Caterpillar 315D Fault Code 39:1598-6: Complete Diagnostic Guide

What is Caterpillar 315D Fault Code 39:1598-6?

Fault Code 39:1598-6 indicates a "Current Below Normal" condition in the Hydraulic Pump Flow Control Solenoid circuit. This diagnostic trouble code (DTC) means the Electronic Control Module (ECM) has detected insufficient electrical current flowing through the hydraulic pump compensator solenoid valve circuit, typically below the expected 200-450 milliamp range.

This code specifically affects the main hydraulic pump control system on the Caterpillar 315D excavator. The pump flow control solenoid regulates hydraulic oil delivery based on operator demand and load conditions. When current levels fall below normal parameters, the ECM cannot properly modulate hydraulic pump output, directly impacting machine performance and fuel efficiency. For used excavators, this fault often signals wiring degradation or connector corrosion rather than component failure.

Common Symptoms

• Reduced hydraulic power across all functions (boom, stick, bucket, and swing operations respond sluggishly)
• Amber warning light illuminated on the instrument cluster with possible audible alarm
• Erratic hydraulic response where implement speed varies unpredictably under load
• Engine over-revving without corresponding hydraulic work output due to improper pump displacement control
• Increased fuel consumption as the system cannot efficiently match pump flow to actual demand

Potential Causes

The most common technical causes for Code 39:1598-6 on used Caterpillar 315D excavators include:

• Open circuit in solenoid wiring harness – particularly at flex points near the main pump where vibration causes conductor fractures
• Corroded or loose connector pins at the pump solenoid connector (C301) or ECM harness connection
• Failed hydraulic pump flow control solenoid with internal winding break or short to ground
• Damaged ECM driver circuit – less common but possible after electrical surges or water intrusion
• Chafed wiring where the harness contacts the pump housing or frame rails (known issue on high-hour machines)

How to Troubleshoot and Fix Code 39:1598-6

Step 1: Visual Inspection Begin by thoroughly inspecting the solenoid wiring harness from the ECM to the hydraulic pump. On used excavators, check for wire chafing at the frame rail near the fuel tank and where the harness enters the pump area. Examine connector C301 at the pump solenoid for corrosion, bent pins, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Solenoid Resistance Test Disconnect the solenoid connector and measure resistance across the solenoid terminals using a digital multimeter. Specification should be 3-7 ohms at 20°C (68°F). Values outside this range indicate solenoid failure requiring replacement (Caterpillar part number 191-2208 or equivalent).

Step 3: Circuit Continuity and Voltage Check With ignition on and solenoid disconnected, check for supply voltage at the harness connector (should read battery voltage, approximately 24V). Test continuity from connector to ECM pin using wiring diagrams. Resistance should be less than 5 ohms. High resistance indicates damaged wiring requiring repair or harness replacement.

Step 4: Current Draw Test Reconnect the solenoid and use a clamp ammeter to measure actual current draw during operation. Normal current should be 250-400 milliamps when energized. Low current with proper voltage confirms solenoid failure; low current with low voltage indicates wiring or ECM issues.

Step 5: Clear Code and Verify After repairs, clear fault codes using Caterpillar Electronic Technician (Cat ET) software and operate the machine through full hydraulic cycles to verify proper system function and confirm the code does not return.

---

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Caterpillar service manual for your specific machine serial number and follow proper lockout/tagout procedures. If unfamiliar with hydraulic or electrical systems, consult a certified Caterpillar technician to avoid injury or further equipment damage.