What is Hyundai R335LC-9T Fault Code 91-19?

Fault Code 91-19 on the Hyundai R335LC-9T excavator indicates a Swing Motor 2 Position Sensor Circuit Malfunction. This diagnostic trouble code (DTC) is triggered when the Electronic Control Unit (ECU) detects an abnormal voltage signal or complete signal loss from the swing motor's secondary position sensor, which monitors rotational position and speed during upper structure rotation.

The R335LC-9T utilizes dual position sensors on the swing motor to provide redundant feedback to the hydraulic control system. When the ECU receives conflicting data or no signal from Sensor 2, it logs Code 91-19 to prevent erratic swing behavior and protect the hydraulic circuit. This is particularly critical for this 33-ton machine, as precise swing control directly impacts operational safety and productivity on job sites.

Common Symptoms

• Swing motion warning lamp illuminated on the instrument cluster with possible audible alarm
• Reduced swing speed or swing function operating in derated/limited mode
• Erratic or jerky rotation of the upper structure during swing operations
• Intermittent swing lockout where the swing motor fails to engage despite joystick input
• ECU error codes displayed on the monitor panel, potentially alongside related hydraulic fault codes

Potential Causes

The most common technical causes for Code 91-19 on used R335LC-9T excavators include:

• Position sensor failure due to internal component degradation or magnetic ring damage on the swing motor shaft
• Wiring harness damage at known rub points near the center joint and swing bearing, where cables experience constant flexing
• Corroded or loose connector pins at the sensor plug (3-pin connector) or ECU interface
• Contaminated sensor face from hydraulic fluid leaks or metal debris accumulation affecting magnetic field reading
• ECU voltage supply issues providing incorrect reference voltage (typically should be 5V ± 0.5V)
• Damaged swing motor seal allowing moisture intrusion into the sensor housing

How to Troubleshoot and Fix Code 91-19

Step 1: Visual Inspection Begin by inspecting the swing motor position sensor located on the swing motor housing. Check the 3-pin connector for corrosion, bent pins, or oil contamination. On used excavators, examine the wiring harness from the sensor to the main harness, paying special attention to flex points near the center joint where cable wear commonly occurs.

Step 2: Electrical Testing Using a digital multimeter, perform these checks with the ignition on but engine off:

• Verify supply voltage at the sensor connector (Pin 1): should read 4.5-5.5V DC
• Check sensor ground (Pin 2): should show less than 0.5Ω resistance to chassis ground
• Measure signal output (Pin 3): should fluctuate between 0.5-4.5V when manually rotating the swing motor shaft

Step 3: Component Replacement Protocol If voltage readings are correct but the signal remains absent or erratic, replace the Swing Motor 2 Position Sensor (Hyundai part number 31N8-02112 or equivalent). Before installation, clean the mounting surface and apply dielectric grease to connector pins. For used machines with over 8,000 hours, inspect the sensor mounting bracket for cracks and the magnetic encoder ring on the motor shaft for damage.

Step 4: System Recalibration After repairs, clear fault codes using Hyundai Hi-MATE diagnostic software or equivalent dealer-level tool. Perform a swing function calibration procedure and cycle the swing motor through full left/right rotations to verify proper sensor operation and confirm Code 91-19 does not return.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Hyundai service manual for your specific serial number and consider professional diagnosis for complex electrical issues. Improper repairs may void warranties or create safety hazards.