CAN-BUSBasic Concepts Contents 1. Introduction; 2. CAN-BUS Network Development; 3. CAN-BUS Concepts; 4. CAN-BUS Operating Principle; 5. CAN-BUS Network Advantages; 6. CAN-BUS Network Diagnosis; 6.1. Diagnosis Procedures 7. Communication Failure in the CAN-BUS Network; 7.1. Failure in the CAN-BUS Lines; 7.2. Failure in an ECU; 7.3. Tension or Ground Failure in an ECU; 7.4. Interference in the CAN-BUS Lines; 7.5. Failure in the CAN-BUS Internal Resistance; 2 1. Introduction • More and more, there is the need to have a larger interaction between the different vehicle systems, and so the number of required functions also increases; • Higher need for safety and security (driving and anti-theft); • In order to reach these goals, the wiring harness quantity in the car was becoming larger and larger, and so it became necessary to create another way to reach those goals: enter the CAN-BUS; 3 2. CAN-BUS Network Development • The communication protocol CAN (Controller Area Network) was developed by INTEL and BOSCH in 1988, to be used in the automotive industry, in order to create a reliable and low-cost normalized communication line between the electronic devices of the car, allowing to reduce the vehicle's wiring harness size and number; 4 3. CAN-BUS Concepts • BUS Line: a communication line with bi-directional data transmission capacity. This means that, in a car, the Electronic Control Units (ECUs) are linked in parallel, implying that all information sent by one ECU is also received by the others. In practical terms, a BUS line operates like a closed ring, where all units send and receive information, constantly communicating between them; 5 3. CAN-BUS Concepts • CAN-BUS: is a local area network of communication, through which all of the car's ECUs connected to that network (via BUS lines), send and receive information. Every information sent by an ECU (rpm, engine temperature, air flow, etc.) has an exclusive ID code, throughout all CAN network. In this way, all ECUs will receive that signal, but only the ECU determining the signal is important to its operation will react to the signal. The other ECUs will simply ignore the signal. The image on the next page illustrates the CAN-BUS network for the new Jazz; 6 3. CAN-BUS Concepts 7 4. CAN-BUS Operating Principle • Each ECU receives information from the sensors connected to the ECU and sends information to the actuators, via the CAN-BUS network; • So that the ECUs are able to communicate between them, each ECU has an integrated digital Encoder/Decoder for the CAN information; • In this way, the logic In and Out signals are converted into digital information, compatible with the CAN communication protocol; 8 4. CAN-BUS Operating Principle • The F-CAN (Fast-CAN) line connects the ECUs of the dynamic systems (ABS/VSA, EPS, SRS, ECM/PCM), through two intertwined copper wires, each one of them carrying an opposing signal with the same information: CAN-High and CAN-Low respectively, with a data transfer rate of 500 kbps; • The purpose of using tow BUS lines (CAN-H and CAN-L) is to prevent any parasitic voltage pulses that might interfere with the information sending and receiving; 9 4. CAN-BUS Operating Principle • The wires are intertwined in order to create an equal and constant distance between them and any electromagnetic fields in the vicinity; • The B-CAN (Body-CAN) line connects the ECUs of the car body systems (MICU, Immobiliser, A/C, Headlights Adjuster), through a copper wire, with a data transfer rate of 33,33 kbps; 10 4. CAN-BUS Operating Principle • The F-CAN B-CAN Gateway is integrated in the indicator control module (tachometer) and allows for the dynamic systems (ABS, SRS, EPS, etc.) and the body systems (MICU, Immobiliser, A/C, etc.) to communicate between them; • In practical terms, this Gateway is a communication portal between the F-CAN and the B-CAN, where there is a “conversion” of the information circulating at the B-CAN line, so that they can be received by those ECUs connected to the FCAN line and vice versa; Note: the CAN-BUS network is only for the communication between the ECUs and their peripherals, that is, through the CAN lines, there's only information and commands circulating. This means that the vehicle power circuit and ground points are still necessary. 11 4. CAN-BUS Operating Principle • The ECU are connected to the CAN network via the integrated Encoders/Decoders; • The information gathered by the sensors circulate in the CAN network and is recognized as valid for the operation of each ECU, by means of its digital encoding. The information is only received by the ECU to which it is intended after this recognition; 1 2 3 4 ECU; Microprocessor; Encoder/Decoder; CAN-BUS network 12 4. CAN-BUS Operating Principle • As we know, every information circulating in the CAN-BUS network has a digital encoding. Part of this encoding comprises a field dedicated to the priority level of the information, so, if two or more ECUs attempt to transmit information at the same time through the CAN-BUS network, the ECU whose information has the lowest level of priority does not send the information at that moment; • The CAN-BUS network is a system that consumes electrical power, and so it also includes Wake-Up and Sleep