Komatsu PC800SE-7 Fault Code E977: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code E977?

Fault Code E977 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 Electronic Control Module (ECM) detects an abnormal voltage, current draw, or communication failure within the pump control circuitry.

The PC800SE-7's hydraulic system relies on precise electronic control of the main pump regulators to optimize fuel efficiency and hydraulic performance. When E977 appears, the ECM may limit pump output to protect the system, directly impacting excavator productivity. This fault is particularly critical because the pump control system manages flow and pressure across all hydraulic functions, from swing to boom operations.

Common Symptoms

When Code E977 is active, operators typically experience:

• Reduced hydraulic power across all functions, with noticeably slower cycle times
• Warning light illumination on the monitor panel, often accompanied by an audible alarm
• Derated engine performance as the ECM enters protective mode to prevent system damage
• Erratic hydraulic response, including inconsistent swing speed or boom movement
• Increased fuel consumption due to inefficient pump regulation and compensatory throttle input

Potential Causes

The most common technical causes for E977 in used PC800SE-7 excavators include:

• Faulty pump regulator solenoid valve due to internal coil failure or mechanical sticking from contamination
• Damaged wiring harness between the ECM and pump solenoid, particularly at known rub points near the pump mounting area
• Corroded or loose electrical connectors at the solenoid or ECM interface, common in machines with high operating hours
• ECM internal fault affecting the pump control output driver circuit
• Hydraulic contamination causing the regulator spool to bind, increasing electrical load beyond normal parameters
• Voltage supply issues from the main power harness or grounding problems at the pump assembly

How to Troubleshoot and Fix Code E977

Step 1: Initial Inspection Connect Komatsu diagnostic software (KOMTRAX Plus or equivalent) to retrieve active and historical fault data. Check for multiple codes that might indicate broader electrical issues. Physically inspect the pump regulator solenoid and its connector for obvious damage, oil contamination, or corrosion.

Step 2: Electrical Testing Using a digital multimeter, measure solenoid coil resistance (typically 8-15 ohms for this model). Check supply voltage at the solenoid connector with the key on—should read approximately 24V DC. Inspect the wiring harness from ECM to solenoid, paying special attention to areas where the harness contacts the pump housing or frame rails, as these are common wear points on used machines.

Step 3: Component Testing If electrical values are correct, perform a functional test of the solenoid by applying direct power (with appropriate precautions). Listen for audible clicking and check for smooth plunger movement. For used excavators, remove and inspect the solenoid for internal varnish buildup or debris that could cause intermittent operation.

Step 4: Advanced Diagnostics If the solenoid tests good, suspect ECM output driver failure or internal hydraulic issues. Check hydraulic oil quality and filter condition—contaminated oil can cause regulator binding. Verify all ground connections at the pump and ECM are clean and tight, as poor grounding is frequently overlooked in older machines.

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