From MACS Technical Think tank
Controller area networks may be the new standard, but even they have three different levels, or speeds, at which they operate. CAN C is the high speed network, able to transmit data at over 500Kbps. Compare that to older systems, like UART, that transmitted at only 8Kbps. CAN C is typically used for vehicle critical systems like the ECM, ABS and VSC – systems that need information now!
CAN B is medium speed network, operating at speeds of 83Kbps, and is used for semi-critical vehicle systems. CAN A operates at the lowest speed, roughly 10Kbps, and is used for convenience systems, like entertainment and HVAC.
CAN uses a dual-wire bus, with each module wired to it in parallel. Unlike older systems, there is no master module. Each module has equal access to the bus and supplies its own power to the network. Messages sent along the bus are seen by all modules, but only those needed for an individual module’s operation are recognized and acted on.
The failure of one module will not necessarily stop all communication between modules. By going to the diagnostic link connector (DLC), you can quickly check the bus itself for opens or wiring faults by measuring the resistance between pins 6 and 14. A reading of 60 ohms indicates the bus itself is intact.
In diagnosing the system, keep in mind that if a module has dropped off the bus, you may not see it on your scan tool when you look for it – it’s as if the module doesn’t exist. Be sure to check the computer data lines schematic to identify all the modules you should be able to communicate with.
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
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The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.