What is Komatsu PC450-7 Fault Code E985?

Komatsu PC450-7 fault code E985 indicates a malfunction in the engine speed sensor circuit, specifically detecting an abnormal signal or lack of signal from the engine revolution sensor to the Engine Control Module (ECM). This code is part of Komatsu's advanced diagnostic system that monitors critical engine parameters to ensure optimal performance and protect the powerplant.

The engine speed sensor (also called the crankshaft position sensor) is essential for the PC450-7's SAA6D125E-3 engine operation. It provides real-time data to the ECM about engine RPM and crankshaft position, which directly affects fuel injection timing, throttle response, and overall engine management. When this sensor fails or sends irregular signals, the excavator may enter a protective derate mode or fail to start entirely. For used excavators, this code often appears due to sensor degradation, vibration damage, or corroded connections from years of operation in harsh conditions.

Common Symptoms

• Engine fails to start or cranks but won't fire up
• Sudden engine shutdown during operation without warning
• Intermittent rough idling or inconsistent RPM fluctuations
• Power derate mode activated, limiting machine performance to protect the engine
• Check engine light illuminated on the dashboard display panel

Potential Causes

The most common technical causes for code E985 on used PC450-7 excavators include:

• Failed engine speed sensor due to internal component wear or heat damage
• Damaged sensor wiring harness, particularly at known rub points near the engine block and starter motor
• Corroded or loose electrical connectors at the sensor plug (common in machines with 5,000+ operating hours)
• Broken or damaged reluctor ring on the crankshaft/flywheel causing erratic signal generation
• ECM internal fault (less common, but possible in high-hour machines)
• Electromagnetic interference from aftermarket electrical accessories or damaged shielding

How to Troubleshoot and Fix Code E985

Step 1: Visual Inspection Begin by locating the engine speed sensor on the flywheel housing (driver's side of the engine block). Inspect the sensor connector for corrosion, moisture, or damaged pins. Check the wiring harness along its entire route for abrasion, particularly where it passes near metal edges or moving components—a common failure point on used excavators.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. Specifications typically range between 150-300 ohms (consult your PC450-7 service manual for exact values). Also check for proper voltage supply from the ECM (usually 8-12V reference voltage). Test harness continuity from sensor to ECM connector.

Step 3: Sensor Replacement and Verification If readings fall outside specifications or the sensor shows physical damage, replace with a genuine Komatsu engine speed sensor (part number 7861-93-1650 or equivalent). On used machines, clean the mounting surface thoroughly and apply anti-seize compound. After installation, clear codes using Komatsu diagnostic software (KOMTRAX or aftermarket tools) and perform a test run. Verify proper air gap between sensor and reluctor ring (typically 0.5-1.5mm).

Disclaimer: This guide provides general troubleshooting procedures. Always consult the official Komatsu PC450-7 service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with ECM systems.