Komatsu PC400-7 Fault Code E931: Complete Diagnostic Guide

What is Komatsu PC400-7 Fault Code E931?

Fault Code E931 on the Komatsu PC400-7 indicates a malfunction in the Revolution Sensor Circuit (engine speed sensor), specifically detecting an abnormal signal or loss of communication between the revolution sensor and the Engine Control Module (ECM).

This diagnostic trouble code points to issues with the crankshaft position sensor or its associated wiring harness that prevents the ECM from accurately reading engine RPM. The revolution sensor is critical for proper fuel injection timing, engine speed control, and overall machine performance. On the PC400-7's SAA6D125E-3 engine, this sensor provides real-time data that the ECM uses to optimize combustion efficiency and regulate hydraulic pump output. Without reliable sensor input, the machine may enter a protective derate mode or fail to start entirely.

Common Symptoms

When Code E931 is active on your Komatsu PC400-7, operators typically experience:

• Engine refuses to start or cranks without firing due to lack of RPM signal to ECM
• Warning light illuminated on the monitor panel with simultaneous fault code display
• Sudden engine shutdown during operation when sensor signal is intermittently lost
• Rough idling or unstable engine speed as the ECM struggles with inconsistent RPM data
• Reduced hydraulic response or power derate as the system enters limp mode for protection

Potential Causes

The E931 code in used PC400-7 excavators typically stems from these common issues:

• Failed revolution sensor (crankshaft position sensor) due to internal component degradation or heat exposure over time
• Damaged wiring harness at known rub points near the engine flywheel housing where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug or ECM connector, especially common in machines operating in wet/muddy conditions
• Broken or damaged sensor mounting causing improper air gap between sensor and flywheel ring gear
• ECM internal fault (less common) preventing proper signal processing from an otherwise functional sensor
• Magnetic contamination on the sensor tip from metal debris accumulation affecting signal quality

How to Troubleshoot and Fix Code E931

Step 1: Visual Inspection and Connector Check Begin by locating the revolution sensor on the flywheel housing (driver's side of engine block). Inspect the sensor connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators. Disconnect and reconnect the harness firmly, checking for physical damage. Examine the wiring from sensor to ECM for chafing, particularly where the harness passes near moving components or sharp edges.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the resistance across the sensor terminals (with connector disconnected). The revolution sensor should typically read 150-300 ohms at room temperature (consult your service manual for exact specifications). A reading of infinite resistance indicates an open circuit within the sensor, while near-zero resistance suggests an internal short. Clean any metal debris from the sensor tip with a lint-free cloth.

Step 3: Wiring Harness Continuity Check Test for continuity between the sensor connector pins and the corresponding ECM terminals (refer to wiring diagrams in Komatsu technical manual). Check for shorts to ground by measuring resistance between each wire and the engine block—readings should be infinite. Pay special attention to harness sections with visible wear on used machines.

Step 4: Air Gap Verification If sensor tests are normal, verify the air gap between the sensor tip and flywheel ring gear teeth using a feeler gauge (typical specification: 0.5-1.5mm). Damaged flywheel teeth or improper sensor mounting can prevent adequate signal generation even with a functional sensor.

Step 5: Signal Output Testing With the sensor connected and engine cranking, use Komatsu diagnostic software (KOMTRAX or PC-based service tool) to monitor live RPM signal data. Alternatively, connect an oscilloscope to view the AC voltage waveform from the sensor during cranking—you should see consistent pulses. Erratic or absent waveforms confirm sensor or reluctor wheel problems.

Step 6: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu revolution sensor (Part #: 7861-93-1651 or equivalent for PC400-7). For used excavators, also replace the connector pigtail if corrosion is evident, as intermittent connections often recur. After replacement, clear codes using diagnostic software and verify proper engine operation.

Professional Tip for Used Equipment: Before ordering expensive components, thoroughly clean all connectors with electrical contact cleaner and apply dielectric grease to prevent moisture ingress. Many E931 codes on high-hour machines resolve with proper connector maintenance rather than parts replacement.

---

Disclaimer: This guide provides general troubleshooting procedures for Komatsu PC400-7 fault code E931. Always consult the official Komatsu service manual for your specific machine serial number and software version. If unfamiliar with electrical diagnostics or working around engines, seek assistance from a certified Komatsu technician to prevent equipment damage or safety hazards.