Modern Tech
George Murphy
 
Considering The CAN Bus
Hint: it’s not a mode of public transportation
 
As demand grew in the 1980s for more and more onboard electronic devices, Mercedes-Benz and all carmakers faced a “wire crisis.” In the effort to comply with various government regulations for fuel economy, emissions control and safety and to achieve industry leadership in providing increasingly sophisticated comfort and convenience features, all auto manufacturers were continually adding sensors, actuators, controllers, and other electronic components.

Developing the electronic devices was one issue, but a more serious problem was to connect the components so they would work together properly. Old-school technology required separate wires to connect every device. Sometimes, multiple wires had to run between two devices. For example, if two sensors reported to a controller, and the controller operated two devices based on sensor inputs, the controller would be the center of a spider web of four to eight wires. The exact number of wires joining the devices depended on the circuitry to provide reference voltage and signal voltage between the sensors and the controller and the connections needed for output and feedback between the controller and the actuators. Whenever a new device was added, entire wiring looms had to be redesigned to accommodate the latest change.

Continually adding more wires to a vehicle causes many problems:

• Manufacturing costs increase because more material and labor are needed to assemble the vehicle. The fact that wire must be uniquely color-coded further increases cost.
• Identifying places to route the wiring becomes more difficult as the amount of wiring increases. According to some industry estimates, a midsize car can contain up to three miles of wire.
• More wire means more weight, which hurts performance and fuel economy.

The wire crisis was solved by CAN bus technology, which was quickly adopted by Mercedes-Benz and is now used in all its vehicles. CAN stands for “Controller Area Network.” “Bus” is an electrical/electronics term for a device or system that links multiple components together.

CAN bus technology was developed in the late 1980s as a cooperative venture between Intel and Robert Bosch. The two companies realized that the wire crisis was not limited to the automotive industry. Any manufacturer of digital devices faced the same problem – trying to fit more wires into less space as components became increasingly complex and sophisticated. CAN bus technology was so successful at reducing wiring use, it is estimated that literally hundreds of millions of devices are manufactured each year using one or more CAN bus networks.

Mercedes-Benz first used a CAN bus in model year 1992 on the W140. Today, every Mercedes-Benz has multiple CAN bus networks. The current C-Class has three of these networks; one for the engine electronics, one for diagnostics, and one for the body (there is a fourth CAN bus, but it supports diagnostics and isn’t a factor in service work). The most extensive use of CAN devices is on the current S-Class, which can have up to eight separate networks including the interior, drive train, diagnostic, chassis, front end, vehicle dynamics, telematics, and the central network. Just about any electron that moves on any circuit in the S-Class passes through one or more CAN bus networks.

According to one Mercedes-Benz service engineer, today’s M-B vehicles, with all of their features and innovations, could not be built without CAN bus technology. It would be impossible to effectively link together all the electronic components and systems that make a Mercedes a Mercedes.

A CAN bus replaces the traditional wiring harness composed of a thick bundle of multicolored wiring wrapped in plastic with just two wires. One is the CAN-high wire, the other is the CAN-low wire. The two wires are twisted or braided along their length to limit the amount of electrical noise that can enter the wiring, and both wires are connected to every device on the network. Two wires are used because, in a CAN bus, signals may be redundantly “pulled up” on one wire or “pulled down” on the other. In electronic engineering, this is known as dominant recessive logic for signal transmission. Because of this setup, CAN bus networks are virtually immune from electrical interference caused by external devices that are not part of the network. Interference can still disrupt a CAN network, but such problems are rare.

In electronic terms, “node” is a generic term for any device on a given CAN bus network. All nodes in an automotive CAN network are capable of both sending and receiving messages. The type of message determines if a node will transmit or receive it. Although nodes can both send and receive, sensors and other devices that provide input data to a control module generally transmit. Actuators that respond to signals from a controller are mostly receivers.

A new Mercedes-Benz can have more than 60 electronic components linked to eight CAN bus networks. 

Mercedes-Benz has quickly adopted CAN bus technology because of its many advantages for the designer.

