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

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

Fault Code 39:1665-6 indicates a moderately severe deviation (Level 6) in the Auxiliary Hydraulic Valve Control Circuit, specifically related to the proportional solenoid valve that controls auxiliary hydraulic flow and pressure. This code is triggered when the Electronic Control Module (ECM) detects an abnormal current or voltage condition in the circuit that operates the auxiliary hydraulic function solenoid.

In the Caterpillar 318D2 L excavator, this code directly impacts the machine's ability to properly regulate auxiliary hydraulic attachments such as breakers, grapples, or augers. The proportional solenoid valve modulates hydraulic flow based on operator input, and any electrical malfunction can compromise attachment performance or render auxiliary functions completely inoperable. For used excavators, this is particularly critical as degraded wiring insulation and connector corrosion are common failure points that affect circuit integrity.

Common Symptoms

When fault code 39:1665-6 is active on your 318D2 L, operators typically experience:

• Auxiliary hydraulic functions become erratic or unresponsive – attachments may operate intermittently or not at all
• Amber warning light illuminates on the instrument cluster indicating a hydraulic system fault
• Reduced auxiliary flow or pressure – noticeable loss of power when operating hydraulic attachments
• Diagnostic display shows fault code 39:1665-6 when accessed through the monitor panel
• Possible derate mode in severe cases, limiting overall machine performance to protect hydraulic components

Potential Causes

The most common technical causes for this fault code on used Caterpillar 318D2 L excavators include:

• Damaged or corroded wiring harness – particularly at flex points near the boom base where constant movement causes insulation wear
• Failed proportional solenoid valve – internal coil failure or mechanical jamming from contamination
• Connector corrosion or moisture intrusion – especially on machines with high hours in wet or corrosive environments
• Short circuit or open circuit in the control wiring between the ECM and solenoid valve
• Poor ground connection at the valve mounting point or ECM ground strap
• ECM internal fault (less common) – controller output driver failure

How to Troubleshoot and Fix Code 39:1665-6

Step 1: Visual Inspection and Connector Check Begin by inspecting the auxiliary hydraulic valve wiring harness from the ECM to the proportional solenoid valve. On used excavators, pay special attention to harness routing near the boom pivot and along the upper frame where abrasion commonly occurs. Disconnect the solenoid connector and inspect pins for corrosion, bent contacts, or moisture. Clean contacts with electrical contact cleaner and apply dielectric grease before reconnection.

Step 2: Electrical Testing with Multimeter Using a digital multimeter (DMM), measure the resistance of the solenoid coil. Typical resistance should be between 8-12 ohms (consult service manual for exact specification). Check for continuity between each wire terminal and ground – there should be infinite resistance (no short to ground). With ignition on but engine off, measure voltage at the solenoid connector; you should see battery voltage (approximately 12-14V DC) when the auxiliary function is activated.

Step 3: Harness Continuity and ECM Output Test Disconnect both the ECM connector and solenoid valve connector. Test continuity through the entire harness – resistance should be less than 1 ohm with no breaks. Check for shorts between wires. If harness tests good but the fault persists, connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor real-time solenoid command values versus actual current draw. A significant deviation indicates either ECM output driver failure or internal solenoid issues requiring replacement.

Step 4: Component Replacement and Verification For used machinery showing harness wear, consider replacing the entire harness section rather than attempting repairs, as temporary fixes often fail quickly. When replacing the proportional solenoid valve, ensure proper torque specifications and verify hydraulic oil cleanliness – contamination accelerates repeat failures. After repairs, clear fault codes using Cat ET or the machine's diagnostic menu, operate auxiliary functions through full range, and verify the code does not return.

---

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 all safety procedures. Complex electrical or hydraulic repairs should be performed by certified Cat dealers or qualified heavy equipment technicians with proper diagnostic tools.