What is Komatsu PC450-7 Fault Code E984?

Fault Code E984 on the Komatsu PC450-7 excavator indicates a Main Relief Valve Solenoid Circuit Malfunction in the hydraulic system. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage or current flow in the electrical circuit controlling the main relief valve solenoid, which regulates maximum system pressure in the hydraulic pump.

This code is critical because the main relief valve prevents catastrophic damage to hydraulic components by limiting peak pressure. On the PC450-7, this electronically-controlled relief valve adjusts dynamically based on machine load and operator demand. When E984 appears, the system typically defaults to a fixed relief pressure setting, reducing operational efficiency and potentially causing the machine to feel sluggish during heavy digging operations.

Common Symptoms

• Amber warning lamp illuminated on the instrument cluster with E984 displayed on the monitor panel
• Reduced hydraulic power during boom, arm, or bucket operations, particularly under heavy loads
• Automatic engine derate to protect the hydraulic system, limiting maximum RPM to approximately 1,800
• Erratic hydraulic response where implement speed varies unexpectedly during operation
• Possible overheating of hydraulic oil due to improper pressure regulation over extended periods

Potential Causes

The most common causes for E984 on used PC450-7 excavators include:

• Failed main relief valve solenoid due to internal coil burnout (resistance typically measures 15-25 ohms when functional)
• Damaged wiring harness between the ECM and solenoid, especially at known rub points near the main hydraulic pump mounting area
• Corroded or loose connector pins at the solenoid connection point, common in machines with 5,000+ operating hours
• ECM internal fault in the solenoid driver circuit (less common but possible in high-hour machines)
• Intermittent short to ground in the harness caused by insulation breakdown from heat exposure near the engine block

How to Troubleshoot and Fix Code E984

Step 1: Initial Inspection Using Komatsu diagnostic software (KOMTRAX Plus) or a compatible scan tool, capture freeze frame data and check for additional active codes. Visually inspect the main relief valve solenoid connector (located on the main hydraulic pump) for oil contamination, corrosion, or bent pins.

Step 2: Electrical Testing Disconnect the solenoid connector and measure solenoid resistance using a digital multimeter. Specification is 15-25 ohms at 68°F (20°C). Next, check for supply voltage at the harness connector with the key on—should read approximately 12V. Measure resistance to ground on both solenoid wires; infinite resistance indicates proper insulation.

Step 3: Harness and Ground Verification On used excavators, carefully trace the wiring harness from the ECM to the solenoid, paying special attention to areas where the harness contacts metal surfaces or passes near hot engine components. Check all ground connections at the pump mounting bracket for tightness and corrosion.

Step 4: Component Replacement If testing confirms solenoid failure, replace with a genuine Komatsu relief valve solenoid (Part #723-56-11501 or equivalent). After replacement, clear codes and perform a hydraulic system function test under load.

Professional Consultation Disclaimer: While this guide provides technical troubleshooting steps, hydraulic system repairs on heavy equipment can be complex. For used excavators with unknown maintenance history, consult a certified Komatsu technician or qualified heavy equipment mechanic to ensure proper diagnosis and prevent further damage to expensive hydraulic components.