Komatsu PC750-7 Fault Code E955: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E955?

Fault Code E955 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 PC750-7, this system is critical for maintaining optimal hydraulic performance across all functions. The pump flow control solenoid modulates pressure and flow based on operator demand and load conditions. When E955 appears, the machine typically enters a protective derate mode to prevent damage to the hydraulic system, significantly impacting productivity and operational efficiency.

Common Symptoms

When Code E955 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Sluggish or unresponsive controls, particularly under heavy load conditions
• Warning light illumination on the monitor panel with accompanying fault code display
• Intermittent loss of hydraulic function that may temporarily restore after machine restart
• Abnormal pump noise or pressure fluctuations during operation

Potential Causes

The most common technical causes for E955 on used PC750-7 excavators include:

• Solenoid valve coil failure due to overheating or internal short/open circuit
• Wiring harness damage at known rub points near the pump assembly or along the main chassis harness
• Corroded or loose connector pins at the solenoid valve or ECM connections (common in machines with high operating hours)
• ECM internal fault affecting the solenoid driver circuit
• Contaminated hydraulic oil causing solenoid valve sticking or mechanical binding
• Voltage supply issues from the machine's electrical system affecting solenoid operation

How to Troubleshoot and Fix Code E955

Step 1: Visual Inspection Begin by inspecting the pump flow control solenoid valve and its wiring harness for obvious damage. Check connector pins for corrosion, bent terminals, or oil contamination. On used excavators, pay special attention to harness routing near moving components where friction wear commonly occurs.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid coil resistance (typically 10-30 ohms for Komatsu solenoids—verify with service manual). Check for supply voltage at the connector (should be 12-24V depending on system). Test for short to ground or open circuits in the harness between the solenoid and ECM.

Step 3: Functional Testing With Komatsu diagnostic software (KOMTRAX or PC-based diagnostic tool), perform active solenoid tests to verify ECM output and solenoid response. Monitor real-time data for voltage drops or irregular current draw patterns.

Step 4: Component Replacement If testing confirms solenoid failure, replace the valve assembly and clear codes. For harness issues on used machines, repair damaged sections with proper gauge wire and heat-shrink connections. Always check hydraulic oil condition—contamination often accelerates solenoid failures.

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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 seek professional assistance for complex repairs involving hydraulic or electrical systems.