Caterpillar 328D Fault Code 39:1598-6: Complete Diagnostic Guide

What is Caterpillar 328D Fault Code 39:1598-6?

Fault Code 39:1598-6 indicates a moderately high voltage condition (current above normal) in the hydraulic pump solenoid circuit on the Caterpillar 328D excavator. This code is triggered when the Electronic Control Module (ECM) detects electrical current flowing through the hydraulic pump control solenoid that exceeds normal operating parameters.

This fault specifically affects the variable displacement hydraulic pump control system, which is critical for regulating hydraulic flow and pressure throughout the machine. The 1598 parameter refers to the pump solenoid circuit, while the "6" modifier indicates a "current above normal" or moderately high electrical condition. On the 328D, this system directly controls excavator performance, affecting everything from boom speed to swing torque. When this circuit malfunctions, the ECM may limit machine functions to prevent hydraulic system damage.

Common Symptoms

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

• Reduced hydraulic performance across all functions, including slower cycle times and weak digging force
• Illuminated malfunction indicator lamp (MIL) on the instrument cluster
• Derated engine power as the ECM enters a protective mode to prevent hydraulic system overpressure
• Erratic hydraulic response with inconsistent pump output during multi-function operations
• Stored fault code in ECM memory even after symptoms temporarily disappear

Potential Causes

The most common technical causes for this fault on used Caterpillar 328D excavators include:

• Shorted hydraulic pump solenoid windings due to heat exposure or internal coil degradation
• Damaged wiring harness with exposed conductors creating short-to-ground conditions, particularly at harness routing points near the hydraulic pump
• Corroded or moisture-contaminated connectors at the pump solenoid connection (common on machines with 3,000+ hours)
• Faulty ECM driver circuit providing incorrect voltage supply to the solenoid
• Pinched or chafed wires where the harness passes through frame brackets or near moving components
• Aftermarket electrical modifications that compromise circuit integrity

How to Troubleshoot and Fix Code 39:1598-6

Step 1: Visual Inspection and Connector Examination

Begin by locating the hydraulic pump solenoid on the main hydraulic pump assembly. Disconnect the electrical connector and thoroughly inspect both the solenoid pins and harness connector for corrosion, bent pins, moisture intrusion, or burnt contacts. On used excavators, this connection point frequently develops corrosion due to environmental exposure. Clean connectors with electrical contact cleaner and dielectric grease.

Step 2: Solenoid Resistance Testing

Using a digital multimeter (DMM), measure the resistance across the solenoid terminals. Typical specification for the 328D pump solenoid is 3-7 ohms at room temperature. A reading significantly below 3 ohms indicates a shorted solenoid that must be replaced. Also check for continuity between each terminal and the solenoid body (ground); any continuity indicates internal short-to-ground.

Step 3: Harness Continuity and Short Circuit Testing

Disconnect the ECM connector (harness side) and test the pump solenoid circuit wiring for short-to-ground and short-to-power conditions. Measure resistance between the solenoid control wire and chassis ground—it should read open circuit (OL). Inspect the entire harness route, paying special attention to areas where the harness contacts frame members, hydraulic lines, or passes through grommets where chafing commonly occurs on older machines.

Step 4: ECM Output Testing

If the solenoid and harness test within specifications, use Caterpillar Electronic Technician (Cat ET) diagnostic software to command the pump solenoid and monitor actual current draw. Normal operating current should be 0.5-1.5 amps. Higher readings with a known-good solenoid indicate an ECM driver circuit failure, requiring ECM replacement or professional repair.

Step 5: Repair and Verification

Replace faulty components (typically the solenoid or damaged harness sections), ensure all connections are clean and properly sealed, then clear fault codes using Cat ET. Operate the machine through full hydraulic cycles and monitor for code recurrence. For used excavators with high hours, consider preventive replacement of the pump solenoid even if testing marginal, as internal wear accelerates failure.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Caterpillar service manual for your specific machine serial number. If unfamiliar with high-voltage electrical systems or hydraulic diagnostics, seek assistance from a certified Caterpillar technician to prevent equipment damage or injury.