readings.

Step 2: Inspect Electrical Connections Physically inspect the pump control solenoid valve connector and associated wiring harness. On used excavators, check for:

• Connector pin corrosion (clean with electrical contact cleaner)
• Harness chafing at mounting brackets near the pump
• Proper ground connections at the ECM

Step 3: Test Solenoid Valve Circuit Using a multimeter, measure solenoid resistance (typically 10-15 ohms). Check supply voltage at the connector (should be 12-24V depending on system). Verify no short circuits to ground.

Step 4: Check Hydraulic Pressure Sensor Test the pump discharge pressure sensor output voltage (typically 0.5-4.5V range). Compare readings against actual hydraulic pressure using a mechanical gauge. Replace sensor if readings are erratic or out of specification.

Step 5: Hydraulic Oil Analysis For used machines, contaminated oil is often overlooked. Check oil condition and filter status. Metal particles indicate internal pump wear requiring deeper investigation.

---

Disclaimer: This guide provides general troubleshooting information. Always consult Komatsu service manuals and qualified technicians for machine-specific procedures. Hydraulic system work requires proper training and safety precautions.