About this guide
We created this tutorial to help our customers solve the most common problems they may encounter when dealing with a communication failure on a CAN bus. Our products are designed to communicate with engine ECUs and other devices over a CAN bus, but this guide is not limited to using the CAN Bus with our products. Having a good understanding of how a CAN bus network is supposed to be configured and how to troubleshoot any problems is vital to our customers continued success. The below video and other guides will equip you with the knowledge and confidence you need to troubleshoot CAN bus problems quickly and easily.
Watch this Video First
Introduction to the CAN Bus
CAN bus nodes are connected over a two-wire bus with 120-ohm nominal twisted-pair cable.
Note: For in-depth detail about the physical layer, we recommend you read Application Report (SLLA270) from Texas Instruments.
CAN Bus Termination
There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.
In some instances, a termination resistor may be located inside a device and switched electronically. In these instances, it cannot be detected by measuring resistance. Instead, follow the manufacturer's instructions to verify an electronically switched termination resistor is enabled (ex. for a display, verify the option is enabled in a settings menu).
If one or more termination resistors are missing, communications may work temporarily but will be unreliable and eventually fail.
CAN Bus Voltage Levels
When CAN voltages are measured with a multimeter, only the average voltage is displayed. Refer to the table and drawings below for common signal measurements.
|CAN Bus Signal Measurements|
|CAN HI (≥ 2.5 VDC)||2.5 VDC||2.5 to 3.5 VDC||2.6 to 3.0 VDC|
|CAN LOW (≤ 2.5 VDC)||2.5 VDC||2.5 to 1.5 VDC||2.4 to 2.0 VDC|
* With respect to GROUND when there is no active data
IMPORTANT: These measurements should only be taken with one device powered on the network. If multiple devices are connected on the network, the CAN voltage measured will be the average of all devices, and you will not be able to determine if one device has failed.
General Troubleshooting Procedure
This procedure is a general guideline. Consult the manufacturer's wiring information to determine CAN connections and pinouts for your specific equipment.
Common CAN Failures
- Device configuration settings
- Missing termination resistors
- CAN Hi and CAN Low wired backwards
- Damaged CAN port due to lightning or welding
Check Device Configuration Settings
If the device has configuration options via display menu, dip switch settings, jumper settings, or software download verify the following are correct.
- Baud Rate – J1939 use 250Kbps but some other networks use 500Kbps
- Device Source Address – verify each device on the network has a unique source address
- Verify your device is configured to receive data from the source address of the desired data source.
Verifying Network Termination Resistance
- With power off unplug connector from any device on the network and measure resistance between CAN Hi and CAN Low.
- Resistance should be 60 ohms if both termination resistors are present.
- If measurement is 120 ohms only one terminating resistor is present, two resistors are required (see note below).
- If 40 ohms is measured a third terminating resistor is installed and should be removed.
- If less than 40 ohms is measured there could be a short in the harness or a damaged CAN Port of one of the devices connected to the network.
NOTE: Some CAN devices may have an internal termination resistor that is switched by software once the unit is powered up, consult manufactures literature to determine if this is the case.
Checking CAN Voltage
- Disconnect all devices except for the device being tested, then power the device on.
- Measure voltage on any of disconnected plugs between CAN HI and GROUND. The resulting voltage should be between 2.5 and 3.0VDC.
- At the same location, measure voltage between CAN LOW and GROUND. The resulting voltage should be between 2.5 and 2.0 VDC.
A low voltage of 1.4VDC or less on either of these indicates a potential failure on the CAN port of the device.
If voltages are exactly 2.50 VDC, and do not change after several seconds, this indicates the device connected is powered but not broadcasting data.
Checking for Reversed Wiring
Perform the CAN voltage test detailed above and verify CAN HI voltage is greater than CAN LOW voltage. If not, the wires are reversed.
Checking Device CAN Port
If the CAN Voltage Test shows a low voltage coming from a device, you can verify the CAN port is damaged by measuring resistance to ground. Damage from lightning or welding typically causes a short to ground on one or both CAN lines.
- Unplug the connector from the device.
- Measure resistance on the connector pins of the device between CAN HI and CAN LOW. The resulting resistance should be between 28k – 50k ohms.
- Measure resistance between CAN HI and GROUND. The result should be Mega ohms or open.
- Measure resistance between CAN LOW and GROUND. The result should be Mega ohms or open.
- If damage to an input has occurred measurement will typically be 10K ohm or less between CAN HI/LOW and GROUND