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CAN bus and multiplexing basics
Multiplexed trucks put a switch on one end of a network and a load on the other, with no wire between them. When the bus goes down the symptoms are spectacular and misleading — and the first useful test takes thirty seconds with an ohmmeter.
Two wires, many messages
CAN uses a twisted pair carrying mirrored signals. Noise hits both wires equally and cancels out at the receiver, which is why a network can run the length of an apparatus without picking up interference.
Every module hears every message and acts on the ones addressed to it. That is how a single pair replaces the enormous looms these vehicles used to carry.
Termination and the 60 ohm check
A CAN bus has a 120 Ω resistor at each end to stop signal reflections. Two 120 Ω resistors in parallel measure 60 Ω, and that is the single most useful bus test there is.
Key off, measure between CAN high and CAN low. About 60 Ω means both terminators are present. About 120 Ω means one is missing or the bus is broken between them. Very high means the pair is open; very low means shorted.
Faults look like everything at once
When a bus fails, modules lose contact with each other and set communication codes everywhere. The instinct is to chase the biggest symptom — but the fault is usually one connector, one chafed pair, or one module dragging the network down.
Unplug modules one at a time and watch whether the bus recovers. On apparatus, remember that bodybuilder-added multiplex systems may be a separate network from the chassis J1939.
How to test it
- Read the codes, but do not chase themCommunication codes tell you who lost contact, not what broke. Note which modules are missing.
- Do the 60 ohm checkKey off, meter across CAN high and CAN low at a diagnostic connector.
- Interpret the readingAbout 60 Ω healthy, about 120 Ω one terminator missing, open means broken pair, near zero means shorted.
- Isolate by unpluggingDisconnect modules one at a time to find one pulling the bus down.
- Inspect the pairCheck for untwisted repairs, chafe points and corroded connectors. The twist matters.
Numbers to work to
| Bus resistance, key off | about 60 Ω | two 120 Ω terminators in parallel |
| One terminator missing | about 120 Ω | or a break between the two ends |
| Termination resistor | 120 Ω each end | prevents reflections |
| J1939 high-speed | 250 or 500 kbit/s | heavy truck standard |
| Repair rule | keep the twist | untwisted repair sections invite noise faults |
Common mistakes
- Measuring bus resistance with the key on — module pull-ups make the reading meaningless.
- Replacing modules that set communication codes when the wiring is the fault.
- Repairing a twisted pair with a straight untwisted splice.
- Assuming one network. Apparatus often runs a bodybuilder multiplex system alongside chassis J1939.
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