I remember when computers made their first appearance on an automobile. Today, uncomplicated tasks like changing the oil or rotating the tires requires knowledge of how the computers tied to these systems work. The advances in car electronics have added protection and convenience to today's cars, and the trend shows no sign of letting up. Since this same technology is also used to furnish occupant ease by controlling the car Hvac system, it is prominent that we become as familiar with diagnosing their problems as we are when dealing with the refrigerant side of the system.
It All Starts With a Computer
Just the word computer strikes fear in the minds of some techs...and it truly shouldn't. Computers can only act on their programming. They take in data from their input sensors and, based on that information, perform the programmed tasks. This can be sharp the compressor clutch, activating a mode door, or setting blower speed...in other words, the computer turns things on or off. Its language is made up of 0s and 1s, called bits. This is useful in diagnostics, because that tells me as a technician that the computer can only see these bits; it can only see on/off, yes/no, or high/low. The data it takes in from the input sensors must be in a language it can understand. The easiest way to do this is to use a sensor that provides a digital signal to the module. These sensors send either a high or low voltage depending on the state of the sensor. An example of this would be an Ac request switch that is either on or off. Sensors that send a varying voltage back to the module are called "analog" sensors. These sensor signals are converted, or translated if you will, in the module to digital signals the module can then process. A high pressure sensor is just one example of this type of input signal. Since both of these types of input signals are electrical, it's prominent to make sure they haven't been corrupted before the signal gets to the computer. Low voltage to the sensor (whether it's direct law voltage or a reference voltage supplied by the module itself) or a bad ground (also direct or module supplied) can cause the signal to be ignored or read incorrectly by the module, and cause symptoms ranging from intermittent performance of personel features of the Hvac law to no performance of the law at all. Since we're on the topic of power and grounds, the module itself is an electrical device. It, too, must have the permissible voltage furnish and a clean ground path to work the way it should. Diagnosing faults in Hvac systems that only use one module to control the varied functions becomes much easier when we understand how it works. As technicians, we can associate to the computer with our scan tool to see what data the module is seeing. We can also test the personel inputs with either a Dvom (digital volt-ohm meter) or scope to make sure the signal from the sensors is correct. In some cases, we can even command the actuation of varied outputs through the module to verify the module's capability to turn them on and off. But what if you are reviewing the schematic and see other computer listed on the diagram?
Enter the Bus
We all know that computer technology has grown exponentially over the last any years. Processors are faster and smaller than ever before, automotive modules are more dependable and able to take the abuse of daily driving, and all of this is allowing the Oems to furnish to the consumer what they want...convenience. Of course, these advances have also resulted in very real protection systems being incorporated into even the low end offerings from the manufacturers. Hang on to your hats, techs, there is even more to come. None of this would be practical without a means of allowing all these different modules to work together and share information. This is where the idea of the bus comes from. The bus is nothing more than a association between modules that allows them to characterize with each other. The network you may have in your shop that allows all the shop computers to way the same data and programs is the same notion built into today's cars. Think of the bus law as a phone line that modules can use to call the other modules. There are any bus systems in use on today's cars, and may incorporate a singular wire or dual wire network between modules. That car in your bay may even use more than one, depending on what systems are networked together. If so, then keep an eye out for the gateway module. This module will have more than one bus law associated to it. The gateway module is the translator that allows different bus systems to share needed information. Each law has its own unique methodology for communicating. Older bus systems typically assigned a master module that was in fee of the whole network, supplying the power to the bus and arbitrating the messages passed along the network. In some designs, the failure of one module could cause the whole network to fail and resulted in the dreaded no communication or no bus codes to be stored. The latest expanding to the list is Can, or Controller Area Network. Can systems first appeared in some models in 2003 and the good news is that this is the protocol now required on every car artificial starting with the 2008 model year.
Can Highlights
Can may be the new standard, but even it has 3 different levels, or speeds, at which it operates. Can C is the high speed network, able to forward data at over 500Kbps. Correlate that to older systems, like Uart, that transmitted at only 8Kbps. Can C is typically used for car essential systems like the Ecm, Abs and Vsc...systems that need data Now. Can B is medium speed Can, operating at speeds of 83Kbps, and is used for semi-critical car systems. Can A is low speed Can, operating at speeds of almost 10Kbps, and is used for convenience systems, like entertainment and Hvac. Can uses a dual wire bus, with each module wired in parallel to it. Unlike older systems, there is no master module. There are, however, 2 terminating resistors that may or may not be incorporated into a module. By going to the Dlc (diagnostic link connector), you can fast 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. Each module has equal way 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 personel module's performance are recognized and acted on. The failure of one module will not necessarily stop all communication between modules. 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...as if it doesn't exist. Be sure to check the computer data lines schematic to recognize all the modules you should be able to characterize with.
A Few Diagnostic Tips
Faults in any bus law are typically associated to communications (U codes) or faults with the modules themselves (B codes). The key here is to test the performance the same as if you were diagnosing a singular module system. Each module still needs good power and ground, still needs correct input, and still needs to carry out its programming. The only variation 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 data to module B? Is module B then carrying out its programming? If 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 fast 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 data 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 personel 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 is coming. Familiarity with these systems is a must for any tech who wants to be able to repair today's...and tomorrow's cars.
The Bus and Hvac heal