From MACS Technical Think tank
Faults in any bus system are typically related to communications (U codes) or faults with the modules themselves (B codes). The key here is to test the operation the same as if you were diagnosing a single module system.
Each module still needs good power and ground, still needs accurate input, and still needs to carry out its programming. The only difference is how these duties are handled by the networked modules. It then becomes a matter of “If/Then” troubleshooting – if module A sees what it’s supposed to see, is it sending that information to module B? Is module B then carrying out its programming?
When you start diagnosing a communications fault, first clear the communications code and cycle the key to see if the code resets. If not, a scope plugged into the bus at the DLC comes in handy. You can quickly tell if the message on the bus is corrupt (by a bad module), shorted (flat line high or low), or open (no signal).
Of course, you have to know what the signal is supposed to look like, and this information is getting easier and easier to find. Take the time to hook up to known good systems and play with them to get comfortable with what normal is. If an individual module is the suspect, most systems have some central point in the wiring you can use to isolate modules one at a time until the culprit is uncovered.
Diagnosing computer network, or bus, systems need not be complicated. They are a fact of life in every tech’s working day, and more are coming. Familiarity with these systems is a must for any tech who wants to be able to repair today’s and tomorrow’s cars.
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.