What is Hyundai HX220 NL EPFC Fault Code 1691 (100-18)?

Fault Code 1691 (100-18) indicates a Main Relief Valve Solenoid Circuit malfunction in the Hyundai HX220 NL EPFC excavator's hydraulic system. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormal electrical signal—either an open circuit, short circuit, or incorrect voltage—from the main relief valve solenoid within the hydraulic pump assembly.

The main relief valve solenoid regulates maximum system pressure in the hydraulic circuit, protecting components from over-pressurization while optimizing fuel efficiency and performance. When this solenoid circuit fails, the ECM cannot properly modulate hydraulic pressure, which is critical for the HX220's precise boom, arm, and bucket control. In used excavators, this code often appears due to wiring degradation or solenoid coil failure after thousands of operating hours.

Common Symptoms

When Code 1691 (100-18) is active, operators typically experience:

• Hydraulic system derate or reduced maximum pressure, causing sluggish movement of all hydraulic functions
• Illuminated malfunction indicator lamp on the dashboard with possible audible alarm
• Loss of lifting power during heavy digging or loading operations
• Erratic hydraulic response or inconsistent pressure during combined functions (e.g., boom and swing simultaneously)
• Increased engine RPM without corresponding hydraulic power output due to pressure regulation failure

Potential Causes

The most common technical causes for this fault code in used HX220 excavators include:

• Damaged or corroded wiring harness between the ECM and main relief valve solenoid, particularly at flex points near the pump
• Failed main relief valve solenoid coil due to heat exposure, moisture ingress, or normal wear (typical lifespan: 6,000-10,000 hours)
• Corroded or loose electrical connectors at the solenoid or ECM termination points
• Shorted or chafed wiring caused by contact with hydraulic lines or structural components
• ECM internal fault (rare) preventing proper solenoid driver circuit operation
• Low supply voltage to the solenoid circuit from battery or charging system issues

How to Troubleshoot and Fix Code 1691 (100-18)

Step 1: Visual Inspection Begin by inspecting the main relief valve solenoid located on the hydraulic pump assembly. Check the electrical connector for corrosion, bent pins, or moisture. On used excavators, examine the wiring harness routing for obvious damage, especially where it contacts metal edges or hydraulic components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the solenoid connector and measure coil resistance across the solenoid terminals. Typical specification is 3-7 ohms; readings outside this range indicate solenoid failure. Next, check for supply voltage (typically 12-24V depending on system) at the harness connector with the ignition on. Verify ground continuity to ensure less than 1 ohm resistance.

Step 3: Circuit Verification With the harness disconnected from both solenoid and ECM, perform a continuity test on all wires to detect opens or shorts. Check for short-to-ground on the power supply wire. Inspect harness sections near articulation points where cable flex causes insulation breakdown—a common issue in machines with 5,000+ hours.

Step 4: Component Replacement If electrical tests confirm solenoid failure, replace the main relief valve solenoid assembly with OEM or equivalent parts. Always apply dielectric grease to connectors during reassembly to prevent future corrosion. Clear the fault code using Hyundai Hi-Mate diagnostic software or compatible scan tool, then perform a function test under load.

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Hyundai service manual for your specific machine serial number and consult qualified technicians for complex hydraulic-electrical diagnostics.