Komatsu PC750SE-7 Fault Code E955: Complete Diagnostic Guide

What is Komatsu PC750SE-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 PC750SE-7, this system is critical for optimizing hydraulic performance across multiple functions simultaneously. 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 hydraulic system damage, significantly impacting productivity on job sites.

Common Symptoms

When Code E955 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Warning light illumination on the monitor panel with accompanying fault message
• Sluggish or unresponsive controls, particularly during combined operations
• Inconsistent hydraulic response where functions work intermittently or at reduced speed
• Engine may not reach full RPM under load due to protective derate programming

Potential Causes

The most common technical causes for E955 on used PC750SE-7 excavators include:

• Solenoid valve coil failure due to heat exposure or internal short/open circuit
• Wiring harness damage at known rub points near the hydraulic pump mounting area
• Corroded or loose connector pins at the pump control solenoid (common in machines with 5,000+ hours)
• ECM internal driver circuit failure affecting solenoid command signals
• Contaminated hydraulic oil causing solenoid valve spool sticking
• Voltage supply issues from damaged power distribution harness or poor grounds

How to Troubleshoot and Fix Code E955

Step 1: Visual Inspection Begin by inspecting the pump control solenoid connector located on the main hydraulic pump assembly. Check for corrosion, bent pins, or oil contamination. On used excavators, examine the wiring harness routing for chafing against the pump housing or frame—a common wear point on this model.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid coil resistance (typically 3-7 ohms for Komatsu solenoids). Check supply voltage at the connector with ignition on (should read 12-24V depending on system). Verify ground continuity to chassis (less than 1 ohm resistance).

Step 3: Solenoid Function Test With Komatsu diagnostic software (KOMTRAX Plus) connected, perform an active solenoid test to verify ECM output and mechanical response. Listen for audible clicking and measure current draw (typically 1.5-2.5 amps).

Step 4: Harness and ECM Verification If solenoid tests normal, trace the harness back to the ECM connector, checking for voltage drops or intermittent connections. For used machines, consider harness replacement if multiple repair splices are present.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnosis for complex electrical issues or ECM replacement procedures.