What is Caterpillar 336D2 Fault Code 39:1665-6?

Fault Code 39:1665-6 indicates a voltage issue with the Hydraulic Pump Solenoid Circuit, specifically reporting "Current Below Normal" or "Open Circuit" condition. This diagnostic trouble code (DTC) is generated by the Electronic Control Module (ECM) when it detects insufficient current flow through the hydraulic pump pressure control solenoid valve circuit, typically reading below the expected 4-20mA range.

This code is critical for the Caterpillar 336D2 excavator because the hydraulic pump solenoid directly controls the machine's hydraulic flow and pressure output. When the ECM cannot properly regulate hydraulic pressure due to electrical circuit problems, the excavator's performance, fuel efficiency, and operational safety are compromised. The 336D2's advanced hydraulic system relies on precise electronic control to deliver optimal digging force and smooth multi-function operations.

Common Symptoms

• Reduced hydraulic power across all functions (boom, stick, bucket, and swing operations feel sluggish)
• Amber warning light illuminated on the instrument cluster with fault code displayed on monitor
• Engine derate or power limitation mode activated to protect the hydraulic system
• Erratic hydraulic response or inconsistent implement speed during operation
• Increased fuel consumption due to inefficient hydraulic pump operation

Potential Causes

The most common causes for Code 39:1665-6 in used Caterpillar 336D2 excavators include:

• Damaged or corroded wiring harness between the ECM and hydraulic pump solenoid (common wear point near the pump mounting area)
• Failed hydraulic pump pressure control solenoid valve due to internal coil failure or mechanical damage
• Corroded or loose electrical connectors at the solenoid or ECM connection points (moisture intrusion is common in older machines)
• Broken or frayed wires caused by harness rubbing against the pump housing or frame members
• ECM internal circuit failure (less common but possible in high-hour machines)
• Poor ground connection at the hydraulic pump mounting area or chassis ground points

How to Troubleshoot and Fix Code 39:1665-6

Step 1: Visual Inspection Begin by thoroughly inspecting the wiring harness running from the ECM to the hydraulic pump solenoid. On used excavators, check for harness abrasion points where the wiring contacts the pump housing, frame rails, or hydraulic lines. Look for pinched wires, melted insulation, or signs of rodent damage.

Step 2: Connector and Ground Verification Disconnect the electrical connector at the hydraulic pump solenoid and inspect both male and female pins for corrosion, bent pins, or moisture. Clean contacts with electrical contact cleaner and check for proper pin retention. Verify the ground connection at the pump mounting bolts registers less than 0.5 ohms resistance to chassis ground using a digital multimeter (DMM).

Step 3: Solenoid Resistance Testing With the connector disconnected and ignition off, measure the solenoid coil resistance across the solenoid terminals. Typical specification for the 336D2 hydraulic pump solenoid is 3-7 ohms. Readings of infinite resistance indicate an open circuit (failed coil), while zero ohms suggests a short circuit.

Step 4: Circuit Voltage Testing Reconnect the solenoid connector and backprobe the circuit with ignition on, engine off. You should measure battery voltage (approximately 12-14V) on the power supply wire. Use Caterpillar Electronic Technician (Cat ET) diagnostic software to command the solenoid on/off and observe voltage changes.

Step 5: Wiring Continuity Check If the solenoid tests good, disconnect both ends of the harness (at ECM and solenoid) and test continuity through each wire. Any reading above 5 ohms indicates excessive resistance from corroded crimps or partial breaks inside the insulation—a common issue in used machines.

Step 6: Component Replacement Replace the hydraulic pump solenoid if resistance values are out of specification. If wiring is damaged, repair or replace the affected harness section using proper gauge wire and weatherproof connectors. For used excavators with multiple harness repairs, consider replacing the entire engine-to-pump harness to prevent recurring issues.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific serial number and follow proper lockout/tagout procedures. Complex electrical diagnostics should be performed by certified Caterpillar technicians with appropriate diagnostic equipment.