What is Caterpillar 318D2 L Fault Code 39:1598-6?

Fault Code 39:1598-6 indicates a voltage problem with the Hydraulic System Temperature Sensor circuit, specifically detecting a voltage level above the normal operating range. This diagnostic trouble code (DTC) means the Electronic Control Module (ECM) is receiving a signal from the hydraulic temperature sensor that exceeds expected parameters, typically above 4.5 volts when normal operation ranges between 0.5-4.5 volts.

On the Caterpillar 318D2 L excavator, this sensor monitors the hydraulic oil temperature to protect critical hydraulic components from overheating damage. The ECM uses this data to adjust system performance and trigger protective measures. When voltage readings are abnormally high, it typically indicates an electrical circuit fault rather than actual hydraulic overheating, though both scenarios must be investigated.

Common Symptoms

• Warning light illuminated on the instrument cluster, often accompanied by a diagnostic code display
• Reduced hydraulic performance or derated machine functions as the ECM enters protective mode
• Intermittent loss of hydraulic power during operation, particularly noticeable in swing or boom functions
• Error messages on the monitor display specifically referencing hydraulic system issues
• Machine may operate normally despite the code being active, indicating an electrical rather than thermal problem

Potential Causes

The most frequent causes of this fault code on used Caterpillar 318D2 L excavators include:

• Open circuit in the sensor wiring harness, often caused by wire breakage or connector separation
• Corroded or damaged connector pins at the hydraulic temperature sensor or ECM connection points
• Failed hydraulic temperature sensor with internal circuit degradation (common in machines with 5,000+ hours)
• Wiring harness chafing near the hydraulic tank mounting area where harnesses contact metal edges
• Damaged ECM pin connections or internal ECM circuit issues (less common but possible in older units)
• Aftermarket repairs using incorrect wire gauge or poor splicing techniques

How to Troubleshoot and Fix Code 39:1598-6

Step 1: Visual Inspection Begin by locating the hydraulic temperature sensor on the hydraulic tank. Inspect the entire wiring harness from sensor to ECM for obvious damage, abrasion points, or connector corrosion. On used excavators, pay special attention to areas where harnesses pass near sharp edges or moving components—this is a known wear point on the 318D2 L model.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. A functioning sensor should read approximately 2,000-3,000 ohms at room temperature. Next, check for continuity in the wiring harness from sensor connector to ECM. Measure voltage at the sensor connector with ignition on—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data. Compare actual hydraulic oil temperature (use an infrared thermometer) with displayed values. If physical temperature is normal but voltage reads high, replace the sensor. If voltage is correct at the sensor but high at the ECM, inspect harness for short to power. For used machines, consider replacing aged connectors even if they appear functional—internal corrosion often causes intermittent high resistance.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific machine serial number and consider professional diagnostic services for complex electrical issues.