A modern car is a complex network of interconnected electronic components, and the search CAN buses often becomes a priority when installing an alarm system, diagnosing an engine, or connecting additional equipment. Unlike older models, where each sensor had its own separate wire, today all information is transmitted over two twisted wires, which requires a special approach to search and identification. Understanding exactly where this line lies in your vehicle is a key skill for any auto electrician or advanced enthusiast.
The main difficulty is that a car can have several such networks with different data transfer rates, and they can be located in completely different places in the body. CAN High and CAN Low often hidden in the depths of the dashboard, under the floor trim or even in the trunk, next to the comfort control unit. Incorrect wire identification can lead to short circuits or damage to expensive electronics, so the approach must be systematic and careful.
In this article we will analyze not only theoretical aspects, but also practical search methods using available tools. You'll learn how to distinguish twisted pair wires from regular power wires, what colors manufacturers most often use, and why a multimeter may not be enough for an accurate diagnosis. A deep dive into on-board network architecture will help you avoid common mistakes.
Theoretical foundations and types of CAN networks in cars
Before picking up a multimeter, you need to clearly understand that CAN bus (Controller Area Network) is not one wire, but a differential pair. The signal is transmitted simultaneously along two lines: CAN High (high level) and CAN Low (low level). This is done to increase noise immunity: any external noise affects both wires equally, and the controller simply subtracts one signal from the other, obtaining clean data.
Modern vehicles typically use high-speed and low-speed networks. High speed CAN bus (High-Speed CAN, up to 1 Mbit/s) integrates critical components: engine, gearbox, ABS and airbags. Low-speed (Low-Speed CAN, up to 125 kbit/s) serves comfort: power windows, climate control and central locking. Sometimes there is a separate bus for multimedia systems, known as MOST or Infotainment CAN.
⚠️ Attention: When locating wires, never rely solely on the color of the insulation. Although standards exist, manufacturers often change color coding depending on the year of manufacture and the assembly plant. An error in identification can lead to the supply of 12 Volts to the controller input, designed for 3.5 Volts.
It is also important to consider the network resistance. At the ends of each bus are installed termination resistors with a nominal value of 120 Ohms, which when connected in parallel give a total resistance of about 60 Ohms. This is a key parameter for the initial line integrity check without power being applied. If you measure the resistance between CAN High and CAN Low with the ignition off, this is the value you should see.
Search for CAN bus via OBD2 connector
The easiest and most accessible place to search for signals CAN buses is the OBD2 (On-Board Diagnostics) diagnostic connector, which is legally required to be located in the driver's access area, usually under the steering column. According to the international standard ISO 15031-3, data contacts are reserved and have fixed numbers, which greatly simplifies the search task.
The vast majority of modern cars built after 2004 use pins 6 and 14. Pin 6 is CAN High, and pin 14 - CAN Low. However, in some older models or specific brands there may be other pinouts, for example, the use of pins 1 and 9 or even 3 and 11, although this is rather an exception to the rule for passenger cars.
To accurately determine the contacts, it is best to use technical documentation or a universal pinout table. Below is the standard pin distribution diagram in the OBD2 connector, which is relevant for most cars:
| Contact number | Signal purpose | Function description | Typical wire color |
|---|---|---|---|
| 1 | Reserved / K-Line | Additional diagnostics | Various |
| 2 | J1850 Bus+ | American Standard (Ford/GM) | Various |
| 6 | CAN High | High level of data | Green/Yellow |
| 14 | CAN Low | Low data level | Green/Yellow |
| 16 | Power (+12V) | Constant plus from the battery | Red/Orange |
Using this table, you can quickly find the desired pins in the connector. Please note that the CAN wires in the OBD2 harness are often twisted pairs, twisted together, which is another visual marker. If you plan to cut into the tire through this connector, use special adapters to avoid damaging the standard contacts.
Methods for finding wires in electrical harnesses
If OBD2 is difficult to access or you need to find CAN bus directly in the wiring harness (for example, behind the fuse box or in the door), a more thorough visual and instrumental search will be required. First, look for twisted pair: two wires that are tightly twisted together along the entire length of the bundle. This is the most reliable visual indication of the CAN protocol.
Often these wires have the same base color with different colored stripes. For example, one wire might be green with a black stripe, and the other wire might be green with a white stripe. Or they can be yellow-black and yellow-white. In VAG cars (Volkswagen, Audi), orange or green twisted pair cables are often found. In BMW cars, the wires are often yellow and green.
To search, you can use an oscilloscope or a high-quality multimeter with a frequency measurement mode. With the ignition on on the wires CAN buses there should be floating voltage. On CAN High it usually varies from 2.5 to 3.5 Volts, and at CAN Low - from 1.5 to 2.5 Volts. If you see stable 0 or 12 Volts, this is not CAN.
☑️ Checking the found twisted pair
Another method is to trace a harness from a known control unit. If you find a connector going to the ABS unit or engine (ECU), there is a 99% chance that there is a CAN bus in this harness. By carefully disassembling the insulation of the harness in an accessible place, you will find the desired pair.
Diagnostics and line integrity testing
Once you have allegedly found the wires, you need to carry out diagnostics to make sure they are working and belong to the desired network. The first step should always be to measure the resistance between the found wires with the system completely de-energized (the terminal has been removed from the battery). As mentioned earlier, a working bus with two terminators will show a resistance of about 60 ohms.
