Caterpillar 320D Fault Code 39:1598-6: Technical Guide

What is Caterpillar 320D Fault Code 39:1598-6?

Fault Code 39:1598-6 indicates a "Fan Speed Sensor Circuit Voltage Above Normal" condition in the Caterpillar 320D excavator's cooling system. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the engine cooling fan speed sensor, exceeding the manufacturer's specified threshold of approximately 4.5-5.0 volts.

The cooling fan speed sensor is a critical component in the 320D's engine thermal management system. It provides real-time feedback to the ECM regarding fan rotation speed, allowing the controller to optimize engine cooling performance and prevent overheating. When this sensor circuit reports excessive voltage, the ECM cannot accurately monitor fan operation, potentially compromising engine protection systems and triggering derate modes or shutdown sequences.

Common Symptoms

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

• Check Engine warning light illuminated on the instrument cluster
• Engine derate or reduced power output as a protective measure
• Cooling fan running continuously at maximum speed regardless of engine temperature
• Elevated engine operating temperatures due to improper fan control
• Secondary fault codes related to engine temperature or cooling system performance appearing simultaneously

Potential Causes

The most common technical reasons for this fault code on used Caterpillar 320D excavators include:

• Open circuit in the fan speed sensor wiring harness, often caused by chafing against the fan shroud or engine components
• Failed fan speed sensor with internal short-to-power condition (common after 5,000+ operating hours)
• Corroded or damaged connector pins at the sensor or ECM connection points—particularly problematic in machines operating in wet or corrosive environments
• Damaged wiring harness near known rub points along the right side engine compartment where harnesses route near moving components
• ECM internal fault (less common) affecting the sensor circuit voltage reference
• Aftermarket sensor installation with incorrect voltage specifications or incompatible design

How to Troubleshoot and Fix Code 39:1598-6

Step 1: Visual Inspection Begin by performing a thorough visual examination of the fan speed sensor located on the fan hub assembly and its associated wiring harness. On used excavators, check for obvious signs of harness wear, particularly where cables contact the fan shroud or route near the hydraulic pump. Inspect all connector terminals for corrosion, bent pins, or moisture intrusion—common issues in older machines.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the fan speed sensor connector and measure resistance across the sensor terminals. A functional sensor typically reads 800-1,200 ohms at ambient temperature. Next, check for short-to-power conditions by measuring voltage at the sensor connector with the key on, engine off—you should see approximately 5.0 volts on the signal wire and 0 volts on the ground. Any reading above 5.2 volts indicates a wiring fault.

Step 3: Wiring Harness Continuity Check With the sensor disconnected at both ends, test continuity from the sensor connector to the ECM connector pins (refer to the 320D electrical schematic for specific pin locations). Check for open circuits or excessive resistance (above 5 ohms indicates damaged wiring). Additionally, perform an insulation resistance test between the signal wire and ground—readings below 10 megohms suggest damaged insulation.

Step 4: Component Replacement and Verification If testing reveals a faulty sensor, install a genuine Caterpillar replacement (part number 227-7698 or equivalent) rather than aftermarket alternatives that may have voltage incompatibilities. For wiring issues common in used equipment, repair or replace the affected harness section using proper marine-grade heat shrink connectors and secure routing away from rub points. After repairs, clear fault codes using Caterpillar Electronic Technician (Cat ET) software and perform a cooling fan function test to verify proper operation.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Caterpillar service manual for your specific machine serial number, and consider engaging a certified Cat technician for complex electrical diagnostics. Improper repairs may void warranties or create safety hazards.