LS-CAN / Communication network

LOW-SPEED / FAULT-TOLERANT CAN

Body, comfort and convenience networks on many platforms

NETWORK GUIDE02/14

TOPOLOGYTwo-wire fault-tolerant network; termination strategy is platform-specific

PHYSICALTwisted pair with transceivers designed to communicate after certain single-wire faults

DATA RATECommonly up to 125 kbit/s

Correct theory and operation

HOW LS-CAN
COMMUNICATES.

Understand the normal architecture, signaling and message rules before deciding that a voltage or waveform is incorrect.

Physical signaling

Fault-tolerant CAN uses two bus wires, but its absolute voltage levels and bias network differ from high-speed CAN. Receivers evaluate the pair while transceivers are designed to retain communication after certain opens or shorts on one conductor.

Message control

It retains CAN framing, identifier-based arbitration, acknowledgement and error detection. Multiple modules share the bus, and the dominant/recessive rules determine access without a central message scheduler.

Degraded single-wire mode

When one wire is open or shorted within the transceiver's fault-tolerant range, compatible nodes may continue communicating on the remaining conductor. Continued scan-tool communication therefore does not prove both wires are healthy.

Topology and termination

Bias and termination are distributed according to the vehicle design and must not be judged by the familiar 60 ohm high-speed-CAN rule. Body networks may use branches, splice packs or star-like distribution points.

Wake, sleep and normal behavior

Body controllers frequently manage wake-up and sleep to limit key-off current. Both wires should show the platform's specified activity during normal communication; a stable but degraded pattern can accompany one-wire operation.

Common mistakes

AVOID THE
FALSE CLUE.

Testing with the connector unplugged when the fault only appears under load.
Using a generic voltage value instead of the vehicle-specific specification.
Skipping power, ground and reference checks before condemning the component.
Moving the harness or clearing codes before capturing the original evidence.
Replacing a part without repeating the test.

Reading the signal

Good waveform / normal communication

The two bus lines follow the bias and switching behavior specified for the installed fault-tolerant transceivers. Compare both lines and decoded traffic under the same operating conditions. Do not substitute a high-speed CAN reference.

Faulty waveform / fault indications

A line stuck at a supply rail, missing transitions or unexpected single-wire operation can indicate a wiring or transceiver fault. Some designs continue communicating after a line fault, so working communication alone does not prove a healthy physical layer.

COMPLETE TEST SEQUENCE

Identify the network by wiring diagram, connector pins and module list—not wire color alone.
Run a full scan and look for clusters of modules sharing the same branch.
Measure both wires key-on and compare to the OEM known-good pattern.
Scope both channels and check for a wire fixed high/low while communication continues on the other.
Inspect door, liftgate, seat and body-harness flex points.
Isolate the failed leg at documented splice or star points.

⚠ SAFETY / TEST CAUTION

Do not apply high-speed CAN voltage or termination rules to low-speed fault-tolerant CAN.

TEST EQUIPMENT

OEM topology and waveformFull-system scan toolTwo-channel oscilloscopeDigital multimeter

KNOWN-GOOD BASELINE

✓ Both wires carry data, but voltage levels and termination differ from high-speed CAN.✓ A fixed 60 Ω expectation is usually inappropriate without the exact topology.✓ A network may continue communicating in a degraded single-wire mode.

COMMON FAULTS

— One wire open with degraded operation— Short to battery or ground— Water intrusion at body connectors— Failed splice pack— Module transceiver fault— Network wake/sleep problem

IDENTIFY BEFORE YOU MEASURE

Confirm the network, topology, awake state and test conditions from vehicle-specific information.

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