Diagnostics and repair: CAN bus

21.02.2006

This is exactly what that “tire” looks like (mostly) CAN ", which we have to deal with more and more lately:

photo 1

This is an ordinary two-wire cable called Twisted Pair .
Photo 1 shows the wires CAN High and CAN Low power unit.
These wires carry out data exchange between control units; they can carry information about vehicle speed, rotation speed crankshaft, ignition timing and so on.
Please note that one of the wires is additionally marked with a black stripe. This is how the wire is marked and visually identified CAN High (orange-black).
Wire color CAN-Low - orange-brown.
For the main color of the tire CAN orange color is accepted.

In pictures and drawings it is customary to depict the colors of the bus wires CAN other colors, namely:

photo 2

CAN-High - yellow
CAN-Low - green

There are several types of tires CAN , determined by the functions they perform:
Powertrain CAN bus(fast channel) .
She allows transmit information at a speed of 500 kbit/s and is used for communication between control units (engine - transmission)
Comfort CAN bus(slow channel) .
She allows transmit information at a speed of 100 kbit/s and is used for communication between control units included in the Comfort system.
Information and command system CAN data bus(slow channel), allowing data transmission at a speed of 100 kBit/s. Provides communication between different service systems (for example, telephone and navigation systems).

New car models are becoming more and more like airplanes - in terms of the number of declared functions for safety, comfort and environmental friendliness. There are more and more control units and it is unrealistic to “pull” from each bunch of wires.
Therefore, in addition to the tire CAN There are already other tires called:
– LIN bus (single-wire bus)
– MOST bus (fiber optic bus)
– Bluetooth wireless bus

But let’s not “spread our thoughts down the tree,” let’s focus our attention for now on one specific tire: CAN (according to the corporation's views BOSCH).

Using the CAN bus as an example power unit, you can view the signal shape:

Photo 3

When on High CAN bus dominant state, the voltage on the wire rises to 3.5 volts.
In the recessive state, the voltage on both wires is 2.5 volts.
When on the line Low dominant state, the voltage drops to 1.5 volts.
(“Dominant” is a phenomenon that dominates, dominates or dominates in any area, from dictionaries).

To increase the reliability of data transmission, the bus CAN a differential method of transmitting signals over two wires called Twisted Pair . And the wires that form this pair are called CAN High and CAN Low .
In the initial state of the bus, both wires are supported constant pressure at a certain (basic) level. For bus CAN power unit it is approximately 2.5 volts.
This initial state is called a “resting state” or “recessive state”.

How are signals transmitted and converted? CAN bus?

Each of the control units is connected to CAN bus through a separate device called a transceiver, which has a signal receiver, which is a differential amplifier installed at the signal input:

photo 4

Coming by wire High and Low The signals enter the differential amplifier, are processed and are sent to the input of the control unit.
These signals represent the voltage at the output of the differential amplifier.
The differential amplifier generates this output voltage as the difference between the voltages on the High and Low wires of the CAN bus.
This eliminates the influence of the base voltage (for the CAN bus of the power unit it is 2.5 V) or any voltage caused, for example, by external noise.

By the way, about the interference. As they say, "tire CAN It is quite resistant to interference, which is why it is so widely used."
Let's try to figure this out.

CAN bus wires power unit are located in engine compartment and they can be affected by various types of interference, for example, interference from the ignition system.

Since the CAN bus consists of two wires that are twisted together, then the interference simultaneously affects two wires:

From the above figure you can see what happens next: in the differential amplifier, the voltage on the Low wire (1.5 V - " Pp ") is subtracted from the voltage
on the High wire (3.5 V - " Pp ") and there is no interference in the processed signal (" Pp" - interference).

Note: Depending on the availability of time, the article may be continued - much remains “behind the scenes”.

Kucher V.P.
© Legion-Avtodata

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CAN bus is a device that makes it easier to control a car by exchanging information with other car systems. Data transfer from one vehicle unit to another is carried out through special channels using encryption.

