Checking the CAN Bus Signal
Checking The CAN Bus Signal
Checking the CAN bus signal
These instructions are used for making sure that the signal levels for CAN High and CAN Low are present and correct on the bus connection.
Cables used
Multifunction cable
Testing instructions
- Voltage test (oscilloscope): The prerequisites for the voltage test are that the battery is connected and the ignition is switched on.
- Resistance measurement: Prior to the resistance measurement, the test specimen must be deenergized. The battery must be disconnected to ensure this. Wait around 3 minutes until all system condensers have discharged.
Information on the CAN bus
The CAN (Controller Area Network) bus system is a linear bus system that is characterized by the following features:
- Signals are broadcast in both directions.
- All bus users receive a message. Each bus user decides whether to process the message or not.
- Additional bus users can be added by connecting them in parallel.
- The bus system constitutes a multimaster system. Each bus user can be a master or slave depending on whether it is connected as a transmitter or receiver.
- The transmission medium is a two-wire connection. the cores are designated CAN Low and CAN High
- In principle, each bus user can use the bus to communicate with all other bus users. An access mechanism controls data exchange on the bus. The main differences between the K-CAN (body CAN) bus, the PT-CAN (powertrain CAN) bus and the F-CAN (chassis CAN) bus are detailed below:
- Body CAN: Data transfer rate of around 100 kBits/s. Single-wire operation possible.
- Powertrain CAN: Data transfer rate of around 500 kBits/s. Single-wire operation not possible.
- F-CAN: Data transfer rate of around 500 kBits/s. Single-wire operation not possible.
Master: the master is the active communicating node, i.e. the one that initiates communication. The master is in control of the bus and controls communication. The master can send messages to the passive bus users (slaves) in the bus system and can receive messages from them on request.
Slave: the slave is the passive communicating node. The slave is instructed to receive and send data.
Multimaster system: a multimaster system is one in which all communication nodes can take on the role of master or slave at a particular time.
Body CAN, powertrain CAN, and suspension CAN oscilloscope measurement
In order to obtain a clear idea of whether the CAN bus is functioning properly, you must be able to observe activity on the bus. You do not need to analyze the individual bits; you simply need to observe whether or not the CAN bus is working. The oscilloscope test can state that, "the CAN bus is probably operating without faults".
When you measure the voltage between the CAN Low line (or CAN High line) and the ground, you receive a rectangle-like signal in the following voltage ranges:
- Body CAN:
CAN Low to ground: U(min) = 1 volts and U(max) = 5 volts
CAN High to ground: U(min) = 0 volts and U(max) = 4 volts
These values are approximate values and can vary by a few hundred mV depending on the bus load.
Oscilloscope settings for the measurement at the K-CAN:
CH1: Probe tip 1, range 2 V/div; DC coupling
CH2: Probe tip 2, Range 2 V/div; DC coupling
Time: 50 micros/div
When you measure the voltage between the CAN Low line (or CAN High line) and the ground, you receive a rectangle-like signal in the following voltage ranges:
- PT-CAN and F-CAN
CAN Low to ground: U(min) = 1.5 volts and U(max) = 2.5 volts
CAN High to ground: U(min) = 2.5 volts and U(max) = 3.5 volts
These values are approximate values and can vary by a few hundred mV depending on the bus load.
Oscilloscope settings for the measurement at the PT-CAN (or F-CAN):
CH1: Test probe 1, range 1 V/div; DC coupling
CH2: Test probe 2, range 1 V/div; DC coupling
Time: 10 micros/div
Procedure for measuring resistance with body CAN, powertrain CAN, and suspension CAN terminating resistor
Inspection procedure for resistance test:
- The CAN bus must be de-energized.
- No other testing equipment must be in use (connected in parallel).
- The measurement is taken between the CAN Low and CAN High lines.
- The actual values may differ from the setpoint values by a few ohms.
Body CAN
No defined resistance test can be carried out on the body CAN data bus as the resistance varies depending on the internal switching logic of the control units.
PT-CAN, F-CAN
In order to prevent signal reflection, 2 CAN bus users (at the extremities of the powertrain CAN network) with 120 ohm each are terminated. The two terminal resistors are connected in parallel and form an equivalent resistance of 60 ohm. When the supply voltage is switched off, this equivalent resistance can be measured between the data lines. In addition, the individual resistors can be tested independently of one another.
Information on the measurement with 60 ohm: Disconnect an easily accessible control unit from the bus. Then measure the resistance on the connector between the CAN Low and CAN High lines.
NOTICE: Not all vehicles have a terminating resistor on the CAN bus. Use the wiring diagram to check whether the connected vehicle has a terminating resistor.
CAN-bus not operative
If the body CAN or powertrain CAN data bus is not working, there may be a short circuit or open circuit on the CAN Low/CAN High line. Alternatively, a control unit might be faulty.
The following procedure is recommended to localize the cause of the fault:
- Disconnect the bus users from the CAN bus one after the other until the cause of the fault (control unit X) is found.
- Check the lines of control unit X for a short/open circuit.
- If possible, check control unit X itself.
- However, this procedure only leads to success if a tap line from a control unit to the CAN bus has a short circuit. If a line in the CAN bus itself has a short circuit, the wiring harness must be checked.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.