Multiplexing, Networks, and Electronic Controls
Multiplexing, Networks, and Electronic Controls
Multiplexing lets modules share information over a few wires instead of dedicated circuits. Know the common buses: J1939 high-speed CAN for powertrain and chassis, J1708/J1587 for legacy diagnostic and body data, LIN for low-speed accessories, and manufacturer-specific body networks. CAN uses a twisted pair, differential signaling (CAN High and CAN Low), and termination resistors-typically two 120-ohm resistors in parallel, giving about 60 ohms at the data link connector. Modules need power, ground, and a valid bus to communicate; a module that is 'dead' may simply have lost battery power or ground.
Diagnose network faults systematically: check battery voltage and grounds, measure termination resistance with power off, check for shorts to ground or battery on CAN H/L, and verify wake-up signals. A shorted bus can shut down multiple modules. After replacing a module, programming or configuration may be required. Network wake-up can cause parasitic draw.
Traps: measuring CAN voltage with a low-impedance test light; expecting 60 ohms with a module still connected on some architectures; misdiagnosing J1708 as CAN; replacing modules without checking power and ground; ignoring the need for software configuration.
Worked example: a DMM across CAN H and CAN L at the DLC reads 60 ohms-normal termination. A reading near 120 ohms suggests one terminator is open; near 40 ohms suggests an extra terminator or a short. With power off, isolate modules to locate the fault.
Rule of thumb: about 60 ohms across the CAN pair at the DLC is the normal baseline for a two-terminator bus.