Hyundai R335LC-9T Fault Code 691: Complete Diagnostic Guide

What is Hyundai R335LC-9T Fault Code 691?

Fault Code 691 indicates a malfunction in the Swing Motor 2 Solenoid Valve circuit, specifically detecting an abnormal electrical signal or open/short circuit in the swing motor's secondary proportional control system. This code is generated when the Electronic Control Unit (ECU) detects voltage or resistance values outside the manufacturer's specified parameters (typically 0.5-4.5V) in the swing motor's secondary solenoid valve circuit.

This fault is critical for the R335LC-9T's operational efficiency because the swing motor solenoid valve controls the hydraulic flow that enables smooth, proportional rotation of the upper structure. When this circuit malfunctions, the excavator's swing function becomes compromised, directly impacting productivity on job sites where precise positioning is essential.

Common Symptoms

Operators may experience the following when Code 691 is active:

• Erratic or complete loss of swing function in one direction or reduced swing speed
• Illuminated warning light on the instrument cluster with accompanying fault code display
• Jerky or unresponsive swing movements when operating the control lever
• Automatic derate mode limiting engine power to protect hydraulic systems
• Abnormal buzzing or clicking sounds from the swing motor solenoid valve area

Potential Causes

The most common technical reasons for Code 691 on used R335LC-9T excavators include:

• Damaged wiring harness near the swing motor housing due to vibration-induced chafing (known wear point on 9-series models)
• Failed swing motor solenoid valve coil from prolonged exposure to heat and hydraulic fluid contamination
• Corroded electrical connectors at the solenoid valve or ECU interface, common in machines operating in wet/marine environments
• Broken or frayed wires in the cable routing near the center joint (rotary union area)
• ECU internal fault or corrupted software requiring recalibration
• Low system voltage from failing alternator or weak batteries affecting solenoid operation

How to Troubleshoot and Fix Code 691

Step 1: Visual Inspection Begin with a thorough inspection of the wiring harness from the ECU to the swing motor solenoid valve. On used excavators, pay special attention to harness routing near the swing bearing and center joint where cables flex during rotation. Check for abrasion marks, exposed copper wire, or heat damage. Inspect all connector pins for corrosion, bent terminals, or hydraulic oil contamination.

Step 2: Electrical Testing Using a digital multimeter (DMM), measure the solenoid coil resistance at the valve connector (disconnect first). Specifications typically range from 3-8 ohms for Hyundai solenoids; consult your service manual for exact values. Test for continuity along the entire harness length. Check supply voltage at the solenoid connector with ignition ON—should read approximately 12V DC. Verify ground circuit integrity (should show less than 0.5 ohms resistance to chassis ground).

Step 3: Component Testing and Replacement If resistance/voltage tests fail, isolate whether the issue is the solenoid valve or wiring. Swap with Swing Motor 1 solenoid (if accessible) to determine if the fault follows the component. For used machines, clean all connector contacts with electrical contact cleaner and apply dielectric grease before reassembly. Replace damaged harness sections using OEM-spec wire and protect with abrasion-resistant sleeving. If the solenoid valve is faulty, replacement requires partial hydraulic system depressurization—use proper lockout/tagout procedures.

Step 4: ECU Diagnostics Connect Hyundai Hi-MATE diagnostic software or equivalent OEM tool to verify code parameters and check for multiple simultaneous faults that might indicate ECU problems. Clear codes after repairs and perform a swing function test under no-load conditions, then gradually increase to full operational testing.

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Disclaimer: This guide provides general diagnostic procedures for experienced technicians. Always consult the official Hyundai R335LC-9T service manual for exact specifications and safety procedures. Complex electrical or hydraulic repairs should be performed by certified heavy equipment technicians to prevent injury or further machine damage.