The Controller Area Network (CAN Bus) functions as the central nervous system of a modern vehicle. When physical damage occurs within the twisted-pair wiring harness (CAN High and CAN Low), the entire communication grid can fail, disabling multiple control modules simultaneously.
1. The Three Primary CAN Bus Electrical Failure Modes
1. Open Circuit
Happens when one wire in the twisted pair breaks (often inside door boot grommets due to repetitive flexing).
- Effect: The differential signaling breaks down and terminal resistance drops to $120\ \Omega$, causing intermittent communication faults (
U-codes) as vibration separates the broken strands.
2. Short to Ground
Occurs when CAN Low or CAN High touches bare metal chassis ground (0V).
- Effect: Differential voltage collapse halts message transmission across all connected transceivers, shutting down network communication.
3. Short to Battery (+12V)
The most destructive fault, happening when harness insulation melts against a constant +12V power feed.
- Effect: High voltage instantly destroys transceiver IC chips (such as TJA1050 or MCP2551) across connected control modules.
2. Diagnostic Testing via Multimeter and Scope
- Static Resistance (Battery Disconnected):
- Healthy network: 60 $\Omega$.
- Reading 120 $\Omega$: Open circuit on one termination resistor branch.
- Reading 0 $\Omega$ or infinity ($\infty$): Direct short or total severed wire.
Conclusion
Understanding CAN Bus electrical failure modes accelerates troubleshooting harness damage in complex collision and wiring repairs.