If the multimeter shows 120 ohms, this means that one of the termination resistors is missing or burned out, or one of the control units is disconnected from the network. A reading at infinity (break) indicates a line break or the absence of terminators. A value close to zero indicates a short circuit between the CAN High and CAN Low wires.
⚠️ Attention: Never carry out resistance (“continuity”) measurements on a switched-on circuit. This can lead to failure of the multimeter and damage to the vehicle's electronic control units. Always remove the negative battery terminal before testing with an ohmmeter.
Next comes a voltage check. With the battery connected and the ignition on (you don’t have to start the engine), measure the voltage relative to the body ground. On the wire CAN High you should see a voltage in the range of 2.5–3.5 V, and CAN Low - 1.5–2.5 V. The sum of the voltages on both wires in a quiet state is often about 5 Volts.
Specifics of search in different car brands
Search CAN buses may vary significantly depending on the vehicle manufacturer. European, American and Asian concerns use different standards for color marking and block arrangement. For example, in cars Toyota and Lexus Often a white twisted pair cable with blue or green stripes is used, located in a bundle under the instrument panel to the left of the steering wheel.
In group cars VAG (Volkswagen, Skoda, Audi) The Comfort CAN bus is often green, and the Drive CAN is orange. However, in new models the color scheme may change. B BMW the wires are often yellow and green, and they may not be twisted in a bundle, but run parallel, which confuses beginners.
Secrets of searching in difficult cases
In some vehicles (for example, certain Ford or Renault models), the CAN bus may be implemented via a single wire (Single Wire CAN) or use non-standard colors such as purple or brown. In such cases, without a wiring diagram, searching for wires at random is extremely risky. Use your vehicle's VIN to find the exact wiring diagram.
It is also worth considering the location of the diagnostic connectors. While in passenger cars OBD2 is usually under the steering wheel, in some trucks or older models it may be in the engine compartment or even in the trunk. Knowing the specifics of the brand helps you narrow your search and avoid having to disassemble unnecessary panels.
Typical errors when connecting equipment
One of the most common mistakes is connecting additional equipment (alarms, telematics) directly to the CAN bus wires without using special interfaces or resistive dividers. CAN bus sensitive to changes in capacitance and line resistance. Insertion by twisting or poor-quality soldering can disrupt the integrity of the signal, which will lead to errors appearing on the dashboard (“Check Engine”, “ABS”, etc.).
The second mistake is using wires that are too large for connection. The current in the CAN bus is minimal, and thick wires create unnecessary rigidity of the harness, which can lead to mechanical damage to the contacts due to vibration. They also often forget about high-quality insulation: moisture getting into the twisted CAN wires causes corrosion and loss of communication between the blocks.
Use special T-splitters or soldering followed by heat shrink to connect to the CAN bus. Avoid using vampire clamps (piercing clamps) on thin twisted pair wires as they can damage the center core.
The third mistake is ignoring shielding. If you are laying new wires to connect the external unit to the CAN bus, use a shielded cable. Long unprotected wires act as an antenna and catch interference from the generator or ignition system, which “clogs” the useful signal and causes electronic malfunctions.
Tools for working with the CAN protocol
For professional work with CAN bus A regular multimeter is not enough. The minimum kit for a specialist should include an oscilloscope that allows you to see the signal shape. On the oscillogram you will see rectangular pulses “floating” relative to the baseline. The absence of a clear rectangular shape or severe distortion indicates problems in the line.
Also an indispensable tool is a CAN analyzer (for example, based on a ELM327 or more professional devices like PCAN). By connecting such an analyzer to a laptop or smartphone, you can see messages passing in real time, determine the bus speed (250 kbit/s or 500 kbit/s) and identify active blocks.
The presence of an oscilloscope and a CAN analyzer allows you not to guess, but to accurately diagnose the state of the network, see breaks, short circuits and interference that are not visible when measured with a conventional voltmeter.
Don't forget about quality stripping and soldering tools. Thin twisted pair wires require care. Use aluminum flux or neutral flux if working with stranded copper wires to ensure reliable contact without future oxidation.
FAQ: Frequently asked questions
Is it possible to connect to the CAN bus anywhere in the car?
Theoretically, the network is distributed throughout the car, but it is better to physically connect at access points, such as the OBD2 connector, behind the fuse box, or directly at the control unit where you mount the equipment. Connecting in the middle of a long harness under the dash can be technically difficult and risky.
What happens if CAN High and CAN Low are swapped?
In most modern systems, data exchange simply will not work, and devices will not see each other. However, in some older or specific implementations, this can lead to incorrect operation or even damage to the controller inputs, since the voltage levels on these lines are different.
How to find a CAN bus if the car has two diagnostic connectors?
The presence of two connectors often indicates the presence of a Gateway between different networks (for example, High-Speed and Low-Speed). One connector can be connected to one bus, the second to another. It is necessary to connect the continuity from the connectors to the control units or use a CAN analyzer on each of them.
Why is the CAN bus resistance not 60 Ohm?
Deviation from 60 Ohms (for example, 58-62 Ohms) is acceptable and is associated with the error of the resistors. If the resistance is significantly different (for example, 40 Ohms or 100 Ohms), this indicates a malfunction: a short circuit, a break in one of the terminators, or the connection of a third, unauthorized device to the network.
Do I need a special adapter to connect the auto scanner?
Yes, to connect to OBD2 you need a corresponding connector. For insertion into harnesses, there are special adapters that allow you to connect to twisted pair without damaging the insulation of the main wires, which is especially important for maintaining the warranty on the car.