Caterpillar 313D2 [PLD] Fault Code 39:1598-6: Complete Diagnostic Guide

What is Caterpillar 313D2 Fault Code 39:1598-6?

Fault Code 39:1598-6 indicates a "Current Below Normal" condition in the Pilot Solenoid Supply circuit. This diagnostic trouble code (DTC) signals that the Electronic Control Module (ECM) has detected abnormally low electrical current flowing through the pilot solenoid supply system, which controls critical hydraulic pilot pressure functions.

The pilot solenoid supply circuit provides regulated electrical power to the hydraulic pilot control solenoids that manage implement movements, swing functions, and auxiliary hydraulics. On the Caterpillar 313D2 with PLD (Pump-Line-Die) fuel injection system, this circuit is essential for proper coordination between electronic controls and hydraulic actuators. When current drops below the manufacturer's specified threshold (typically 0.5-2.0 amps depending on load), the ECM triggers this fault to prevent erratic hydraulic behavior and potential safety hazards.

Common Symptoms

When fault code 39:1598-6 is active, operators typically experience:

• Intermittent or complete loss of hydraulic functions including boom, stick, bucket, or swing operations
• Amber warning lamp illuminated on the instrument cluster with possible reduced engine power
• Erratic or sluggish hydraulic response during multi-function operations
• Hydraulic functions that work intermittently or only at certain operating temperatures
• ECM may enter derate mode limiting engine RPM to protect the hydraulic system

Potential Causes

The most common technical causes for this fault code on used Caterpillar 313D2 excavators include:

• Damaged or corroded wiring harness at known rub points near the hydraulic pump mounting area or along the main chassis harness
• Failed pilot solenoid supply relay or poor connections at the relay socket (common failure point after 5,000+ hours)
• Worn or internally shorted pilot control solenoid valves drawing excessive current or creating short-to-ground conditions
• Corroded connector pins at the ECM C2 connector or pilot valve harness connectors (moisture intrusion common on used machines)
• Failing ECM internal driver circuit providing power to the pilot solenoid supply (less common but possible on high-hour machines)
• Loose or damaged ground connections at G101 or G102 ground points on the frame

How to Troubleshoot and Fix Code 39:1598-6

Step 1: Visual Inspection and Preliminary Checks

Begin with a thorough harness inspection focusing on areas where wiring runs near moving components or sharp edges. On used excavators, check for chafed insulation along the right-side main harness near the hydraulic tank and pump area. Inspect all connectors for corrosion, bent pins, or moisture. Clean and apply dielectric grease to suspect connections.

Step 2: Electrical Testing with Diagnostic Tools

Using Caterpillar Electronic Technician (Cat ET) diagnostic software, monitor live data for pilot solenoid supply voltage and current values while cycling hydraulic functions. Normal supply voltage should read 12-14 VDC with current varying between 0.8-2.5 amps under load. Use a digital multimeter (DMM) to back-probe the pilot solenoid supply circuit at ECM connector C2, pin 47 (verify with your specific wiring diagram). Measure resistance to ground; readings below 5 ohms indicate a short circuit.

Step 3: Component Testing and Replacement

Test the pilot solenoid supply relay (typically located in the cab fuse/relay panel) by swapping with an identical relay or bench-testing with a 12V power supply. Measure individual pilot solenoid resistance values; they should read 15-25 ohms when cold. On used machines, solenoids often develop internal shorts causing current draw issues. Inspect ground connections G101 and G102 for tightness and corrosion—clean contact surfaces and apply anti-corrosion compound.

Step 4: Advanced Diagnostics for Persistent Issues

If the fault persists after harness and component checks, perform an ECM power supply test. Measure voltage at the pilot solenoid supply output with the ECM commanding the circuit ON (use Cat ET to force actuate). Voltage below 11.5V indicates internal ECM driver failure. For used excavators with 8,000+ operating hours, consider ECM connector refurbishment—corrosion inside connector bodies is a frequent root cause requiring connector replacement rather than just pin cleaning.

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Professional Disclaimer: This guide provides general troubleshooting procedures for informational purposes. Hydraulic and electrical systems operate under high pressure and voltage. Always consult the official Caterpillar service manual for your specific machine serial number, use proper lockout/tagout procedures, and consider professional diagnostic assistance for complex electrical issues. Improper repairs may cause equipment damage or personal injury.