Electronic networking a necessity driving vehicle systems

CAN controls variety of vehicle systems on today's automobiles.

CAN CAN bus diagnosis electrical vehicle electrical systems repair shop training technician training automotive aftermarket As the use of computers (control modules) became more common to a variety of vehicle systems, it became obvious that some means of allowing these computers to share information was needed to avoid redundant wiring. Imagine having to hard wire each sensor to every computer that may need its input and the complexity of the wiring harness that would result.

Enter the bus — a single or dual wire system that allows these computers to be interconnected, with information shared along this system that all could use. In its most basic interpretation, a bus is nothing more than a phone line allowing each computer connected to it to call another, asking for the shared information needed to carry out its programming.

These networks are nothing new, and there are a variety of them in use today. Effective with the 2008 model year, however, the communication standard for emissions related computers is CAN, short for Controller Area Network. Even this protocol has been around awhile, first developed in the late 1980s by Intel and the Robert Bosch companies, and making its first appearance in the early 1990s on Mercedes Benz and BMW vehicles. Phase-in as a standard protocol began on some models as early as the 2003 model year.

But CAN is not limited to engine control, being used in a variety of vehicle systems. Whether you focus on drivability issues or HVAC repair, brakes or entertainment systems, CAN bus diagnosis lies in your future.

A Closer Look at CAN

While CAN is one protocol, there are four types. The first three are CAN C, CAN B and CAN A. The difference between them is the speed with which they can pass data from one module to another. CAN C is the fastest, with average transmission speeds of 500 kbps (kilobytes per second) but capable of speeds up to 1 Mbps (megabyte per second). It is used primarily for engine control and those systems that need real-time data to operate. CAN B is next, able to transmit at speeds of 83.3 kbps, and then CAN A, the slowest of the three, transmitting at 10 kbps. The faster the transmission speed, the less tolerant the system is to interference and network faults.

In addition to the three speeds of CAN, you'll still find older bus networks operating some vehicle systems. To allow the different speed CAN networks or older networks to share information with each other, they meet at a central point called a gateway. A gateway acts as a translator between networks and is usually incorporated into a module that already exists. You can identify the gateway by checking the data lines schematic, looking for the module where more than one network connects. Look close — there may be more than one.

The fourth CAN is Diagnostic CAN, typically a dedicated bus line between the gateway and the Diagnostic Link Connector (DLC). This dedicated line allows easier access to all the modules on any of the connected networks. When a scan tool is connected to CAN, it too becomes a network module and can cause codes to appear that weren't there before you started if the scan tool has problems of its own.

CAN Advantages

There are other advantages to CAN besides its speed. CAN networks can be single wire systems, but the majority are two wire systems in a twisted-pair configuration. The topography, or the way modules are connected, is a tree-type with the modules wired in parallel to the bus. CAN C usually has two dominant modules, but only in the sense that each has a 120 ohm termination resistor used to provide the voltage bias on the two lines.

Wired in parallel, the two will provide a total resistance reading of 60 ohms that you can check at the DLC (pins 6 and 14) or other connector specified in your service information system. Just be sure to disconnect the battery first and remember that this test tells you only about the bus wiring between the two dominant modules, not the "branch" legs leading to the other modules on the network.

Another advantage to CAN is in how data is transferred. It's called a message based system, because each type of message (engine coolant temperature, for example) sent over the bus is assigned a unique identifier that is transmitted at the beginning of the message. Standard CAN is able to assign more than 2,048 different message identifiers, while enhanced CAN is able to assign millions of different identifiers, literally! Each module on the bus has a list of those identifiers it needs to do its job and responds only to those messages, ignoring all the others.

Messages can be sent only when the bus is idle, and if two modules try to talk at the same time, the message with the highest priority will go first. The second module goes into receive mode, listening to what is being said before trying to transmit its message again. This method ensures that no data transmission is lost when a higher priority message takes over; the second just has to wait an extra microsecond or two.

CAN is also a multi-master system, meaning there is no Master module the network is dependent on to function. Each module on the bus provides its own termination and bias, and is able to both transmit and receive data on the network. While a fault in any module can cause the bus to go down, because of the enhanced error detection ability of CAN a more likely scenario would be a record of the fault and the bus ignoring the bad module while allowing others to continue to operate.

U Codes

If the bus does go down, or a module on the network is no longer able to communicate with its neighbors, a communications (U-XXXX) code will be recorded. Depending on what caused the fault, there may be more than one U code stored in more than one module.

Problems with the bus itself generally are a result of an open, short to power or short to ground in one or both of the bus wires. Another common source of communications codes are the addition of accessories that are inadvertently wired to the bus or one of its modules. And while CAN is pretty resistant to electromagnetic interference, improper routing of the bus wiring or excessive AC bleed through from a failed alternator can lead to sporadic and intermittent communications issues.

On CAN C systems, you can see these signals using a Digital Storage Oscilloscope (DSO), accessible at the individual modules or the same test point specified for checking bus resistance. (If the vehicle uses Diagnostic CAN, you cannot measure either at the DLC).

The two lines of CAN C are referred to as CAN H (High or +) and CAN L (Low or -). When data is transmitted, CAN H starts at approximately 2.5 volts and is pulled upwards, or high while CAN L starts at roughly the same voltage and is pulled down, or low. The two signals mirror one another, and are part of the error detection capability of CAN. Spikes in one or the other will be recognized as interference and ignored. In order to measure line voltages, be sure to measure each to a ground reference; either pin 4 or 5 will provide this for you when testing at the DLC.

The modules themselves can lead to communications codes. Each module needs good power and ground, just like any other electrical component. Modules that are not dominant also contain termination resistors that can fail. Typically, these resistors are between 2,500 and 3,000 ohms, but check your service information system to be sure. Individual modules can cause the bus to short to ground or power and should be one of the first possibilities you consider when dealing with these codes.

You can test for failed modules by disconnecting them one at a time and monitoring bus voltages with a Digital Multimeter (DMM) or DSO. When the bad module is disconnected, the bus voltages will return to normal and communications will begin again. Many designs make it easier to perform this testing by incorporating junction block connectors in the wiring that allows you to isolate sections of the bus at one time and then narrow your focus from there. If the fault is in the wiring, be sure that any repair adds no additional resistance to the circuit and the number of twists per inch is as specified by the manufacturer.

As with so many systems today, there are very few generalizations you can make when diagnosing these systems. Having access to a factory equivalent scan tool is a plus, making the job easier, but is not mandatory. What is mandatory is taking the time to understand the system as it is used on the specific vehicle in your bay, so take the time to study up and you'll have an easier time of repairing U codes and keeping the bus on the road.

Pete Meier, Motor Age technical editor, is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com.