functions to avoid draining the battery too fast; 13 4. CAN-BUS Operating Principle • In “Sleep” mode, the MICU stops working, that is, the CAN network communication stops and the power consumption drops from 200 mA to 30 mA, but there is still power consumption!!!; • The “Sleep” mode will not function if there is one door open or if the key is in the ignition switch; • When the ignition is turned off and the driver's door is opened, and then closed using the power door lock, there will be a delay of about 40 seconds before the network goes from “Wake-Up” mode to “Sleep” mode; 14 4. CAN-BUS Operating Principle • If we turn ignition off and open the driver's door, there will be a time delay of about 10 minutes before the network goes from the “Wake-Up” mode into the “Sleep” mode; • The difference between the time periods needed to change from “Wake-Up” mode into “Sleep” mode, when the power door lock is, or not, activated, is explained because the "door closed" signal "tells" the MICU that the vehicle systems no longer need to remain in operation; • When the "closed door" signal is not transmitted, the MICU does not "know" if the systems still need to be "live" and that's the reason why it "waits" longer 15 4. CAN-BUS Operating Principle • The “Wake-Up” mode is activated the moment any action is requested to the vehicle. Opening a door is enough to take the CAN network out of “Sleep” mode and immediately resume its operation; • F-CAN line is: => activated ("Wake-Up" mode) by turning the ignition switch ON; => deactivated (“Sleep” mode) by turning the ignition switch OFF; • B-CAN line is: => activated ("Wake-Up" mode) by turning the ignition switch ON or opening a door; => deactivated (“Sleep” mode) by turning the ignition switch OFF, closing the vehicle doors and waiting 40 seconds; 16 5. CAN-BUS Network Advantages • All ECU have access to all information circulating through the CAN network; • Possibility of more complex functions, without the need to further increase the wiring harness number; • Software updates for the vehicles' ECUs without need to replace any ECUs; • Better self-diagnosis capacity; 17 6. CAN-BUS Network Diagnosis • The CAN network diagnosis is complex and lengthy because all units operate within the network, communicating to each other; • The K line is used by the HDS to communicate with the ECU via the DLC connector; • HDS allows reading the ECU in terms of communication loss, Diagnostic Troubleshoot Codes (DTCs) and input and output signals (for some ECUs); • The DLC (or OBD) connector is a 16 pin connector to connect the HDS diagnostic equipment; 18 6. CAN-BUS Network Diagnosis • The function of each of the 16 pins at the DLC connector is defined according to one of the several international norms ruling the car building activity. 1 2 3 4 5 6 7 8 Ignition +; BUS + line; Defined by the manufacturer; Body ground; Signal ground; CAN-H; K line; Defined by the manufacturer; 9 10 11 12 13 14 15 16 Defined by the manufacturer; BUS – line ; Defined by the manufacturer; Defined by the manufacturer; Defined by the manufacturer; CAN-L; L line or 2nd K line; Battery +; 19 6. CAN-BUS Network Diagnosis • The DTC codes memorized at the car also obey to international norms regulating its structure; DTC: X Y Z K W (ex: P1101) Type of failure (what is not functioning correctly in the affect sub-system) affected sub-system: 1 2 3 4 Fuel and Air Metering (MAF); Fuel and Air Metering (Injection Circuit); Ignition Systems and Misfire; Emissions Control (Catalyst); 6 7 8 DTC Type (0 According to the SAE norm; 1 5 Vehicle Speed Control and Idle Control System; Microprocessor; Transmission; Transmission; Defined by the manufacturer); Affected System: P B Engine; Body (Interior); C U Body (Exterior); Not Assigned; 20 6.1. Diagnosis Procedures Even after the DTCs have been deleted from the different units where they were memorized, the DTCs reoccurred; Before starting any complex diagnostics, you should start by checking and confirming that all fuses, relays, ground points relating to the ECUs with DTCs and their wiring harnesses connectors are all in good working order; Check whether all fuses and relays are OK, all ground points are tight and making good electrical contact and all wire harness connectors are correctly connected and installed and their terminals are free from moisture and/or corrosion; 21 6.1. Diagnosis Procedures Moisture at the electrical contacts and subsequent corrosion increases the wiring electrical resistance and this causes some of the problems normally attributed to damages ECUs; 22 6.1. Diagnosis Procedures Fuse Checking: Use a multimeter (set it to resistance measurement/continuity check and touch the probes to the visible fuse terminals) or a test lamp (connect it to a ground point of the vehicle and check for continuity between ground and each terminal of each fuse). 