Komatsu PC800-7 Fault Code E986: Complete Diagnostic Guide

What is Komatsu PC800-7 Fault Code E986?

Fault Code E986 indicates a malfunction in the hydraulic pump control system, specifically related to the main pump regulator solenoid circuit or its feedback signal. 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 and pressure.

In the PC800-7, this system is critical because it directly controls the variable displacement of the main hydraulic pumps. When E986 appears, the machine typically enters a protective derate mode to prevent damage to the hydraulic system. This fault compromises the excavator's ability to match hydraulic power to load demands, resulting in reduced productivity and potential component wear.

Common Symptoms

• Reduced hydraulic power across all functions (boom, arm, bucket, swing)
• Warning light illumination on the monitor panel with accompanying alarm
• Erratic or sluggish response when operating multiple hydraulic functions simultaneously
• Engine RPM fluctuations during loaded operations as the pump fails to regulate properly
• Intermittent loss of hydraulic pressure that may come and go with machine temperature changes

Potential Causes

The most common technical causes for E986 on used PC800-7 excavators include:

• Solenoid coil failure in the pump regulator valve due to heat cycling and age-related degradation
• Wiring harness damage at known rub points near the main pumps, particularly where harnesses pass through the swing bearing area
• Corroded or loose connector pins at the pump control solenoid (common in machines with 5,000+ hours)
• ECM internal driver circuit failure affecting the solenoid output channel
• Contaminated hydraulic oil causing the regulator spool to stick, preventing proper solenoid response
• Voltage supply issues from the main power harness or ground connections

How to Troubleshoot and Fix Code E986

Step 1: Initial Inspection Using Komatsu KOMTRAX or compatible diagnostic software, record active and stored codes. Check the solenoid connector at the main pump for corrosion, bent pins, or oil contamination. On used excavators, this connector frequently shows wear from vibration and environmental exposure.

Step 2: Electrical Testing Disconnect the solenoid connector and measure coil resistance with a multimeter—specification is typically 8-12 ohms at 20°C. Check for supply voltage (should be 24V DC) and verify ground continuity (less than 1 ohm resistance to chassis ground). Inspect the wiring harness along its entire length, especially at flex points.

Step 3: Functional Testing If electrical values are correct, perform a solenoid swap test with the opposite pump's solenoid (if equipped) to isolate component versus circuit failure. Monitor live data while operating the machine to check for command signal versus actual response discrepancies.

Step 4: Advanced Diagnostics For persistent codes after component replacement, test ECM output driver function using an oscilloscope to verify proper PWM signal characteristics. Check hydraulic system pressure to ensure mechanical binding isn't preventing solenoid operation.

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 electrical or hydraulic system repairs.