Komatsu PC750LC-7 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code S-2?

Fault Code S-2 on the Komatsu PC750LC-7 indicates a malfunction in the engine speed sensor circuit or an abnormal signal from the crankshaft position sensor system. This code is triggered when the Electronic Control Module (ECM) detects irregular or missing pulses from the speed sensor, which is critical for proper fuel injection timing and engine management.

The engine speed sensor monitors crankshaft rotation and sends real-time data to the ECM. This information is essential for the PC750LC-7's SAA6D140E-3 engine to maintain optimal performance, control fuel delivery, and coordinate with the hydraulic system. When this sensor fails or provides inconsistent signals, the machine's operational efficiency and reliability are severely compromised.

Common Symptoms

When Code S-2 is active, operators typically experience:

• Engine cranks but fails to start, or starts with extreme difficulty and rough idling
• Sudden engine shutdown during operation without warning
• Loss of power or inability to reach rated RPM under load
• Erratic engine performance with inconsistent throttle response
• Monitor panel warning light illuminated with S-2 displayed on the diagnostic screen

Potential Causes

The most common technical reasons for S-2 on used PC750LC-7 excavators include:

• Failed or contaminated engine speed sensor due to oil leaks, debris accumulation, or internal component degradation
• Damaged sensor wiring harness from rubbing against engine components, particularly near the flywheel housing where vibration is constant
• Corroded or loose electrical connectors at the sensor or ECM interface, common in machines with high operating hours
• Worn flywheel ring gear teeth causing irregular signal generation (typical in older units)
• ECM internal faults or corrupted software, though less common than sensor-related issues
• Inadequate air gap between sensor and flywheel due to improper installation or mounting bracket wear

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection Begin by locating the engine speed sensor on the flywheel housing (driver's side of the engine block). Inspect the wiring harness for obvious damage, chafing, or oil contamination. Check all connector pins for corrosion, bent terminals, or moisture intrusion—critical on used excavators with extended service hours.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance (typically 190-250 ohms at 20°C for Komatsu sensors). Check supply voltage at the connector (should be approximately 8-12V with key on). Inspect signal wire continuity from sensor to ECM pin, noting any resistance above 5 ohms indicates harness issues.

Step 3: Air Gap Verification Verify the air gap between sensor tip and flywheel teeth using feeler gauges (specification: 0.5-1.5mm). Incorrect gap causes weak or erratic signals. On used machines, check for flywheel runout or damaged teeth that may have developed over time.

Step 4: Component Replacement If testing confirms sensor failure, replace with genuine Komatsu parts (Part #: 7861-93-1650 or equivalent). After installation, clear codes using Komatsu diagnostic software (KOMTRAX) and perform a test run under load.

Disclaimer: This guide provides general diagnostic information. Complex electrical issues or ECM programming should be performed by certified Komatsu technicians with proper diagnostic equipment. Always consult the official service manual for your specific machine serial number.