What is Hyundai R335LC-9T Fault Code 1590-2?

Fault Code 1590-2 on the Hyundai R335LC-9T excavator indicates an Engine Coolant Temperature (ECT) sensor circuit intermittent or erratic voltage signal. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an inconsistent or fluctuating voltage reading from the coolant temperature sensor that falls outside the expected operating range during engine operation.

The ECT sensor is critical for this Cummins QSB6.7-powered excavator because it directly influences fuel injection timing, engine protection protocols, and hydraulic system temperature management. When the ECM receives unreliable temperature data, it cannot properly optimize engine performance or activate necessary protective shutdowns, potentially leading to overheating or inefficient fuel combustion.

Common Symptoms

When fault code 1590-2 becomes active on your R335LC-9T, operators typically experience:

• Check Engine light or fault indicator illuminated on the instrument cluster
• Intermittent engine derate (reduced power output) as the ECM enters failsafe mode
• Erratic temperature gauge readings that fluctuate between normal and abnormal ranges
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Difficult cold starts or rough idling when the engine is warm

Potential Causes

The most common technical causes for this intermittent signal code on used R335LC-9T excavators include:

• Corroded or loose connector pins at the ECT sensor harness (common on machines with 5,000+ operating hours)
• Damaged sensor wiring due to harness rubbing against the engine block near the thermostat housing
• Failing ECT sensor with internal resistance breakdown causing intermittent signal dropout
• Poor ground connection at the ECM or sensor ground circuit
• Water intrusion into the sensor connector causing intermittent short circuits
• Worn ECM connector terminals creating inconsistent contact (especially on pre-2015 models)

How to Troubleshoot and Fix Code 1590-2

Step 1: Visual Inspection Begin by locating the ECT sensor on the engine block (typically near the thermostat housing on the driver's side). Inspect the sensor connector for corrosion, pushed-back pins, or moisture intrusion. On used excavators, check the wiring harness routing for abrasion points where the harness contacts metal surfaces during engine vibration.

Step 2: Electrical Testing Using a digital multimeter, backprobe the ECT sensor connector with the key on, engine off. Verify reference voltage from the ECM (should read approximately 5 volts). Check sensor resistance values with the engine cold versus hot—resistance should decrease smoothly as temperature increases (typically 2,400-2,700 ohms at 68°F, dropping to 270-300 ohms at 194°F).

Step 3: Signal Testing Under Load With a professional diagnostic tool (like Hyundai Hi-MATE or compatible J1939 scanner), monitor the ECT sensor signal in real-time while wiggling the harness connector. If the signal drops out or spikes during harness movement, you've confirmed a wiring or connector issue rather than sensor failure.

Step 4: Repair or Replace For used machines, thoroughly clean all connector terminals with electrical contact cleaner and apply dielectric grease before reconnection. Replace damaged harness sections using OEM-spec heat-resistant wire. If testing confirms sensor failure, install a genuine Hyundai or OEM-equivalent ECT sensor with proper thread sealant.

Disclaimer: This guide provides general troubleshooting information. Always consult the Hyundai R335LC-9T service manual and consider professional diagnostics for complex electrical issues, especially on machines still under warranty or service agreements.