What is Caterpillar Fault Code 1590-6?

Caterpillar Fault Code 1590-6 indicates a "Transmission Output Speed Sensor - Voltage Above Normal" condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the transmission output speed sensor circuit, typically exceeding 4.5-5.0 volts when normal operating range should be 0.5-4.5 volts.

This sensor is critical for monitoring the rotational speed of the transmission output shaft, which directly affects the machine's ground speed calculations, shift points, and overall powertrain management. In Caterpillar excavators and heavy equipment, the ECM relies on this data to coordinate transmission shifting, torque converter lockup, and engine load management. When voltage readings are abnormally high, the system cannot accurately determine actual machine speed, potentially causing transmission control issues and reduced operational efficiency.

Common Symptoms

When fault code 1590-6 is active, operators typically experience:

• Check Engine Light or transmission warning lamp illuminated on the instrument cluster
• Erratic or delayed transmission shifting patterns, including failure to upshift properly
• Speedometer malfunction showing incorrect ground speed or no reading at all
• Transmission may enter limp mode or default to a fixed gear for protection
• Possible derate condition where engine power is reduced to protect the drivetrain

Potential Causes

The most common technical reasons for this fault code in used Caterpillar excavators include:

• Open circuit in the transmission output speed sensor wiring harness due to wire breakage or connector damage
• Corroded or damaged sensor connector pins causing intermittent high resistance or open circuits
• Failed transmission output speed sensor with internal electrical faults
• Chafed or abraded wiring at known rub points near the transmission housing or frame rails (common in older machines)
• Damaged ECM pins or internal ECM circuit issues (less common)
• Improper sensor installation after previous repairs, causing air gap issues that affect signal quality

How to Troubleshoot and Fix Code 1590-6

Step 1: Visual Inspection Begin with a thorough inspection of the transmission output speed sensor and its wiring harness. On used excavators, focus on areas where the harness routes near moving components, sharp edges, or heat sources. Check the sensor connector for corrosion, bent pins, moisture intrusion, or physical damage. Look for oil contamination that may indicate seal leaks affecting the sensor.

Step 2: Electrical Testing Using a digital multimeter, disconnect the transmission output speed sensor connector and measure resistance across the sensor terminals. Typical resistance values range from 190-250 ohms for most Caterpillar speed sensors (consult your specific service manual). Next, check for continuity between each sensor wire and ground—there should be no continuity (infinite resistance), indicating no short to ground. Measure voltage at the ECM side of the harness with ignition on; you should see approximately 5-8 volts supply voltage.

Step 3: Sensor and Harness Repair If electrical tests reveal an open circuit or the sensor resistance is out of specification, replace the transmission output speed sensor. Before installation, clean the mounting area thoroughly and verify proper air gap (typically 0.020-0.040 inches). If wiring damage is found—particularly common at harness support brackets or where the harness passes through the transmission tunnel—repair using appropriate gauge wire and weatherproof connectors. For used machines, apply dielectric grease to all connectors and secure harnesses away from wear points using new cable ties and protective loom.

Step 4: Clear Codes and Verify After repairs, use Caterpillar Electronic Technician (Cat ET) diagnostic software or equivalent to clear fault codes. Perform a test drive under various load conditions and monitor live data to confirm the output speed sensor signal remains within normal voltage parameters (0.5-4.5V during operation).

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Disclaimer: This guide provides general troubleshooting information for Caterpillar fault code 1590-6. Always consult your machine's specific service manual and consider engaging a certified Caterpillar technician for complex electrical diagnostics, especially on used equipment where multiple underlying issues may exist.