What is Caterpillar Fault Code EID1935?

Caterpillar Fault Code EID1935 indicates an "Engine Coolant Temperature Sensor Circuit - Voltage Above Normal or Shorted High." This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the Engine Coolant Temperature (ECT) Sensor circuit, typically above 4.5 volts.

The ECT sensor is a critical component that monitors engine coolant temperature and sends real-time data to the ECM. This information directly affects fuel injection timing, engine idle speed, and cooling fan operation. When EID1935 appears, the ECM cannot accurately determine engine temperature, which can lead to improper engine operation, reduced fuel efficiency, and potential overheating risks. For used excavators, this code often points to age-related wiring deterioration or sensor failure rather than actual overheating conditions.

Common Symptoms

When fault code EID1935 is active, operators may experience:

• Check Engine Light or malfunction indicator lamp illuminated on the dashboard
• Engine running in derate mode or limp mode with reduced power output
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Difficulty starting the engine, especially in cold conditions
• Inaccurate temperature gauge readings showing extremely cold temperatures (often reading -40°F/-40°C)

Potential Causes

The most common technical reasons for EID1935 in used Caterpillar excavators include:

• Open circuit in the ECT sensor wiring harness due to wire breakage or poor connections
• Failed ECT sensor with internal circuit damage or contamination
• Corroded or damaged connectors at the sensor or ECM connection points
• Wiring harness chafing against engine components, frame rails, or brackets—common wear points on older machines
• Damaged or pinched sensor signal wire during previous repair work
• Moisture intrusion into the sensor connector causing high resistance or open circuits
• Faulty ECM (rare, but possible in high-hour machines)

How to Troubleshoot and Fix Code EID1935

Step 1: Visual Inspection Begin by locating the ECT sensor (typically installed in the engine block or cylinder head coolant passage). Inspect the sensor connector for corrosion, bent pins, moisture, or physical damage. On used excavators, check the entire wiring harness routing from sensor to ECM for signs of rubbing, cracking, or previous repair attempts with electrical tape.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure the resistance across the sensor terminals. A functional sensor typically reads 2,000-3,000 ohms at room temperature (refer to Caterpillar specifications for your specific engine model). If the reading shows infinite resistance (open circuit) or significantly out-of-range values, replace the sensor.

Step 3: Circuit Verification With the sensor disconnected, check for voltage at the harness connector. You should see approximately 5 volts on the signal wire (reference voltage from ECM). If voltage is absent or incorrect, trace the wiring back toward the ECM, checking for breaks or shorts to ground or power. Pay special attention to harness routing near hot exhaust components or sharp edges where insulation may have deteriorated.

Step 4: Connector and Wiring Repair Clean all connector contacts with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Repair any damaged wiring using proper gauge wire and heat-shrink connections—avoid simple wire nuts or tape on used machines operating in harsh environments.

Step 5: Clear Code and Verify After repairs, use Caterpillar Electronic Technician (Cat ET) diagnostic software or an appropriate scan tool to clear the fault code. Run the engine through various temperature ranges and monitor live data to confirm the ECT sensor readings are within normal parameters (typically 180-220°F during operation).

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Disclaimer: This guide provides general troubleshooting information for Caterpillar fault code EID1935. Always consult the specific service manual for your excavator model and engine configuration. If you're unfamiliar with electrical diagnostics or lack proper tools, consult a certified Caterpillar technician to prevent further damage to your equipment.