Caterpillar 318D2 Fault Code 39:1665-6: Complete Diagnostic Guide

What is Caterpillar 318D2 Fault Code 39:1665-6?

Fault Code 39:1665-6 indicates a voltage problem with the Auxiliary Hydraulic Control Solenoid Circuit, specifically detecting a current below normal or shorted to ground condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally low electrical current flowing through the auxiliary hydraulic proportional control solenoid valve circuit, typically reading below the manufacturer's specified threshold of approximately 0.5 amps.

This fault directly impacts the 318D2's auxiliary hydraulic system performance, which controls attachments like breakers, grapples, and shears. The proportional solenoid regulates hydraulic flow based on electrical signals from the ECM, and when voltage anomalies occur, the system cannot accurately modulate hydraulic pressure. For used excavators, this code is particularly critical because degraded wiring insulation and connector corrosion are common failure points that compromise circuit integrity.

Common Symptoms

• Auxiliary hydraulic functions become inoperative or erratic – attachments won't respond to joystick inputs or operate intermittently
• Amber warning light illuminated on the instrument cluster with possible audible alarm
• Loss of proportional control – auxiliary hydraulics may operate only at full flow or not at all, with no variable speed control
• Error code stored in ECM memory – retrievable through Caterpillar Electronic Technician (Cat ET) diagnostic software
• Possible derate mode activation in severe cases, limiting machine performance to protect hydraulic components

Potential Causes

Wiring harness damage is the most frequent culprit in used 318D2 excavators, particularly where harnesses route near the swing bearing or along the boom where constant flexing occurs. The insulation deteriorates from UV exposure, hydraulic fluid contamination, and mechanical abrasion.

Corroded or loose electrical connectors at the solenoid valve or ECM create high resistance paths that reduce current flow below threshold levels.

Failed auxiliary hydraulic solenoid valve with internal coil short-to-ground or open circuit condition, though less common than harness issues.

ECM internal faults affecting the driver circuit for the auxiliary solenoid, though this is rare and typically accompanied by multiple fault codes.

Damaged pins or moisture intrusion in the main chassis harness connector (often located behind the cab) from years of environmental exposure.

How to Troubleshoot and Fix Code 39:1665-6

Step 1: Visual Inspection and Connector Examination Begin by disconnecting the battery negative terminal for safety. Locate the auxiliary hydraulic solenoid valve (typically mounted on the main control valve near the hydraulic pump). Inspect the entire wiring harness from the solenoid back to the ECM, paying special attention to areas where the harness contacts metal edges, passes through grommets, or flexes during machine operation. On used excavators, check for chafed insulation, exposed copper wire, or oil-saturated sections that indicate deterioration. Remove and inspect all connectors in the circuit, cleaning contacts with electrical contact cleaner and examining for bent pins, corrosion (green or white deposits), or moisture.

Step 2: Electrical Testing with Multimeter Using a digital multimeter (DMM), perform resistance and continuity checks. With the solenoid disconnected, measure coil resistance across the solenoid terminals – it should typically read between 8-15 ohms (consult service manual for exact specifications). Next, check for short-to-ground by measuring resistance between each solenoid terminal and machine ground; readings should be infinite (open circuit). Test harness continuity from the ECM connector to the solenoid connector with all components disconnected – each wire should show less than 2 ohms resistance. Check for intermittent shorts by wiggling the harness while monitoring the multimeter.

Step 3: Advanced Diagnostics and Repair Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live solenoid command signals and actual current draw. Command the auxiliary hydraulics to activate and observe whether the ECM is sending proper voltage (typically 12V PWM signal). If voltage is present at the ECM but not at the solenoid, the harness has an open circuit. If current draw reads abnormally low despite proper voltage, replace the solenoid valve. For used machines, preventive harness replacement in high-wear areas often prevents recurring issues. After repairs, clear fault codes, cycle the system multiple times, and verify the code doesn't return before returning the machine to service.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Caterpillar service manual for your specific machine serial number and follow proper lockout/tagout procedures. If you lack proper diagnostic equipment or technical expertise, consult a certified Caterpillar dealer or qualified heavy equipment technician to prevent injury or further machine damage.