• It replaces thick wiring harnesses with just two wires, with several messages transmitted in succession on the same line, saving weight, space, and installation costs. There are fewer plug connections and fewer pins on the control units.
• It saves money on wiring and assembly. Lines from sensors must only be extended to the nearest control unit where the measured values are processed to form a data telegram and are placed on the CAN data bus. Actuators can also receive control signals from a different control unit that receives the data telegram through the CAN bus and then uses this information to calculate a control parameter for the actuator.
• It not only reduces the wiring used, but can reduce the total number of components required. With links between CAN bus networks, a message can be sent from any component to any other location on the vehicle. Signals from one sensor can be used for several purposes, reducing the number of required sensors.
• It is energy efficient – a CAN bus network operates on low voltage – the exact voltage varies from vehicle to vehicle and by the CAN bus type. When no signals are being sent, the network goes into a sleep mode, immediately waking up when a signal is sent.
• It’s more reliable because less wiring and fewer components equals fewer things that can go wrong. The double-twisted-wire setup is extremely fault tolerant. Transmission failures or errors are very rare in a CAN bus network. In particular, the electronics have improved electromagnetic compatibility, reducing sensitivity to interference.
• It’s easy to troubleshoot. The system is programmed to identify any node or component that is not operating properly and will set a trouble code that can be retrieved for diagnosis.
• Problems can more easily be isolated to one component. Because signals from one sensor can be received by several receivers, it can be assumed if a fault is displayed by all systems that use a particular signal, the sensor is defective or the control unit processing the signal is faulty. If only one system displays a fault, even though the signal is used several times, the fault is usually at the processing control unit or at the actuator.
• It makes adding new factory-approved devices easy. All that needs to be done is connect the device to the network and reprogram the controller.
   

Notice the diversity of sensors and actuator components linked by just one CAN bus.

CAN bus is not multiplexing

A CAN bus should not be confused with a multiplex network. Although a multiplex network shares the benefits of connecting multiple devices to a single network, the technologies operate differently.

In a multiplex network, a single wire connects all the devices. Each device has a unique address on the network. A signal from one device is sent directly to another unit on the network based on the receiving network’s unique address. The message skips or bypasses all other components.

On a CAN bus, all nodes or components send and receive data equally. The message goes to all components on the network. In theory, a node on a CAN bus could send a signal to all of the other nodes at the same time. But each message that is sent contains an identifier code. Each receiving component opens the message and immediately looks for the identifier code. If the code isn’t the correct one for the specific node or component, the device ignores the message.

The distinction between unique addresses in a multiplex system versus unique identifier codes in a CAN bus network might seem to be splitting hairs. But using identifier codes is a key reason why it is easy to add components or modify a CAN bus network. Sending messages to all nodes, but only allowing a node to open messages that pertain to its function makes it much easier to program a CAN bus network than a multiplex system with unique addresses.

Included with the CAN identifier code is a message priority status, known as the arbitration process. Greatly simplified, arbitration occurs when two messages are sent to the same node at the same time. As the transmitter node sends a message, it compares the information sent/transmitted with the information received back from the network. If the transmitter detects that the received and transmitted information are different, then it ceases to transmit because it knows a message with higher priority is being sent at the same time. The message with higher priority will be transmitted without interruption. It will try again after an idle time is reached.
In addition to priorities for each node, messages can be ranked by the frequency by which the CAN bus receives them. For example, a signal to initiate the anti-lock brake system would be sent with increased frequency (or at quicker intervals) than a message to the climate control system to adjust the interior temperature.

At maximum speed, a CAN bus network sends signals at speeds of up to 1M (mega) bits per second (bps). A bit is a digital signal for 0 or 1, the basis for all digital communications.

Mercedes-Benz does not operate its CAN bus networks at maximum potential speed. Speed is limited to a balance between what is required for speed and how much noise (interference) the network can tolerate. The faster the network, the more susceptible it is to electronic noise that will interfere with its operation.

