I remember when computers made their first appearance on an automobile. Today, simple tasks like changing the oil or rotating the tires requires knowledge of how the computers tied to these systems work. The advances in vehicle electronics have added safety and convenience to today’s cars, and the trend shows no sign of letting up. Since this same technology is also used to provide occupant comfort by controlling the vehicle HVAC system, it is important 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 really shouldn’t. Computers can only act on their programming. They take in information from their input sensors and, based on that information, perform the programmed tasks. This can be engaging 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 information 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 important to make sure they haven’t been corrupted before the signal gets to the computer. Low voltage to the sensor (whether it’s direct system 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 operation of individual features of the HVAC system to no operation of the system at all. Since we’re on the topic of power and grounds, the module itself is an electrical device. It, too, must have the proper voltage supply and a clean ground path to work the way it should. Diagnosing faults in heating repaIR BEDFORD NY that only use one module to control the various functions becomes much easier when we understand how it works. As technicians, we can connect to the computer with our scan tool to see what information the module is seeing. We can also test the individual 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 various outputs through the module to verify the module’s ability to turn them on and off. But what if you are reviewing the schematic and see another computer listed on the diagram?
Enter the Bus
We all know that computer technology has grown exponentially over the last several years. Processors are faster and smaller than ever before, automotive modules are more reliable and able to take the abuse of daily driving, and all of this is allowing the OEMs to provide to the consumer what they want…convenience. Of course, these advances have also resulted in very real safety 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 connection between modules that allows them to communicate with each other. The network you may have in your shop that allows all the shop computers to access the same information and programs is the same concept built into today’s cars. Think of the bus system as a phone line that modules can use to call the other modules. There are several bus systems in use on today’s cars, and may incorporate a single 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 system connected to it. The gateway module is the translator that allows different bus systems to share needed information. Each system has its own unique methodology for communicating. Older bus systems typically assigned a master module that was in charge of the entire 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 entire network to fail and resulted in the dreaded no communication or no bus codes to be stored. The newest addition 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 manufactured 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 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 CAN, operating at speeds of 83Kbps, and is used for semi-critical vehicle systems. CAN A is low speed CAN, operating at speeds of roughly 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 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. 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. 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 identify all the modules you should be able to communicate with.
A Few Diagnostic Tips
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? 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 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 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.
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