Komatsu PC750SE-7 Fault Code E979: Complete Diagnostic Guide

What is Komatsu PC750SE-7 Fault Code E979?

Fault Code E979 indicates a malfunction in the hydraulic pump control system, specifically related to the pump flow control solenoid valve circuit or its associated sensors. This code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage, current draw, or response time from the pump control solenoid that regulates hydraulic flow distribution.

In the PC750SE-7, this system is critical for maintaining optimal hydraulic performance across all functions. The pump flow control solenoid adjusts displacement based on load demands, directly affecting fuel efficiency and operational responsiveness. When E979 activates, the machine typically enters a protective derate mode to prevent hydraulic system damage.

Common Symptoms

When Code E979 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Illuminated warning lamp on the monitor panel with fault code display
• Sluggish or unresponsive controls, particularly under heavy load conditions
• Increased engine RPM without corresponding hydraulic speed (pump not responding to demand)
• Intermittent loss of auxiliary hydraulic functions or attachment performance issues

Potential Causes

The most common technical causes for E979 in used PC750SE-7 excavators include:

• Solenoid valve coil failure due to overheating or internal short circuits (common after 8,000+ hours)
• Wiring harness damage at known rub points near the pump mounting bracket and main frame pivot
• Corroded or loose connector pins at the pump control solenoid (C-connector series)
• ECM voltage supply issues from degraded power supply circuits or ground faults
• Contaminated hydraulic oil causing solenoid valve spool sticking or restricted movement
• Failed pump control sensor providing incorrect feedback to the ECM

How to Troubleshoot and Fix Code E979

Step 1: Visual Inspection Begin by inspecting the pump control solenoid harness for physical damage, particularly where it routes along the pump housing. Check all connector pins for corrosion, bent terminals, or oil contamination—common in used machines with compromised seals.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid coil resistance (typically 8-12 ohms for this model). Check supply voltage at the connector with ignition on (should read 12-24V depending on system). Verify ground continuity is below 1 ohm resistance.

Step 3: Solenoid Function Test With Komatsu diagnostic software (KOMTRAX Plus) connected, perform an active solenoid test. Monitor current draw (should be 1.5-2.0 amps). Listen for audible clicking indicating valve actuation.

Step 4: Hydraulic System Check Inspect hydraulic oil condition and filter status. Contaminated oil can cause valve sticking. For used excavators, consider removing the solenoid valve for manual inspection of the spool for scoring or debris buildup.

Step 5: Component Replacement If testing confirms solenoid failure, replace with genuine Komatsu parts (Part #: 702-21-55901 or equivalent). Always apply dielectric grease to connectors during reassembly to prevent future corrosion.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and consider professional technician assistance for complex hydraulic system repairs.