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What is CAN bus

The electronic CAN interface in a car is a network of controllers used to combine all control modules into a single system.

This interface is a block with which the following blocks can be connected via wires:

• an anti-theft system equipped with or without an auto-start function;
• machine motor control systems;
• anti-lock unit;
• safety systems, in particular airbags;
• management automatic transmission gears;
• control panel, etc.

Device and where the bus is located

Structurally, the CAN bus is a block made in a plastic case or a connector for connecting cables. The digital interface consists of several conductors called CAN. One cable is used to connect blocks and devices.

The installation location of the device depends on the vehicle model. Usually this nuance is indicated in the service manual. The CAN bus is installed in the vehicle interior, under the control panel, and can sometimes be located in the engine compartment.

How does it work?

Principle of operation automatic system consists of transmitting coded messages. Each of them has a special identifier that is unique. For example, “the temperature of the power unit is 100 degrees” or “the vehicle speed is 60 km/h.” When transmitting messages, all electronic modules will receive relevant information, which is verified by identifiers. When data transmitted between devices is related to a specific block, it is processed; if not, it is ignored.

The length of the CAN bus identifier can be 11 or 29 bits.

Each information transmitter simultaneously reads the data transmitted to the interface. Device with more low priority should release the tire, since the dominant level with high rate distorts its transmission. At the same time, the package with the increased value remains untouched. A transmitter that has lost connection restores it after a certain time.

Interface connected to the signaling device or module automatic start, can operate in different modes:

1. Background, which is called sleeping or standalone. When it is running, all major systems of the machine are disabled. But at the same time, the digital interface receives power from the mains. The voltage is minimal, which prevents discharge battery.
2. Startup or wake-up mode. It begins to function when the driver inserts the key into the lock and turns it to activate the ignition. If the machine is equipped with a Start/Stop button, this occurs when it is pressed. The voltage stabilization option is being activated. Power is supplied to controllers and sensors.
3. Active. When this mode is activated, the data exchange procedure is carried out between the regulators and actuators. The circuit voltage parameter increases because the interface can draw up to 85 mA of current.
4. Deactivation or falling asleep. When the powertrain stops, all systems and components connected to the CAN bus stop functioning. They are deactivated from electrical network vehicle.

Characteristics

Technical properties of the digital interface:

• the general information transfer speed is about 1 Mb/s;
• when sending data between control units various systems this figure is reduced to 500 kb/s;
• the information transfer speed in the “Comfort” type interface is always 100 kb/s.

The “Electrical Engineering and Electronics for Programmers” channel talked about the principle of sending packet data, as well as the characteristics of digital adapters.

Types of CAN buses

Conventionally, CAN buses can be divided into two types according to the identifiers used:

1. KAN2, 0A. This is how digital devices that can operate in an 11-bit data exchange format are marked. This type of interface, by definition, cannot detect errors on signals from modules operating with 29 bits.
2. CH2, 0V. This is how digital interfaces operating in 11-bit format are marked. But key feature is that error data will be transmitted to microprocessor devices if a 29-bit identifier is detected.

CAN buses can be divided into three categories according to their type:

1. For a car's power unit. If you connect this type of interface to it, this will allow for fast communication between control systems via an additional channel. The purpose of the bus is to synchronize the operation of the engine ECU with other components. For example, a gearbox anti-lock braking system etc.
2. Comfort type devices. This type of digital interface is used to connect all systems in this category. Eg, electronic adjustment mirrors, heated seats, etc.
3. Information and command interfaces. They have a similar information transfer speed. They are used to ensure high-quality communication between the nodes necessary to service the vehicle. For example, between electronic unit management and navigation system or smartphone.

The “Electrical Engineering and Electronics for Programmers” channel spoke about the principle of operation, as well as about the types of digital interfaces.

Instructions for connecting alarm via CAN bus

During installation anti-theft system a simple option for connecting it to the on-board network is to connect security installation with digital interface. But this method is possible if there is a CAN bus in the car.

To install a car alarm and connect it to the CAN interface, you need to know the installation location of the system control unit.

