What is Komatsu PC120-6 Fault Code S-2?

Komatsu PC120-6 Fault Code S-2 indicates a malfunction in the engine speed sensor circuit or signal abnormality. This code specifically relates to the primary engine RPM sensor that monitors crankshaft rotation and sends critical timing data to the Engine Control Module (ECM).

The speed sensor is essential for proper fuel injection timing, governor control, and overall engine performance monitoring on this mid-sized excavator. When this sensor fails or its signal is interrupted, the ECM cannot accurately regulate engine speed, potentially causing operational failures or preventing startup. For the PC120-6's SAA6D102E-2 engine, this sensor works in conjunction with the fuel injection system to maintain smooth power delivery across all operating ranges.

Common Symptoms

• Engine fails to start or cranks but won't fire due to lack of speed signal confirmation
• Intermittent stalling during operation, especially under load or at idle
• Erratic engine RPM behavior with unstable idle or sudden speed fluctuations
• Warning indicator illuminated on the instrument panel (typically the engine malfunction light)
• Loss of power or derating as the ECM enters limp-home mode to protect the engine

Potential Causes

The most common technical reasons for S-2 code activation on used PC120-6 excavators include:

• Failed or contaminated speed sensor due to age-related deterioration or exposure to oil/debris in the bell housing area
• Damaged sensor wiring harness particularly at known rub points near the flywheel housing where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug, common in machines with poor maintenance history
• Excessive air gap between sensor tip and flywheel teeth caused by incorrect installation or mounting bracket wear
• ECM internal issues or corrupted software, though less common than physical sensor problems
• Damaged flywheel ring gear teeth creating irregular signal patterns

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, lower section). Inspect the sensor mounting for looseness, check the wiring harness for visible damage, chafing, or oil contamination. On used excavators, pay special attention to connector corrosion—clean with electrical contact cleaner if necessary.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance (typically 190-250 ohms for this model when cold). Check for voltage output while cranking—you should see AC voltage pulses of 0.5-2V. Inspect harness continuity from sensor to ECM connector, checking for shorts to ground or open circuits.

Step 3: Air Gap Verification Verify the sensor air gap using feeler gauges—specification is typically 0.5-1.5mm from sensor tip to flywheel teeth. Incorrect gaps prevent proper signal generation.

Step 4: Component Replacement If testing confirms sensor failure, replace with genuine Komatsu parts (Part #: 7861-93-1651 or equivalent). After installation, clear codes using Komatsu diagnostic software (KOMTRAX) and test operation under load.

Disclaimer: This guide provides general diagnostic information. Complex electrical issues or ECM reprogramming should be performed by certified Komatsu technicians with proper diagnostic equipment.