Komatsu PC750LC-7 Fault Code E981: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E981?

Fault Code E981 indicates a malfunction in the machine's hydraulic system pressure sensor circuit, specifically related to abnormal voltage readings from the main pump pressure sensor. This code is triggered when the Engine Control Module (ECM) detects voltage signals outside the manufacturer's specified range (typically below 0.5V or above 4.5V) from the pressure monitoring system.

In the PC750LC-7, this sensor plays a critical role in monitoring hydraulic system performance and protecting the main pumps from over-pressure conditions. The ECM uses this data to regulate pump output, optimize fuel efficiency, and prevent catastrophic hydraulic system damage. When E981 appears, the machine may enter a protective derate mode to prevent further damage.

Common Symptoms

• Hydraulic system warning light illuminated on the operator display panel
• Noticeable loss of hydraulic power or reduced digging force across all functions
• Erratic boom or arm movement with inconsistent response to joystick inputs
• Machine entering derate mode, limiting engine RPM to approximately 1,500-1,800 RPM
• Intermittent code activation, especially during cold starts or high-load operations

Potential Causes

The most common causes of E981 in used PC750LC-7 excavators include:

• Pressure sensor failure due to internal component degradation (common after 8,000+ operating hours)
• Corroded or damaged wiring harness connections at the sensor plug, particularly where the harness routes near the hydraulic tank
• Chafed wiring at known rub points along the main frame rail near the left-side track motor
• Contaminated hydraulic oil causing sensor membrane damage or blockage
• ECM internal fault or corrupted calibration data (less common but possible in high-hour machines)
• Loose or corroded ground connections affecting the sensor's reference voltage

How to Troubleshoot and Fix Code E981

Step 1: Visual Inspection Begin by inspecting the main pump pressure sensor located on the hydraulic pump assembly. Check for physical damage, oil contamination on the connector, or corrosion on pins. On used excavators, pay special attention to the wiring harness routing—look for wear marks where cables contact the frame.

Step 2: Electrical Testing Using a digital multimeter, check the sensor's supply voltage (should read 5V ±0.25V with key on, engine off). Test resistance across sensor terminals (typically 250-350 ohms at room temperature). Inspect ground continuity from the sensor ground wire to chassis ground (should be less than 1 ohm).

Step 3: Signal Testing Connect Komatsu diagnostic software (KOMTRAX Plus) or a compatible scan tool. Monitor live sensor voltage while operating hydraulics under load. Voltage should fluctuate smoothly between 0.5V-4.5V. Erratic readings or frozen values indicate sensor failure.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu pressure sensor (Part #: 7861-93-1650). For used machines, always replace the connector pigtail simultaneously, as corrosion often damages both components. Clear codes and perform a system calibration using diagnostic software.

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