Subjects:
- Preface
- Oscilloscope measuring
- Multimeter measuring
Preface:
If there is a suspicion that there is a malfunction in the CAN bus, a diagnosis can be made by, among other things, measuring the voltage levels on the wires. The content of the CAN bus message is not yet important in the first instance. We can take measurements on the CAN bus wires with both the multimeter and the oscilloscope. The measurements with the multimeter do have a limitation; only an average value is indicated when measuring the voltages. The multimeter is sufficient when measuring an interruption or short circuit. The oscilloscope is required to measure the voltage levels.
How a CAN bus system works and how the structure of the messages is established is explained on the page CAN bus. This page focuses on measuring the CAN bus with the oscilloscope and the multimeter and possible faults with causes are described.
Measuring with the oscilloscope:
With a two-channel oscilloscope, CAN high and CAN low can be measured simultaneously relative to ground. The two scope images below show the CAN bus signal of the comfort bus. The voltages are as follows:
- CAN-low: idle 0 volts, active 4 volts;
- CAN-high: idle 5 volts, active 1 volt.
When we set the zero lines of both measurement channels to the same height of the Y-axes, the signals overlap. It is therefore advisable to move the Y-axis of the CAN-low upwards for reading. In the second image below, the zero lines have been changed in height, so that the voltage curve of CAN high and low can be compared with each other.
CAN high shorted to ground:
There is a ground short in the CAN high. If the insulation is damaged, the wiring can make contact with the body, or in an ECU a short circuit is made to ground.
In the measurement below, we see a constant voltage line on channel B that is 0 volts.
CAN-low shorted to ground:
There is a ground closure in the CAN-low. If the insulation is damaged, the wiring can make contact with the body, or in an ECU a short circuit is made to ground.
In the measurement below, we see a constant voltage line on channel A that is 0 volts.
CAN high shorted to plus:
In the CAN-high there is a positive circuit. If the insulation of several wires in a wiring harness is damaged, the wiring can make contact with each other, or in an ECU a short circuit is made with positive.
In the two measurements below we see:
- Channel overrange: the voltage range of channel B (red) must be increased;
- On channel B we see (in the 20 V range) a constant voltage line that is equal to the battery voltage.
CAN-low shorted to plus:
There is a positive circuit in the CAN-low. If the insulation of several wires in a wiring harness is damaged, the wiring can make contact with each other, or in an ECU a short circuit is made with positive.
In the two measurements below we see:
- Channel overrange: the voltage range of channel A (blue) must be increased;
- On channel A we see (in the range 20 V) a constant voltage line that is equal to the battery voltage.
CAN-high shorted with CAN-low:
The CAN low changes to the CAN high voltage ramp when they connect to each other. A short circuit between CAN high and CAN low can occur in the wiring, where the insulation of both CAN bus wires is worn through, or due to a defect in the printed circuit board of an ECU.
In the picture below we see the two channel measurement where CAN high and low are shorted together.
On CAN-high, communication is occasionally lost:
Communication with one control unit in CAN-high is interrupted. This control unit no longer sends and receives data via the CAN-high, but the CAN-low still functions. As a result, communication and reading are still possible.
When the plug of the relevant control unit is disconnected, the CAN-low data is also lost and there is no longer a visible difference between CAN-high and CAN-low.
In the image below we see that the CAN-high remains recessive at one point, while data is sent on the CAN-low.
On CAN-low the communication is occasionally lost:
Communication with one control unit in the CAN-low is interrupted. This control unit no longer sends and receives data via the CAN-low, but the CAN-high still functions. As a result, communication and reading are still possible.
When the plug of the relevant control unit is disconnected, the CAN-high data is also lost and there is no longer a visible difference between CAN-high and CAN-low.
In the image below we see that the CAN-low remains recessive at one point, while data is sent at the CAN-high.
Measuring the CAN bus with the multimeter:
Measuring CAN bus voltage levels with the multimeter is unwise. The multimeter displays average values when voltages fluctuate frequently, so that it is not possible to make a good diagnosis. The oscilloscope must be used to measure the voltages.
We can use the multimeter to measure the resistances of a high-speed CAN network with terminating resistors. The measurements below show the ohmic resistance in three different situations: with a correctly functioning system, an open wire and a short circuit between CAN-high and CAN-low.
Interference-free:
On the page CAN-bus it is described that there are two terminating resistors in the network. The terminating resistors both have a resistance of 120 ohms. In a fail-safe system, we will measure a 60 ohm replacement resistance between CAN-high and CAN-low.
Note: we can only measure this if the power supply of all control units is switched off!
Interruption:
In the event of an interruption in a CAN-high or CAN-low wire, we no longer measure the replacement resistance of 60 ohms. In the picture we are only measuring the value of resistor R2 (120 ohms).
Short circuit:
In the situation where the CAN bus wires connect to each other (ie are shorted to each other), we measure a resistance value of approximately 0 ohms.
At the next fault, both CAN wires are interrupted. There will now be a lot of interference (noise) on the bus. Nodes 1, 3 and 4 can communicate with each other provided the interference and reflection is too great, causing the messages to distort. Similarly, node 2 and 5 can communicate with each other subject to the same problem.
Some CAN networks also function when one wire is interrupted. Error codes will be stored and the driver will be informed with warning lamps by messages from various systems. These are the networks equipped with a Fault Tolerant CAN transceiver. Depending on the transceiver used, different types of errors can occur without loss of communication between the nodes. These CAN transceivers can also function normally with the aforementioned faults with the short circuits to plus and ground (of course with various error messages).
Related page:
