Komatsu PC35MR-2 Fault Code S-15: Complete Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code S-15?

Fault Code S-15 on the Komatsu PC35MR-2 indicates a malfunction in the hydraulic oil temperature sensor circuit. This code triggers when the Engine Control Module (ECM) detects abnormal voltage readings from the temperature sensor monitoring hydraulic fluid temperature in the main circuit.

The hydraulic oil temperature sensor plays a critical role in protecting the PC35MR-2's hydraulic system from overheating damage. When operating properly, it allows the ECM to monitor fluid temperature and adjust system performance accordingly. On this compact excavator model, maintaining proper hydraulic temperature is essential because the smaller oil reservoir volume makes the system more susceptible to rapid temperature changes during heavy digging operations.

Common Symptoms

When Code S-15 is active on your PC35MR-2, you may experience:

• Hydraulic temperature warning light illuminated on the instrument cluster
• Reduced hydraulic response or sluggish boom/arm/bucket movements as the system enters protective mode
• Intermittent code activation during cold starts or after extended idle periods
• Erratic temperature gauge readings showing unrealistic temperature values (extremely high or low)
• Machine derate limiting travel speed and auxiliary hydraulic functions to prevent potential damage

Potential Causes

The most common technical failures triggering S-15 on used PC35MR-2 excavators include:

• Faulty hydraulic oil temperature sensor due to internal element degradation (common after 4,000+ operating hours)
• Corroded or loose connector pins at the sensor plug, especially on machines exposed to wet conditions
• Damaged wiring harness between the sensor and ECM, particularly where the harness runs near the slew bearing (known rub point)
• Shorted or open circuit in the sensor signal wire caused by insulation breakdown
• ECM internal fault affecting the temperature sensor input circuit (rare, but possible on high-hour machines)
• Contaminated hydraulic oil causing sensor probe fouling with metallic particles or sludge

How to Troubleshoot and Fix Code S-15

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or main valve body. Inspect the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check for oil seepage around the sensor mounting point, which indicates seal failure. Clean the connector with electrical contact cleaner and ensure proper seating.

Step 2: Electrical Testing Using a digital multimeter, perform resistance checks on the sensor. Disconnect the sensor harness and measure resistance across the sensor terminals. At 20°C (68°F), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). If readings show infinite resistance (open circuit) or zero ohms (short circuit), replace the sensor.

Step 3: Wiring Harness Verification Trace the sensor wiring from the connector back to the ECM, paying special attention to routing near moving components. Check for harness chafing against the slew frame or hydraulic lines. Measure continuity between the sensor connector pins and corresponding ECM terminals. Inspect for voltage drops that indicate corroded connections or damaged conductors.

Step 4: Live Data Monitoring Connect Komatsu diagnostic software (KOMTRAX) or a compatible scan tool to monitor real-time temperature sensor voltage. At ambient temperature, expect approximately 4.5-4.8 volts on the signal wire. Compare displayed temperature values against actual hydraulic oil temperature measured with an infrared thermometer. A variance greater than 10°C indicates sensor drift.

Step 5: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor (Part #7861-93-1651) or quality aftermarket equivalent. Apply anti-seize compound to threads and torque to 15-20 Nm. For used machines with over 5,000 hours, consider replacing the entire sensor harness if insulation shows age-related cracking.

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Disclaimer: This guide provides general troubleshooting procedures. Hydraulic system work involves high pressures and electrical components. Always consult the official Komatsu PC35MR-2 service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with excavator systems.