Komatsu PC800-7 Fault Code E979: Complete Diagnostic Guide

What is Komatsu PC800-7 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 PC800-7, this system is critical for maintaining optimal hydraulic pressure and flow to all work functions. The pump flow control solenoid modulates pump displacement based on operator demand and load conditions. When E979 activates, the machine typically enters a protective derate mode to prevent hydraulic system damage, significantly impacting productivity and operational efficiency.

Common Symptoms

When Code E979 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Sluggish or unresponsive controls during combined operations
• Warning light illumination on the monitor panel with fault code display
• Intermittent loss of hydraulic flow that may worsen as the machine warms up
• Automatic engine derate limiting RPM to protect the hydraulic system

Potential Causes

The most common technical causes for E979 on used PC800-7 excavators include:

• Solenoid valve coil failure due to internal short circuits or open windings (resistance typically should read 8-12 ohms)
• Wiring harness damage at known rub points near the main pump mounting area where vibration causes insulation wear
• Corroded or loose connector pins at the pump solenoid harness connection (C-series connectors are prone to moisture intrusion)
• ECM power supply issues providing incorrect reference voltage to the solenoid circuit
• Contaminated hydraulic oil causing mechanical sticking of the solenoid spool valve
• Failed pump control sensor providing incorrect feedback to the ECM

How to Troubleshoot and Fix Code E979

Step 1: Initial Inspection Using Komatsu KOMTRAX or compatible diagnostic software, record active and stored codes. Check freeze frame data for operating conditions when the fault occurred. Physically inspect the pump control solenoid connector for corrosion, bent pins, or oil contamination—common issues in used machines operating in harsh environments.

Step 2: Electrical Testing Disconnect the solenoid connector and measure coil resistance using a quality multimeter. Specification should be 8-12 ohms at 20°C. Check for short to ground (should read infinite resistance). With ignition on, verify the ECM supplies 12V reference voltage to the solenoid circuit. Inspect the entire wiring harness from ECM to pump, paying special attention to areas where the harness contacts the frame or pump housing.

Step 3: Component and System Verification If electrical values are correct, test the solenoid valve mechanically by applying direct 12V power (briefly) to verify clicking and movement. Check hydraulic oil condition—contaminated oil can cause valve sticking. For used excavators, examine harness routing for previous repairs or modifications that may have created new rub points. Replace the solenoid valve if mechanical or electrical tests fail, ensuring proper torque specifications (typically 25-30 Nm) during installation.

Step 4: System Reset and Validation After repairs, clear fault codes and perform a complete operational test under load. Monitor hydraulic response times and verify no code recurrence.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional assistance for complex hydraulic system repairs. Improper diagnosis can lead to costly component damage.