Komatsu D85EX-15 Fault Code S-6: Complete Diagnostic Guide

What is Komatsu D85EX-15 Fault Code S-6?

Fault Code S-6 on the Komatsu D85EX-15 bulldozer indicates a transmission oil temperature sensor circuit malfunction. This code triggers when the ECM (Engine Control Module) detects abnormal voltage signals from the transmission oil temperature sensor, which monitors hydraulic fluid temperature in the torque converter and transmission system.

This sensor plays a critical role in protecting the powershift transmission from thermal damage. The D85EX-15's hydrostatic drive system relies on precise temperature monitoring to prevent overheating during heavy dozing operations. When this code activates, the machine may enter a protective derate mode to prevent catastrophic transmission failure, directly impacting productivity on the jobsite.

Common Symptoms

• Transmission warning light illuminated on the instrument cluster (amber or red depending on severity)
• Reduced engine power or inability to achieve maximum ground speed, even under light load
• Intermittent transmission shifting issues or delayed engagement when changing direction
• Audible warning alarm sounding in the operator cab during startup or operation
• Erratic temperature gauge readings on the monitor panel, showing incorrect transmission temps

Potential Causes

The S-6 code on used D85EX-15 machines typically stems from these age-related issues:

• Faulty transmission oil temperature sensor (resistance drift or internal failure after 5,000+ operating hours)
• Damaged wiring harness between the sensor and ECM, especially near the transmission tunnel where vibration causes wire chafing
• Corroded electrical connectors at the sensor plug—common on machines operated in wet, muddy, or coastal environments
• Low transmission oil level causing actual overheating and sensor exposure to air pockets
• ECM internal fault or poor ground connection at the controller (less common but possible on 15+ year old machines)

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin by checking transmission oil level with the engine at operating temperature. Locate the temperature sensor on the transmission case (typically near the oil filter). Inspect the sensor connector for corrosion, moisture, or damaged pins. On used excavators, check the entire wiring harness from sensor to ECM for abrasion points against the frame.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across its terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specs). Check continuity from the sensor connector back to the ECM pins to identify broken wires. Measure supply voltage at the sensor connector with ignition on—should read 5V reference.

Step 3: Component Replacement If sensor resistance is out of range, replace the transmission temperature sensor (Komatsu part number specific to D85EX-15). Clean all connector terminals with electrical contact cleaner before reassembly. Clear the code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then test-operate the machine under load to verify repair.

Disclaimer: This guide provides general troubleshooting steps. Always consult the official Komatsu D85EX-15 service manual and consider professional diagnostic assistance for complex electrical issues or if unfamiliar with high-voltage systems.