If the alarm was installed by specialists, then you need to seek help with this issue at a service station. Typically the device is located behind dashboard car or under it. Sometimes installers install a microprocessor module in free space behind the glove compartment or car radio.

What will you need?

To complete the task you will need:

• multimeter;
• stationery knife;
• insulating tape;
• screwdriver.

Step by Step Actions

Connection procedure anti-theft installation to the CAN bus is done as follows:

1. First you need to make sure that all elements security complex installed and working. We are talking about a microprocessor unit, an antenna module, service button, siren, as well as limit switches. If the alarm system has an auto-start option, you need to make sure that this device is installed correctly. All elements of the anti-theft installation are connected to the microprocessor unit.
2. A search is performed for the main conductor going to the CAN bus. It is thicker and its insulation is usually orange.
3. The main car alarm unit is connected to this contact. To perform the task, the digital interface connector is used.
4. The control unit is being installed security system, if it has not been installed. It should be placed in a dry place inaccessible to prying eyes. After installation, the device must be properly fixed, otherwise during movement it will be negatively affected by vibrations. As a result, this will lead to rapid failure of the module.
5. The junction of the conductors is carefully insulated; the use of heat-shrinkable tubes is allowed. It is recommended to additionally wrap the wires with electrical tape. This will increase their service life and prevent abrasion of the insulating layer. When the connection is completed, a check is carried out. If problems arise in transmitting packet data, you should use a multimeter to diagnose the integrity of the electrical circuits.
6. At the final stage, all communication channels are configured, including additional ones, if available. This will ensure uninterrupted operation security system. Used for setting service book, included in the anti-theft installation.

User Sigmax69 talked about connecting a security system with a digital interface using the example of a Hyundai Solaris 2017 car.

Malfunctions

Since the CAN interface is connected to many vehicle systems, in the event of a breakdown or incorrect operation one of the nodes may experience problems. Their presence will affect the functioning of the main units.

Signs and causes

The following “symptoms” may indicate the occurrence of malfunctions:

• several icons lit up on the dashboard at the same time for no reason - airbags, steering, pressure in the lubrication system, etc.;
• light appeared Check indicator Engine;
• There is no information on the control panel about the temperature of the power unit, the fuel level in the tank, speed, etc.

Reasons why malfunctions in the operation of the CAN interface may occur:

• broken wiring in one of the systems or damage to power lines;
• short circuit in the operation of the units to the battery or ground;
• damage to the rubber jumpers on the connector;
• oxidation of contacts, as a result of which signal transmission between systems is disrupted;
• discharge of the car battery or drop in voltage in the electrical network, which is associated with improper functioning of the generator set;
• closing of CAN-high or CAN-low systems;
• occurrence of malfunctions in the operation of the ignition coil.

The KV Avtoservis channel spoke in more detail about digital interface failures and testing using a computer.

Diagnostics

To determine the cause of the problem, you will need a tester; it is recommended to use a multimeter.

Verification process:

1. Diagnostics begins with searching for the twisted pair conductor of the CAN bus. The cable has black or orange-gray insulation. The first is the dominant level, and the second is the secondary level.
2. Using a multimeter, the voltage level on the contact elements is checked. When performing a task, the ignition must be turned on. The testing procedure will reveal voltage in the range from 0 to 11 volts. In practice this is usually 4.5 V.
3. The ignition is turned off. The conductor with the negative contact is disconnected from the battery; first, use a wrench to loosen the clamp.
4. The resistance parameter between the conductors is measured. You can know that the contacts are closed if this value tends to zero. When the diagnostics showed that the resistance is infinite, then there is a break in the power line. The problem may lie directly in the contact. It is necessary to check the connector and all wires in more detail.
5. In practice, a short circuit usually occurs due to a breakdown of the control devices. To find a failed module, turn off each unit one by one and check the resistance value.

User Filat Ogorodnikov talked about diagnosing the CAN bus using an oscilloscope.

How to make an analyzer with your own hands?

