Komatsu D85EX-15 Fault Code E-20: Complete Diagnostic Guide

What is Komatsu D85EX-15 Fault Code E-20?

Fault Code E-20 on the Komatsu D85EX-15 bulldozer indicates a transmission output speed sensor malfunction or circuit abnormality. This code is triggered when the Electronic Control Module (ECM) detects an irregular signal from the output speed sensor that monitors the transmission's output shaft rotation.

The transmission output speed sensor is critical for the D85EX-15's Hydrostatic Transmission (HST) system performance. This sensor provides real-time data to the ECM for calculating ground speed, controlling shift points, and managing torque converter lockup. When this sensor fails or sends erratic signals, the transmission cannot operate efficiently, potentially causing power loss, improper gear selection, or complete transmission protection shutdown.

Common Symptoms

Operators experiencing fault code E-20 typically notice:

• Warning lamp illumination on the instrument cluster with reduced machine responsiveness
• Erratic speedometer readings or complete speedometer failure during operation
• Transmission derate mode activating, limiting machine to low speed or specific gear ranges
• Rough shifting patterns or delayed engagement when changing between forward and reverse
• Loss of transmission efficiency with higher-than-normal engine RPM for given ground speed

Potential Causes

The most common technical causes for E-20 on used D85EX-15 machines include:

• Output speed sensor failure due to internal component degradation or bearing contamination from metallic debris in transmission fluid
• Damaged sensor wiring harness especially near the transmission housing where vibration causes wire chafing against metal brackets
• Corroded or loose electrical connectors at the sensor plug or ECM connections (common in machines operating in wet/muddy conditions)
• Worn sensor tone ring (reluctor wheel) on the transmission output shaft creating irregular magnetic field patterns
• ECM software glitches or outdated calibration files not properly interpreting sensor data
• Low transmission fluid levels causing sensor exposure during operation on slopes

How to Troubleshoot and Fix Code E-20

Step 1: Visual Inspection and Basic Checks Begin by inspecting the output speed sensor location on the transmission case. Check the wiring harness for obvious damage, especially where it routes near moving components or sharp edges. Inspect the electrical connector for corrosion, bent pins, or moisture intrusion. On used excavators, harness friction points are common failure areas—look for worn insulation.

Step 2: Electrical Testing Using a digital multimeter, measure the sensor's resistance. The output speed sensor should typically read between 800-1200 ohms (consult your service manual for exact specifications). Check for supply voltage at the connector (usually 8-12V DC). Test continuity from the sensor connector back to the ECM pins to identify potential open circuits or high resistance caused by corrosion.

Step 3: Dynamic Testing and Component Replacement Connect Komatsu diagnostic software (KOMTRAX or PC-based diagnostics) to monitor real-time sensor output during engine idle. The sensor should produce a consistent AC voltage signal that increases with shaft rotation. If readings are erratic or absent, remove the sensor and inspect the tone ring for damage or debris buildup. Replace the sensor if testing confirms failure, ensuring proper air gap (typically 0.5-1.5mm) during installation.

Step 4: System Verification After repairs, clear fault codes and perform a test cycle including forward/reverse operation at various speeds. Monitor for code recurrence and verify speedometer functionality.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu D85EX-15 service manual for model-specific specifications. Complex transmission issues should be evaluated by certified Komatsu technicians with proper diagnostic equipment.