A Clear-Flow Explanation of CAN System Issues in a Volvo EC240B LR

[MikePhua]

Troubleshooting a Sudden CAN System Breakdown
A perfectly functional Volvo EC240B LR excavator unexpectedly failed to crank after operating for years without issue. Readings revealed:
• 0 V to the Engine ECU (EECU), despite correct supply voltage at other pins.
• A CAN-LO line measuring approximately 2.3 kΩ to ground via the harness.
Let’s break down what might have happened—and how to avoid or resolve similar failures.
What’s Going On? Understanding the CAN Bus
The Controller Area Network (CAN) is the digital communication backbone in modern equipment, linking ECUs (Electronic Control Units) across systems.
• CAN-LO (low-speed line) typically sits at ~2.5 V when idle.
• An abnormally low resistance to ground (2.3 kΩ) suggests the line is shorted or pulled low—blocking vital communication.
In this case, the machine didn’t crank because communication to the Engine ECU was disrupted—effectively isolating it from commands to start.
Key Terms Clarified
• CAN Bus System – A digital communication network that allows modules (e.g., engine, hydraulics, monitoring) to talk to each other via twisted-pair wiring.
• ECU (Engine Control Unit / EECU) – Manages engine-related functions such as fuel timing, safety interlocks, and ignition—often dependent on CAN messaging.
• CAN-LO Short – When the low-speed communication wire accidentally makes contact with ground or another low point, corrupting signals.
• Ground Fault – Any unintended connection between a live circuit and ground, often causing signal disruptions or component damage.
Possible Causes for the Sudden Failure
• Damaged ECU – A local forum follow-up revealed that the technician had inadvertently burned out the EECU, necessitating replacement.
• Wiring Harness Damage – Abrasion, rodent chewing, pinching, or moisture intrusion could have created a short between CAN-LO and ground.
• External Component Failure – Sensors, CAN junction blocks, or modules connected downstream may have failed, dragging the line to ground.
Diagnostic and Repair Workflow

1. Perform a Visual Harness Inspection – Look for cuts, resin, burn marks, or pin terminal damage near connectors, sensor junctions, or wear points.
   
2. Measure Line Resistance
   • With power off, measure between CAN-LO and ground; a healthy line shows open circuit or >10 kΩ.
     
   • Then isolate each module (unplug) and remeasure to detect where the short originates.
     
3. Check for ECU Damage – If only the ECU pin shows odd behavior while disconnected from the bus, the ECU may be grounded internally or have failed components.
   
4. Repair or Replace
   • Replace burned or shorted ECU.
     
   • Repair or replace harness sections with insulation damage.
     
   • Ensure plugs, connectors, shields, and sealing boots are intact and corrosion-free.
     
5. Restore and Re-test Communication – After repair, reconnect and confirm CAN-LO floats correctly, and the ECU regains power and network responses.
   

Real-World Lesson
Even a well-maintained machine can develop instant failures due to latent damage—like frayed wiring or a vulnerable ECU. Proper diagnosis requires both careful measurement of communication lines and awareness that high-voltage surges, incorrect wiring, or harsh handling can instantly disable ECU hardware.
Takeaways to Avoid Future Issues
• Routinely inspect CAN harnesses for wear, especially near moving components or engine access points.
• Handle ECUs with ESD (electro-static discharge) protection and keep them dry.
• Always monitor CAN-LO idle voltage and resistance during electrical diagnostics.
• Keep working spare ECUs and harness segments on-hand if operating older machines in remote areas.
If you’d like, I can help you draft a CAN-bus diagnostic checklist or a wiring repair guide tailored to EC240-series equipment—just say the word!