Do the assembly yourself of this device Only a professional in the field of electronics and electrical engineering can do this.

Main nuances of the procedure:

1. In accordance with the diagram in the first photo in the gallery, you need to purchase all the elements to develop the analyzer. The components are labeled on it. You will need a board with an STM32F103С8Т6 controller. You will need an electrical circuit of a stabilized control device and a CAN transceiver MCP2551.
2. If necessary, a Bluetooth module is added to the analyzer. This will allow you to record basic information on your mobile device when using the device.
3. The programming procedure is performed using any utility. It is recommended to use KANHacker or Arduino programs. The first option is more functional and has the option of filtering packet data.
4. To carry out the firmware, you will need a USB-TTL conversion device; it will be needed for debugging. A simple option is to use ST-Link version 2.
5. After downloading the program to your computer, the main EXE file must be flashed into the controller using a programmer. After completing the task, the bootloader jumper is installed, and the manufactured device is connected to the PC via a USB output.
6. You can upload the firmware to the analyzer using software MPHIDFlash.
7. When the software update is completed, you need to disconnect the wire and remove the jumper. Drivers are being installed. If the device is assembled correctly, it will be detected on the computer as a COM port; this can be seen in the task manager.

Photo gallery

Scheme for developing a CAN analyzer Main board for device assembly

Pros and cons of CAN buses

Advantages of the digital interface:

1. Performance. The device can quickly exchange packet data between different systems.
2. High resistance to electromagnetic interference.
3. All digital interfaces have a multi-level control system. Thanks to this, you can prevent errors when transmitting and receiving information.
4. During operation, the tire itself spreads the speed across the channels in automatic mode. Thanks to this it is ensured effective work vehicle electronic systems.
5. The digital interface is secure. If someone tries to gain illegal access to the electronic components and systems of the car, the bus will automatically block this attempt.
6. The presence of a digital interface makes it possible to simplify the installation of a security system on a car with minimal interference in the standard on-board network.

Disadvantages of the CAN bus:

1. Some interfaces have restrictions on the amount of information that can be transferred. This drawback will be significant for a modern car, “stuffed” with electronics. When adding additional devices the tire bears more than high load. Because of this, response time is reduced.
2. All packet data that is transmitted over the bus has a specific purpose. A minimal portion of traffic is allocated for useful information.
3. If the protocol is used higher level, this will cause a lack of standardization.

Video “Do-it-yourself CAN interface repair”

User Roman Brock talked about the procedure for restoring the dashboard tire in a Ford Focus 2 restyling car.

In order to streamline the operation of all controllers, which facilitate control and increase control of driving a car, a CAN bus is used. You can connect such a device to your car alarm with your own hands.

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What is a CAN bus and how it works

CAN bus is a network of controllers. The device is used to combine all vehicle control modules into one working network with a common wire. This device consists of one pair of cables called CAN. Information transmitted through channels from one module to another is sent in encrypted form.

Scheme for connecting devices to the CAN bus in Mercedes

What functions can the CAN bus perform:

• connection to car on-board network any devices and devices;
• simplification of the connection and operation algorithm auxiliary systems cars;
• the unit can simultaneously receive and transmit digital data from different sources;
• the use of a bus reduces the impact of external electromagnetic fields on the functioning of the main and auxiliary systems of the machine;
• The CAN bus allows you to speed up the procedure for transmitting information to certain devices and components of the vehicle.

This system operates in several modes:

1. Background. All devices are disabled, but power is supplied to the bus. The voltage is too low, so the bus will not be able to discharge the battery.
2. Launch mode. When the car owner inserts the key into the lock and turns it or presses the Start button, the device is activated. The option to stabilize the power supplied to controllers and sensors is enabled.
3. Active mode. In this case, data is exchanged between all controllers and sensors. When operating in active mode, the energy consumption parameter can be increased to 85 mA.
4. Sleep or shutdown mode. When the power unit is turned off, the KAN controllers stop functioning. When the sleep mode is turned on, all components of the machine are disconnected from the on-board network.