23 6.1. Diagnosis Procedures Relay Checking: Use a multimeter (set it to resistance measurement/continuity check and touch the probes to the visible relay terminals). It must be said that the relays fitted to our cars are basically from two different types: Connect the power (BAT+) to terminal 3 and ground to 4 and check for continuity between the terminals 1 and 2; With the power disconnected, there should be no continuity between terminals 1 and 2; Connect the power (BAT+) to terminal 3 and ground to 5 and check for continuity between the terminals 1 and 2; With the power disconnected, there should be continuity between terminals 1 and 4; 24 7. Communication Failure in the CAN-BUS Network; • The communication failure in the CAN-BUS network can have several causes: CAN-BUS lines (F-CAN e B-CAN) failure; ECU (Electronic Control Unit) failure; Tension or Ground Failure in an ECU; Interference in the CAN-BUS Lines; Failure in the CAN-BUS Internal Resistance; 25 7.1. Failure in the CAN-BUS Lines; • The CAN-BUS communication lines can display the following failures: * CAN-H/CAN-L line damaged; * CAN-H/CAN-L line shorted to the battery power; * CAN-H/CAN-L line shorted to ground; * CAN-H line shorted to CAN-L line; * Poor contact at the connections (damage, corrosion or poor fitting). 26 7.2. Failure in an ECU; • Each ECU connected to the CAN-BUS network integrates a communication module which allows that ECU to send and receive data within the CAN-BUS network. Normally, a failure in one ECU originates DTCs in other units communicating with that ECU via the CAN-BUS network. • In order to isolate the failing ECU, the ECU can be disconnected, one at a time, while the CAN network status is monitored with a multimeter or an oscilloscope. This procedure can be complemented by erasing the DTCs from the other units still connected and re-reading those units for DTCs. 27 7.2. Failure in an ECU; • If the ECU disconnected from the CAN-BUS network is the one with the failure, then the DTCs found after a new health check will only be "communication failure with that ECU" DTCs. 28 7.3. Tension or Ground Failure in an ECU; • The CAN-BUS network tension is split in 5 V to the F-CAN and 5 V to the BCAN. • Because the F-CAN is further split into two lines (CAN-H and CAN-L), then we have the 5 V split as 2,7 V to the CAN-H (measured between the CAN-H and ground – IGN ON) and 2,3 V to the CAN-L (measured between the CAN-L and ground – IGN ON). Ground CAN-H K-Diag Ground CAN-H K-Diag CAN-L BAT+ • U=2,7 V CAN-H tension OK • U=2,3 V CAN-L BAT+ CAN-L tension OK 29 7.3. Tension or Ground Failure in an ECU; • Because the CAN-H and CAN-L lines read this tension figures does not mean that the CAN line being tested if free from failures. It only means that there is tension available for data communication. • A battery voltage slowly decreasing or a dead battery can cause occasional communication failures in several ECUs connected to the CAN-BUS network. So, the battery must ALWAYS be checked for full charge and the charging system (ACG or IMA) must be free from problems, before we do any diagnosis to the CAN-BUS network. 30 7.3. Tension or Ground Failure in an ECU; • Because not all ECUs stop communicating at the same tension level, if the battery voltage drops below a point when one or several ECU are deprived from the power they need to operate, then there may be DTCs memorized. • This can happen, for example, when the starter is cranking the engine. With a fully charged good condition battery, at the cranking time, there is a voltage drop of about 4 V. • The poorer the battery condition, the bigger the voltage drop will be, thus the higher the possibility to occur an occasional failure within the CAN-BUS network. 31 7.4. Interference in the CAN-BUS Lines; • A faulty alternator or electronic devices such as cellular phones, audio amplifiers or portable DVD players, might induce voltage pulses into the CAN-BUS network and, occasionally, deprive the network from the necessary voltage, up to a point where all communications might be temporarily interrupted. Many times, this type of interruption is intermittent and only some ECUs might memorize DTCs, and such DTCs are, normally, hard to reproduce. • First, we should make sure that there's no problem with the CAN-BUS network wiring harnesses and check the ACG (alternator or IMA system) for proper operation. Disconnect all extra electronic equipment installed after the vehicle leaves the factory and check for further DTCs. 32 7.5. Failure in the CAN-BUS Internal Resistance; • There is a resistor of about 120 Ω fitted to two ECUs, between the CAN-H and CAN-L lines (F-CAN), in order to absorb electromagnetic "echoes" that might cause communication failures between the ECUs; • The F-CAN line has its ECUs parallel-connected, so the real resistance value shall be about 60 Ω (IGN OFF); • R=60 Ω F-CAN circuit OK; Poor contact at Ground CAN-H K-Diag • 60 Ω<R<120 Ω the F-CAN circuit connectors; • R=120 Ω F-CAN circuit wiring CAN-L BAT+ harness damaged; 33 7.5. Failure in the CAN-BUS Internal Resistance; • The F-CAN circuit is quite stable and will continue to allow the communication between the ECUs, even if the resistance is not within the normal specifications. Nevertheless, there might be temporary DTCs memorized related to communication failures; • Any ECU whose internal resistance is not 120 Ω can be checked by disconnecting it from the harness and measuring the resistance between the CAN-H and CAN-L terminals; this measurement shall fall within 2,4 kΩ and 2,6 kΩ; • In the next images, we can see the measurement results for some models' ECUs: 34