Komatsu D85EX-15 Fault Code S-2: Complete Diagnostic Guide

What is Komatsu D85EX-15 Fault Code S-2?

Fault Code S-2 on the Komatsu D85EX-15 crawler dozer indicates a transmission oil temperature sensor circuit malfunction or abnormally high transmission fluid temperature reading. This diagnostic trouble code is triggered when the Electronic Control Module (ECM) detects either an out-of-range signal from the transmission oil temperature sensor or when actual fluid temperatures exceed safe operating thresholds (typically above 120°C/248°F).

This code is critical for the D85EX-15's hydrostatic transmission system because excessive heat can rapidly degrade transmission fluid, damage seals, and cause premature failure of hydraulic components. The transmission relies on precise temperature monitoring to protect the torque converter, hydraulic pumps, and planetary gear sets from thermal damage.

Common Symptoms

When S-2 is active on your D85EX-15, operators typically experience:

• Flashing transmission warning lamp on the instrument cluster with possible audible alarm
• Reduced machine performance or automatic power derate to protect transmission components
• Sluggish travel response or delayed engagement when shifting between forward and reverse
• Elevated transmission case temperatures detectable during physical inspection
• Intermittent code appearance during heavy load operations like dozing or climbing grades

Potential Causes

The S-2 fault code on used D85EX-15 dozers commonly stems from:

• Failed transmission oil temperature sensor (common failure point after 8,000+ operating hours)
• Damaged or corroded wiring harness between the sensor and ECM, particularly at connection points near the transmission case where vibration and heat exposure are constant
• Plugged transmission oil cooler restricting coolant flow and causing genuine overheating
• Low transmission fluid level due to external leaks at seals or hydraulic line connections
• Contaminated or degraded transmission fluid that has lost thermal transfer properties
• Faulty ECM ground connections creating erratic sensor readings

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection Check transmission fluid level using the dipstick with the engine at operating temperature. Inspect the transmission oil cooler for external debris blockage and verify all cooling fins are clean. Examine wiring harnesses for obvious damage, particularly where they route along the transmission housing.

Step 2: Sensor Circuit Testing Locate the transmission temperature sensor (typically mounted on the transmission case near the filter assembly). Disconnect the sensor connector and use a digital multimeter to measure resistance. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms depending on sensor specification. Compare readings against Komatsu's specification chart. On used excavators, check connector pins for corrosion or bent terminals—a common issue causing intermittent codes.

Step 3: Wiring Harness Verification With the sensor disconnected, check for continuity between the sensor connector and the ECM connector (typically located in the operator cab). Measure resistance; it should be less than 5 ohms. Check for short circuits to ground on both signal and reference wires. Pay special attention to harness routing near the transmission where vibration wear and heat damage are prevalent on older machines.

Step 4: Live Temperature Monitoring Using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, monitor live transmission temperature data while operating the machine. Compare sensor readings to actual fluid temperature measured with an infrared thermometer at the transmission case. A variance greater than 10°C indicates sensor drift or failure.

Step 5: Component Replacement If the sensor tests faulty, replace it with a genuine Komatsu OEM part to ensure proper ECM communication. After replacement, clear the fault code and perform a transmission oil and filter service if the fluid shows signs of thermal breakdown (dark color, burnt smell). On high-hour used machines, also inspect the transmission cooler core for internal restriction.

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Disclaimer: This guide provides general diagnostic procedures for the Komatsu D85EX-15. Always consult the official Komatsu service manual for your specific machine serial number and software version. Complex electrical diagnostics and transmission repairs should be performed by certified Komatsu technicians with proper diagnostic equipment.