Caterpillar Excavator Fault Code CID1935: Complete Diagnostic Guide

What is Caterpillar Fault Code CID1935?

Caterpillar Fault Code CID1935 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) bus system. This code specifically signals that the Electronic Control Module (ECM) is not receiving proper communication from one or more control modules on the network, or the data transmission is corrupted or intermittent.

The CAN bus serves as the central nervous system for modern Caterpillar excavators, allowing the engine controller, hydraulic controller, display module, and other systems to share critical operational data. When CID1935 is active, the machine's ability to coordinate functions between systems becomes compromised. This is particularly critical because the ECM relies on this network to monitor engine performance, adjust hydraulic flow, and manage emissions controls—all essential for optimal machine performance and fuel efficiency.

Common Symptoms

When CID1935 is active on your Caterpillar excavator, operators typically experience:

• Malfunction indicator lamp (MIL) or check engine light illuminated on the dashboard
• Intermittent or complete loss of dashboard display information, including temperature, pressure, or hour meter readings
• Reduced engine power or derate mode activation as a protective measure
• Erratic hydraulic response or inconsistent implement movement due to communication delays
• Multiple fault codes appearing simultaneously as various modules lose communication with each other

Potential Causes

CID1935 typically stems from these common issues, especially in used excavators:

• Damaged or corroded CAN bus wiring harness, particularly at known rub points near the swing bearing or along the boom
• Loose or corroded connector pins at module connection points, often caused by moisture intrusion
• Failed terminating resistor at either end of the CAN bus network (120-ohm resistors)
• Faulty ECM or secondary control module with internal communication circuit failure
• Voltage supply issues causing intermittent module resets or power fluctuations
• Physical damage to wiring from previous repairs, rodent damage, or harness chafing against frame components

How to Troubleshoot and Fix Code CID1935

Step 1: Visual Inspection Begin with a thorough physical examination of all wiring harnesses and connectors in the CAN bus circuit. On used excavators, focus on high-flex areas where the harness routes across the swing bearing and along the boom cylinders. Look for abraded wire insulation, crushed connectors, or evidence of moisture (white or green corrosion). Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion.

Step 2: CAN Bus Resistance Testing Using a digital multimeter (DMM), disconnect both ends of the CAN bus network and measure resistance between the CAN High and CAN Low wires. You should read approximately 60 ohms with both terminating resistors in place. If the reading is infinite (open circuit), one or both terminating resistors have failed. If below 60 ohms, there's likely a short in the harness. Caterpillar ET (Electronic Technician) diagnostic software is invaluable here for identifying which specific module has dropped from the network.

Step 3: Module Communication Testing Connect Caterpillar ET software to the machine's diagnostic port and monitor active communication. The software will display which modules are responding. If a specific module (such as the hydraulic controller or instrument cluster) repeatedly drops offline, check voltage supply to that module—should be stable 24V DC. Test the module's ground connection for continuity and low resistance (less than 1 ohm). For used excavators, swap suspected modules with known-good units to isolate electrical versus module failure.

Step 4: Advanced Diagnostics If basic checks don't resolve the issue, use an oscilloscope to examine CAN bus signal quality. Proper signals show distinct high and low voltage differentials (approximately 2.5V differential). Distorted waveforms indicate EMI interference, poor grounds, or failing modules. On older machines, check the main power supply and chassis grounding points—corrosion at major ground straps commonly causes intermittent communication faults.

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Disclaimer: This guide provides general diagnostic procedures for CID1935 on Caterpillar excavators. Specific repair procedures may vary by model year and configuration. Always consult the official Caterpillar service manual for your specific machine. If you're uncomfortable with electrical diagnostics or lack the proper tools, consult a certified Caterpillar technician to prevent further damage to electronic systems.