What is Komatsu PC138USLC-8 Fault Code DA2RMC?

Fault Code DA2RMC indicates a communication error between the machine's Engine Controller (ECM) and the Hydraulic System Controller, specifically related to data transmission failure on the CAN (Controller Area Network) bus. This code is part of Komatsu's diagnostic system for the PC138USLC-8 model and signals that critical operational data cannot be properly exchanged between the engine management system and hydraulic control modules.

The CAN bus serves as the nervous system of modern excavators, allowing multiple controllers to communicate simultaneously. When DA2RMC triggers, the ECM cannot receive or send vital information about engine load, hydraulic demand, or operational parameters. For the PC138USLC-8's integrated engine-hydraulic system, this communication breakdown directly impacts machine efficiency, potentially causing the excavator to enter derate mode or experience uncoordinated hydraulic response.

Common Symptoms

When fault code DA2RMC is active, operators typically experience:

• Warning lights illuminated on the instrument panel, often accompanied by audible alarms
• Reduced engine power or automatic derate limiting machine performance to protect components
• Sluggish or erratic hydraulic response, particularly during combined functions (boom + swing)
• Intermittent loss of monitor display information showing engine parameters or system status
• Inability to access full machine capabilities, with the system defaulting to safe-mode operation

Potential Causes

The most common technical reasons for DA2RMC on used PC138USLC-8 excavators include:

• Corroded or damaged CAN bus connectors at the ECM or hydraulic controller (C201, C202 connection points)
• Harness chafing or wire breaks along the main chassis harness near the swing bearing or behind the cab
• Failed termination resistors on the CAN network (120-ohm resistors at network endpoints)
• ECM or hydraulic controller internal failure, though less common than wiring issues
• Voltage supply problems affecting controller power (low battery voltage or poor ground connections)
• Water intrusion into controller housings, common on machines operating in wet environments

How to Troubleshoot and Fix Code DA2RMC

Step 1: Initial Inspection and Connection Verification Begin by visually inspecting all CAN bus harness connectors between the ECM (located near the engine) and the hydraulic controller (typically mounted under the operator platform). Check for corrosion, bent pins, or moisture in connectors C201 and C202. On used excavators, pay special attention to areas where harnesses route near moving components or sharp edges.

Step 2: CAN Bus Network Testing Using a digital multimeter, measure the resistance between CAN-High and CAN-Low terminals at the ECM connector with all power disconnected. You should read approximately 60 ohms if termination resistors are functioning correctly (two 120-ohm resistors in parallel). Readings significantly higher suggest open circuits or failed resistors; readings near zero indicate a short circuit.

Step 3: Voltage and Signal Quality Check With the key on but engine off, measure voltage on the CAN-High line (should read approximately 2.5-3.5V) and CAN-Low line (approximately 1.5-2.5V). Use Komatsu KOMTRAX diagnostic software or compatible scan tools to monitor live CAN bus traffic. Look for communication dropouts, excessive error frames, or missing messages from either controller.

Step 4: Harness Continuity and Repair Trace the main harness from ECM to hydraulic controller, checking for wear points particularly where it passes through the cab mounting area and near the swing post. On used machines with 3,000+ hours, harness abrasion at these points is extremely common. Repair any damaged wires using proper shielded twisted-pair cable matching OEM specifications, ensuring proper strain relief.

Disclaimer: This guide provides general troubleshooting information. Complex electrical diagnostics should be performed by qualified technicians with proper Komatsu diagnostic equipment. Always consult the official service manual and follow lockout/tagout procedures before performing electrical work.