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Locate faults in the CAN network with these tips and techniques

Friday, February 1, 2019 - 09:00
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The exchange of information and data has been key from the beginning of mankind. In order to make good decisions, one needs good information. Information exchange is imperative in order for the decision-making process to be carried out. The lack of information one has limits the decisions that can be made correctly. Just as you and I need information to make decisions confidently, so will the modern vehicle. In order for the modern vehicle to run and drive correctly, the information must be exchanged quickly with accuracy.

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In order to exchange information, you must have a transmitter, a medium and a receiver. When we communicate with one another we use sound. When you speak, you become the transmitter; the air becomes the medium; and the person you are speaking to becomes the receiver. Since you can both speak and listen, you are a transceiver. In the modern vehicle, information exchange will occur using electricity. When a module speaks (transmits) it becomes the transmitter; the wiring becomes the medium; and the module that the message is sent to becomes the listener (receiver).  Therefore, if the module can transmit and receive it is a transceiver. Thus, using electrical on-off signals allows information to be transferred through the wiring between various vehicle modules.

Figure 1

This on-off digital information is sent at different speeds on different bus networks within the vehicle. Since each communication speed will use different rules on different networks, a means will be needed that allows communication between various vehicle networks. This will be accomplished by having a common module that connects each of these networks together. This common module is referred to as a gateway or bridge and is illustrated in Figure 1. The gateway has each of the different network communication transceivers within it. In this way the gateway will isolate the different networks from one another, while bridging the communications between the different modules. In order to gather data from the vehicle, an interface is used. This interface or scan tool will allow a connection to the vehicle networks. Once a connection is established with the vehicle data can be transmitted and received by the scan tool. If there are communication problems or no communications present you will need to connect to the communications wiring with an oscilloscope in order to test the circuits.

Figure 2

It is important to check a wiring diagram in order to understand how the scan tool will interface with the vehicle under test. In Figure 2, a block diagram of one method used to interface with the vehicle is illustrated. In this example, the scan tool is connected to the gateway module (CEM). It is important to understand that in this configuration the scan tool can be connected to the system by two different methods. In the first method the scan tool interface is not connected directly to the vehicle network. If the engineering team that designs the vehicle network deems it is necessary to protect the network from the scan tool interference, the gateway will isolate the scan tool interface from the network. The gateway when used in this method will bridge the scan tool communications to the vehicle networks. This means the data that is exchanged from the Diagnostic Link Connector (DLC) to the scan tool is not on the vehicle network. So, if you were to check these signals with an oscilloscope these signals are not the vehicle network communications, but the scan tool communications. In this case you will need to connect the oscilloscope directly to the vehicle network wiring under test.

With the second method, the gateway will connect the scan tool interface to the vehicle network. This will allow the scan tool interface to directly access the vehicle communication network. If an oscilloscope is connected to the DLC the data that is displayed on the oscilloscope is the data that is being exchanged on the actual vehicle network. When diagnosing the vehicle network, it is important to understand that these two methods are different. If one did not understand this you may connect to the gateway with the scan tool and see communications exchange on the oscilloscope and think the system is working. Where in actuality the scan tool communications to the gateway is all that is being displayed. In order to know which system you are working on, connect the oscilloscope to the DLC and the wiring at one of the modules. If the oscilloscope display shows two different waveforms then the gateway is isolating the scan tool from the network. If the oscilloscope displays the waveforms and they overlaying one another then the scan tool is directly connected to the network. If the DLC wiring is not connected to a gateway but connected directly to the network wiring then the scan tool will be connected directly to the network.

If there are communications between the vehicle and the scan tool, and there are communications codes set, get all codes from all of the modules. This will include the codes from the high-speed network, medium-speed network and low-speed network. Now that you have the codes, look over the codes to see if there are similarities between the modules of the same network, and if there are similarities between the codes from different networks. Now you will need to become a detective and analyze the data at hand. For example, if the high-speed network (engine and transmission) has anti-lock brakes (ABS) module codes set, the medium speed network (driver information module) has ABS codes set, and the low speed network (windshield wiper system) has ABS codes set, then the ABS system is the most likely culprit.  In this example all of these systems need the vehicle speed in order to operate. In many of the communication problems on the vehicle there will be many different codes set. It will be important to relate each code that is produced and to try to find some commonality between them.

Figure 3

The low-speed network will most likely use the Local Interconnect Network (LIN). This network is a master slave scheme. This means that the main control module (e.g. CEM) that the other modules connects to is the master and all the other modules are slaves. The LIN communication protocol is based on the SCI (UART) data format, which uses a single-master/multiple-slave concept, on a single-wire (plus ground) 12 V bus. The clock synchronization for nodes does not have a precise time base (e.g., without a crystal or resonator) but uses a capacitance resistive timing circuit that lowers the cost of each module. Therefore the codes will be stored in the master module. An example of a LIN waveform is shown in Figure 3.

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