The fastest any current Mercedes-Benz CAN bus network operates is at 500K (500,000) bps. Some networks operate as slowly as 83K bps. The 500K bps CAN bus networks are used for drive train and other critical systems that require higher throughput. The slower networks are used for various interior control systems, such as climate control, which are not as time critical.

The higher and lower priority messages and transmission intervals are in “electronic time,” which is measured in milliseconds over a relatively short distance. A human observing the operation of a CAN bus without the use of special test equipment would not be able to sense any difference in priority or speed. To humans, a CAN bus does everything all at one time, and all of them are very, very, fast. For example, the single-bit transmit time for a 500K-bps transmission is 2 microseconds and a single-bit transmission at 83.3K bps is still only 12 microseconds.

In addition to the CAN bus networks, Mercedes-Benz uses a Local Interconnect Network (LIN) that is a slow, 19.2K bps, and consequently less expensive system to control some interior functions. Another type of network is Media Oriented Systems Transport (MOST), a very high-speed network used for the fiber optics that control audio and navigation systems. MOST replaces the older Domestic Digital Bus (D2B) networking technology.

This schematic shows why Mercedes-Benz CAN bus networks are called “linear.” Each of the devices (codes keyed to the workshop manual) connects to the same two wires of the CAN bus network.

Trouble-shooting the CAN bus

These networks are reliable and getting better. On the latest Mercedes-Benz vehicles, CAN network problems are rare. In fact, when troubleshooting, a problem with the network should be the last thing, not the first thing, that the technician focuses on.

A key concept in troubleshooting is that external physical problems such as corrosion, abrasion or physical breaks, and issues with the battery and charging system can appear to be problems from within the CAN bus. As a consequence, any owner with reasonable mechanical aptitude may be able to identify and correct an apparently electronic problem by checking the system visually, and running a standard charging test on the battery before taking the car to the dealer.

If the problem still exists after a visual check and check of battery voltage and correct operation of the charging system, then most of the time the problem will be with one or more nodes or components on the network. Typically, a component is either not sending signals or it fails to respond properly to messages that are addressed to it.

A technician will proceed to test the CAN bus if troubleshooting has eliminated the components on a network and the presence of necessary voltage is confirmed. A visual inspection can spot most of the common physical problems with the network.

• Corrosion can be caused by water leaks or other moisture making contact with the wiring.
• Physical breaks in the wiring may have been caused by failure of another component, such a broken bracket, or abrasion from rubbing against a worn metal part or loose part.
• Shorts to battery voltage or ground, which are usually found using a digital multimeter.
• Circuits that have been tampered with.

In particular, if another technician has tried to fix a CAN bus by rewiring it, or has added a component that was not originally part of the system, the network won’t work properly. Splices, replaced wires, wires that do not have the correct turns per inch in the braiding, etc., will change the length of the wiring, which will interfere with network operation. With a good quality multimeter and lab oscilloscope, plus the specific service information for the vehicle being diagnosed, the technician can test a network for signal patterns, voltage drop across the components, and system resistance. All resistance readings should be within specifications.

A CAN bus may operate with the wrong resistance, but signal problems are more likely when the resistance isn’t within limits. This is more critical on the 500K bps networks where there are terminating resistors that maintain this balance. Interference, although rare, can still occur. Interference can be confusing because the problems may mimic a short or communication failure among nodes. A defective alternator is a common cause of interference, which is one reason why a technician must verify a good charging system operation before troubleshooting a CAN bus. The technician will look for a clean signal free of any glitches in the pattern that may indicate transient interference entering the network or control modules that may be sending errors. If needed, compare the signal of the suspect vehicle with that of a known trouble-free vehicle.

The other major cause of system interference is an aftermarket component that has been improperly installed. To locate this problem, the technician has to disconnect each such device, one at a time, and check to see if the inference problem is eliminated.
Overall, the CAN bus systems are robust in operation, and highly cost-effective in use, and with reasonable care should rarely be a source for operational problems.

This article was adapted by George Murphy, MBCA Technical Committee chair, with permission of MBUSA from “StarTuned,” a magazine published by the company to assist Mercedes-Benz technicians in understanding the technologies and diagnostic tools they work with every day.