The Vialon Sushka channel in its video talked about the CAN bus and what you need to know about its operation.

Advantages and disadvantages

What are the advantages of the CAN bus:

1. Easy to install the device in the car. The owner of the car will not have to spend money on installation, since this task can be completed independently.
2. Device performance. The device allows you to quickly exchange information between systems.
3. Resistance to interference.
4. All tires have a multi-level control system. Its use makes it possible to prevent errors during the transmission and reception of data.
5. During operation, the bus automatically distributes speed across different channels. This ensures optimal performance of all systems.
6. High security of the device; if necessary, the system blocks unauthorized access.
7. Large selection of devices various types from different manufacturers. You can choose an option designed for a specific car model.

What disadvantages are typical for the device:

1. Devices have limitations on the amount of data transferred. IN modern cars Many electronic devices are used. Their large number leads to high congestion of the information transmission channel. This causes an increase in response time.
2. Most of the data sent on the bus has a specific purpose. On useful information a small part of the traffic is allocated.
3. When using the protocol top level a car owner may face the problem of lack of standardization.

Types and markings

The most popular type of tires are devices developed by Robert Bosch. The device can operate sequentially, that is, signal is transmitted after signal. Such devices are called Serial BUS. You can also find Parallel BUS parallel buses on sale. In them, data transmission is carried out through several communication channels.

You can learn about the types, operating principle, and capabilities of the CAN bus from the video filmed by the DIYorDIE channel.

Taking into account different types There are several types of devices that can be identified:

1. CH2, 0A Active. This is how devices that support 11-bit data exchange format are marked. These nodes do not indicate errors on 29-bit node pulses.
2. CH2, 0V Active. This is how devices operating in 11-bit format are marked. The main difference is that when they detect a 29-bit ID in the system, they will report an error message to the control module.

It must be taken into account that in modern cars These types of devices are not used. This is due to the fact that the operation of the system must be consistent and logical. And in in this case it can operate at several pulse transmission rates - 125 or 250 kbit/s. More low speed used to control additional devices such as lighting in the cabin, electric windows, windshield wipers, etc. High speed needed to ensure the operating condition of the transmission, power unit, ABS systems etc.

Variety of bus functions

Let's look at what functions exist for various devices.

Device for car engine

When connecting the device, a fast data transmission channel is provided, through which information is distributed at a speed of 500 kbit/s. The main purpose of the bus is to synchronize the operation of the control module, for example, the gearbox and the motor.

Comfort type device

The data transfer rate over this channel is lower and is 100 kbit/s. The function of such a bus is to connect all devices belonging to this class.

Information and command device

The data transfer speed is the same as in the case of Comfort type devices. The main task of the bus is to ensure communication between serving nodes, for example, a mobile device and a navigation system.

Tires from different manufacturers are shown in the photo.

1. Device for automobile internal combustion engine 2. Interface analyzer

Could there be problems with the operation of CAN buses?

IN modern car digital bus is used constantly. It works simultaneously with several systems, and information is constantly transmitted through its communication channels. Over time, problems with the device may occur. As a result, the data analyzer will not function correctly. If problems are detected, the car owner must find the cause.

For what reasons do malfunctions occur:

• damage or breakage of the electrical circuits of the device;
• there is a short circuit in the system to the battery or ground;
• could close the KAN-Hai or KAN-Lo systems;
• damage to the rubberized jumpers occurred;
• battery discharge or voltage drop in the on-board network caused by incorrect operation of the generator device;
• The ignition coil has failed.

When searching for causes, keep in mind that the malfunction may be due to incorrect operation auxiliary devices, installed additionally. For example, the reason may be the incorrect functioning of the anti-theft system, controllers and devices.

You can learn about repairing the dashboard CAN bus in a Ford Focus 2 from a video made by user Brock - Video Corporation.

The troubleshooting process is carried out as follows:

1. First, the car owner diagnoses the state of the system. It is advisable to carry out a computer check to identify any problems.
2. At the next stage, the voltage level and resistance of electrical circuits are diagnosed.
3. If everything is in order, then the resistance parameter of the rubberized jumpers is checked.

