Komatsu D85PX-15 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu D85PX-15 Fault Code S-2?

Fault Code S-2 on the Komatsu D85PX-15 bulldozer indicates a Transmission Oil Temperature Sensor Circuit malfunction, specifically detecting abnormal voltage signals or an open/short circuit in the transmission temperature monitoring system.

This code triggers when the Electronic Control Module (ECM) receives voltage readings outside the normal operating range (typically 0.5-4.5V) from the transmission oil temperature sensor. The D85PX-15's hydrostatic transmission system relies heavily on precise temperature monitoring to prevent overheating and transmission damage. When this sensor circuit fails, the ECM cannot accurately monitor transmission fluid temperature, potentially leading to costly transmission failures if left undiagnosed. This is particularly critical on bulldozers performing heavy dozing or slope work where transmission loads are extreme.

Common Symptoms

When Code S-2 is active on your D85PX-15, operators typically experience:

• Transmission warning lamp illuminated on the instrument cluster (amber/yellow indicator)
• Transmission overheat alarm may activate intermittently or remain on constantly
• Derated engine power or limited ground speed as the ECM enters protective mode
• Erratic temperature gauge readings showing abnormally high, low, or fluctuating temperatures
• Transmission may default to high-temperature shutdown even when fluid is cool to touch

Potential Causes

The most common technical failures causing S-2 on used D85PX-15 machines include:

• Transmission temperature sensor failure due to internal thermistor degradation (common after 8,000+ hours)
• Damaged sensor wiring harness from rubbing against transmission case mounting brackets near the left-side hydraulic pump assembly
• Corroded or loose connector pins at the 3-pin transmission sensor connector (particularly C-142 connector)
• Shorted sensor ground wire caused by harness chafing on track frame cross-members
• ECM internal circuit board damage affecting the temperature input module (rare but occurs in high-vibration applications)
• Contaminated transmission fluid causing sensor tip fouling with metallic particles or varnish buildup

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection and Connector Check

Begin by locating the transmission oil temperature sensor on the left side of the transmission case, near the charge pump housing. Disconnect the 3-pin connector (C-142) and inspect for corrosion, bent pins, moisture intrusion, or oil contamination. On used machines, check the entire wiring harness route from sensor to ECM for abrasion points, especially where it passes near the hydraulic tank mounting bracket—this is a known wear point on D85PX-15 models with 5,000+ operating hours.

Step 2: Sensor Resistance Testing

Using a digital multimeter (DMM), measure the sensor's resistance between the signal pin and ground pin with the sensor disconnected. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms. Heat the sensor gently with a heat gun while monitoring resistance—it should decrease smoothly as temperature increases. Erratic readings, infinite resistance (open circuit), or near-zero resistance (short circuit) confirm sensor failure. Replace with Komatsu part number 421-S62-3271 or verified OEM equivalent.

Step 3: Wiring Harness Voltage and Continuity Tests

With the sensor disconnected and ignition ON, measure voltage at the 5V reference wire (typically the red/white wire) at the harness connector—it should read 4.8-5.2 volts. Check ground continuity on the black wire; resistance to chassis ground should be less than 1 ohm. Measure signal wire continuity from connector to ECM pin (consult wiring diagram for specific terminal)—resistance should be under 5 ohms. For used excavators, perform a wiggle test while monitoring voltage to detect intermittent opens caused by cracked wire strands inside the insulation.

Step 4: ECM Input Circuit Verification

If sensor and wiring test satisfactory, the ECM temperature input circuit may be faulty. Using Komatsu diagnostic software (KOMTRAX or compatible scan tool), observe live sensor voltage while artificially varying sensor resistance with a variable resistor (potentiometer). The ECM should display corresponding temperature changes. No response indicates ECM internal failure requiring replacement or professional ECM repair/reprogramming.

Critical Note for Used Equipment: Before replacing the sensor, drain a small sample of transmission oil and inspect for metal contamination or burnt odor—internal transmission damage can cause sensor fouling that mimics sensor failure. On high-hour machines, consider replacing both the sensor AND connector pigtail simultaneously, as corrosion often spreads internally within sealed connectors.

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Disclaimer: This guide provides general diagnostic procedures for Code S-2. Transmission system repairs involve pressurized hydraulic components and electrical systems. Always consult the official Komatsu D85PX-15 service manual and consider professional diagnosis for complex electrical faults or if you lack proper diagnostic equipment. Improper repairs may cause transmission damage or void remaining warranty coverage on used equipment.