Controller Area Network (CAN)
Controller Area Network (CAN)
Control Module Connections to the CAN Harness
Control modules are connected to the CAN harness either in a 'loop' or 'spur' configuration. In the 'loop' type configuration
the CAN harness loops into the module (via two connector pins) and then loops out of the module (via another two connector
pins). In the 'spur' type configuration, a harness spur is spliced into the main 'backbone' of the CAN harness and the module
is connected to the harness spur via two connector pins.
CAN Harness Architecture
For a detailed description of the CAN Networks and architecture, refer to the relevant Description and Operation section in
the Workshop Manual.
CAN Network Integrity Tests
If a control module is suspected of non-communication, the Network Integrity test application available on the manufacturer
approved diagnostic system can be used to confirm if communication is possible between the control modules on the vehicle
and the manufacturer approved diagnostic system (via the J1962 diagnostic connector ). The results from the test can be
used to determine if either a single module or multiple modules are failing to communicate.
CAN Terminating Modules
If the Network Integrity test indicates that one or more module on one of the CAN networks (HS or MS) are failing to
communicate, there are several checks that can be made. The first step is to identify if both of the CAN terminating modules
on each individual CAN Bus are communicating. If both CAN terminating modules for each individual CAN Bus are
communicating (identified via the Network Integrity test), then it can be confirmed that the main 'backbone' of the CAN
harness is complete. The main 'backbone' of the CAN harness consists of all the modules connected to the CAN harness via a
'loop' configuration and also includes the two terminating modules.
Communication with both CAN terminating modules via the Network Integrity test confirms the physical integrity of the main
'backbone' of the CAN harness (and the harness spur to the J1962 diagnostic connector). This means that there is no
requirement to check the resistance of the CAN Network. This is because the standard check for 60 ohms across the CAN
High and CAN Low lines will not provide any additional information regarding the physical condition of the CAN harness,
beyond what has already been determined from the Network Integrity test.
Non-Communication of a Terminating Module
If a Network Integrity test reveals a terminating module is failing to communicate it can indicate a break in the main
'backbone' of the CAN harness. The first checks should always be to confirm the power and ground supplies to the noncommunicating
module are correct. Providing these are correct, the resistance between the CAN High and CAN Low lines at
the J1962 connector can be checked to determine the integrity of the main 'backbone' of the CAN harness. After
disconnecting the battery a reading of 120 ohms would indicate an open circuit in the main 'backbone' of the CAN harness.
Alternatively, a reading of 60 ohms would indicate that there is no open circuit fault with the main 'backbone' of the CAN
harness.
It is worth noting that even if one of the terminating modules is disconnected from the CAN harness, communications
between the modules still connected may still be possible. Therefore communication between the manufacturer approved
diagnostic system and the connected modules may also be possible.
Locating CAN Harness Open Circuits
In the case where multiple modules, including a terminating module, are failing to communicate, having first confirmed the
power and ground supplies are correct, the approximate location of the open circuit can be identified from analysis of the
Network Integrity test results and reference to the relevant CAN network circuit diagrams. For example, if an open circuit
existed in a certain position on the CAN harness, any module positioned on the Network between the J1962 connector and
the open circuit should return a response during the Network Integrity test. No responses would be returned from any
modules past the open circuit fault in the Network.
CAN Harness 'Spur' Type Configuration Circuits
If, after the initial checks (Network Integrity test using the manufacturer approved diagnostic system, and power and ground
supplies to the module have been checked and confirmed as correct), a module that is connected to the CAN harness via a
'spur' type configuration is suspected of not communicating, then the physical integrity of the CAN harness 'spur' can be
checked.
This is most easily undertaken by individually checking the continuity of the CAN High and CAN Low lines between the noncommunicating
module connector (with the module disconnected) and the J1962 diagnostic connector.
'Lost Communications' DTCs
As well as the methods described so far in this document, which can be used to determine the location of an open circuit in
the CAN harness, 'Lost Communications' DTCs can also be used for this purpose. Lost communication DTCs mean that a module is not receiving CAN information from another module.
For example, if a global DTC read were to be carried out, only DTCs stored in the modules that the manufacturer approved
diagnostic system could communicate with would be displayed. If there was an open circuit fault in a certain position on the
CAN harness, the modules that could display DTCs would all be prior to the open circuit on the Network, and these modules
should display 'Lost Communications' DTCs with all the modules located on the Network past the open circuit fault.
'Bus off' DTCs
The references to bus and its condition refer to the network concerned and the modules on that network.
If a module logs a 'Bus Off' DTC, it means that the module has detected CAN transmission errors and has disabled it's own
CAN transmissions and disconnected itself from the network in an attempt to allow the rest of the network to function. At this
point the 'Bus Off' DTC is set. A common cause of 'Bus Off' DTCs can be a short circuit in the CAN network.