Diagnosing the performance of the CAN bus requires certain skills and experience, so it is better to entrust the troubleshooting procedure to specialists.

How to connect an alarm via CAN bus

To connect the CAN bus with your own hands to the car alarm system of a car with or without auto start, you need to know where the anti-theft system control unit is located. If the alarm installation was carried out independently, then the search process will not cause difficulties for the car owner. The control module is usually located under the dashboard near the steering wheel or behind the control panel.

How to perform the connection procedure:

1. The anti-theft system must be installed and connected to all components and elements.
2. Find the thick orange cable; it connects to the digital bus.
3. The anti-theft system adapter is connected to the contact of the found bus.
4. The device is installed in a safe and convenient place, the device is fixed. Everything needs to be insulated electrical circuits to prevent chafing and current leakage. The correctness of the completed task is diagnosed.
5. At the final stage, all channels are configured to ensure the operating state of the system. You also need to set the functional range of the device.

Often the main cause of malfunction is electronic system management vehicle- are mechanical damage CAN bus or failure of control units hanging on the CAN bus.

Below in the article are ways to diagnose the CAN bus when various malfunctions. A typical example is shown CAN diagram tires on the Valtra T" series tractor.

Legend:

• ICL- Instrumental Cluster (Dashboard)
• TC1/TC2- Transmission controller (Transmission control unit 1/2)
• E.C.- Electronic controller (Engine control unit)
• PCU- Pump Control Unit

CAN BUS measurements

120 Ohm termination resistors (Sometimes these resistors are called terminators) inside the EC control unit and a resistor located next to the TC1 unit

If the display (on the side pillar) shows a fault code related to the CAN bus, this indicates a fault in the CAN bus wiring or the control unit.

The system can automatically report which control unit cannot receive information (control unit monitors transmit information to each other).

If the display flashes or the CAN bus message cannot be transmitted through the bus, a multimeter can be used to locate the faulty CAN bus wiring (or faulty control unit).

The CAN bus has no physical damage

If the resistance between the Hi and Lo wires of the CAN bus (at any point) is approximately 60 ohms, then the CAN bus is not physically damaged.

- The EC and TC1 control units are working properly, since the end-of-line resistors (120 Ohms) are located in the EC unit and next to the TC1 unit.

The TC2 control unit and the ICL instrument panel are also intact as the CAN bus passes through these units.

CAN bus is damaged

If the resistance between the Hi and Lo wires of the CAN bus (at any point) is approximately 120 ohms, then the CAN bus wiring is damaged (one or both wires).

The CAN bus is physically damaged

If the CAN bus is damaged, the location of the damage must be determined.

First, the resistance of the CAN-Lo line is measured, for example between the EC and TC2 control units.

Therefore, measurements must be made between Lo-Lo or Hi-Hi connectors. If the resistance is approximately 0 Ohm, then the wire between the measured points is not damaged.

If the resistance is approximately 240 ohms, then the bus is damaged between the measured points. The picture shows damage to the CAN-Lo wire between the TC1 control unit and the ICL instrument panel.

Short circuit in CAN bus

If the resistance between the CAN-Hi and CAN-Lo wires is approximately 0 ohms, then a short circuit has occurred in the CAN bus.

Disconnect one of the control units and measure the resistance between the contacts of the CAN-Hi and CAN-Lo connectors on the control unit. If the device is working properly, reinstall it.

Then disconnect the next device, take measurements. Continue in this manner until the faulty device is detected. The unit is faulty if the resistance is approximately 0 ohms.

If all blocks are checked and the measurements still indicate a short circuit, the CAN bus wiring is faulty. To find where the wires are damaged, they should be checked visually.

CAN bus voltage measurement

Turn on the power and measure the voltage between the CAN-Hi, CAN-Lo wires and the ground wire.

The voltage should be in the range of 2.4 - 2.7 V.