Komatsu PC750SE-7 Fault Code E985: Complete Diagnostic Guide

What is Komatsu PC750SE-7 Fault Code E985?

Fault Code E985 indicates a malfunction in the hydraulic pump control system, specifically related to the main pump regulator solenoid circuit or its associated control signals. This code is triggered when the Engine Control Module (ECM) detects an abnormal voltage, current draw, or communication failure within the pump control circuitry.

The PC750SE-7 relies on precise hydraulic pump regulation to maintain optimal flow and pressure across all systems. When E985 appears, the ECM may limit pump output to protect the hydraulic system, directly impacting machine performance. This fault is particularly critical because the pump regulator controls fuel efficiency and prevents hydraulic overheating during heavy operations.

Common Symptoms

When Code E985 is active, operators typically experience:

• Reduced hydraulic power across all functions (boom, arm, bucket, and swing movements become sluggish)
• Warning light illumination on the monitor panel, often accompanied by an audible alarm
• Engine derate or limited RPM as the ECM enters protective mode
• Inconsistent hydraulic response during combined operations or under load
• Increased fuel consumption due to inefficient pump regulation

Potential Causes

The most common technical causes for E985 on used PC750SE-7 excavators include:

• Faulty pump regulator solenoid valve (internal coil failure or mechanical sticking)
• Damaged wiring harness between the ECM and pump solenoid, especially at flex points near the pump mounting area
• Corroded or loose electrical connectors at the solenoid or ECM interface
• Failed ECM output driver circuit controlling the solenoid
• Low system voltage from aging batteries or failing alternator affecting solenoid operation
• Contaminated hydraulic oil causing mechanical binding in the regulator assembly

How to Troubleshoot and Fix Code E985

Step 1: Visual Inspection Begin by inspecting the pump regulator solenoid and its wiring harness for obvious damage. On used excavators, check for harness chafing against the pump housing or frame rails. Examine all connectors for corrosion, bent pins, or oil contamination—common issues after years of operation.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid resistance (typically 10-30 ohms when cold). Check for supply voltage at the solenoid connector with the key on (should read battery voltage). Test the ground circuit for continuity to the ECM. Verify system voltage exceeds 24V during cranking.

Step 3: Solenoid Function Test With Komatsu diagnostic software (KOMTRAX Plus) connected, command the solenoid on/off while monitoring current draw. Normal operation shows 0.8-1.2 amps. Erratic readings indicate internal solenoid failure.

Step 4: Component Replacement If testing confirms solenoid failure, replace with genuine Komatsu parts. For used machines, always replace the connector pigtail simultaneously, as aged connectors cause repeat failures. Clear codes and perform a pump calibration procedure using diagnostic software.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnostic assistance for complex electrical issues.