General Information About Fault-Tracing In The CAN Network

General information about fault-tracing in the CAN network

General
The CAN network is a pure communications network and contains no fuses or sensors. The network is divided into two parts: the low speed network (LS-CAN, Low Speed CAN) and the high speed network (HS-CAN, High Speed CAN). The differences between the networks are their physical position in the car and the transfer rates. In general terms, HS-CAN is restricted to the engine compartment and LS-CAN to the passenger compartment and the cargo compartment.
Both the low speed and the high speed networks have two network resistors installed in two of the control modules. The resistors each have a resistance of 120 ohms. When taking resistance readings between the two CAN cables, a normal value is approximately 60 ohms. This value can vary between different vehicles. Permitted values are approximately 55-65 ohms.

Note! Resistance 60 ohms can only be measured if the main artery is intact and connected in the part of the CAN network in which the reading is taken. If there is an open circuit anywhere along the main artery or if any control module through which the main artery passes (a control module with four CAN connections) is not connected, resistance will be 120 ohms when the reading is taken. For more information, see the wiring diagram.

Diagnostic trouble code (DTC) types
Diagnostic trouble codes (DTCs) which can be stored by control modules connected to the controller area network (CAN) can be split into two categories:

Diagnostic trouble codes (DTCs) DFXX
These diagnostic trouble codes (DTCs) can only be stored in the central electronic module (CEM). This diagnostic trouble code (DTC) is stored when the central electronic module (CEM) has detected an electrical fault in the controller area network (CAN) wiring, a short-circuit between the controller area network (CAN) wiring or short-circuit between the controller area network (CAN) wiring and voltage or ground. These diagnostic trouble codes (DTCs) are not stored if there is an open-circuit in the CAN cable. These diagnostic trouble code do not, in any way, indicate that a fault has occurred in a control module.

Diagnostic trouble codes (DTCs) 1AXX (Node not alive)
These diagnostic trouble codes can only be stored in the central electronic module (CEM). A trouble code of this type indicates that the central electronic module (CEM) cannot "hear" a control module on the CAN network from which it anticipates communication. This could be due to an open circuit on one or both of the CAN cables to a control module or the control module that the central electronic module (CEM) is trying to hear not having power.

Diagnostic trouble codes (DTCs) E000 (HS-CAN)/E001 (LS-CAN)
These diagnostic trouble codes (DTCs) can be stored by all control modules. A control module that has stored this diagnostic trouble code (DTC) has discovered that some sort of interference has occurred with communication. The diagnostic trouble code (DTC) does not point out a specific fault source. Examples of fault causes can be intermittent short-circuits between the controller area network (CAN) wiring, intermittent short-circuits in the controller area network (CAN) wiring to supply/ground or a control module which transmits incorrect messages via the controller area network (CAN).

Note! Diagnostic trouble codes (DTCs) E000 and E001 may also be stored in one or more control modules for the same reasons as diagnostic trouble codes (DTCs) DFXX in the central electronic module (CEM) (i.e. a " short circuit" to supply voltage or ground in one or both of the CAN cables). However, no control modules other than the central electronic module (CEM) can store diagnostic trouble codes (DTCs) caused specifically by a short circuit.

Diagnostic trouble codes (DTCs) E003
The diagnostic trouble codes (DTCs) can be stored by all control modules in both the HS-CAN and LS-CAN except for the central electronic module (CEM). The control modules on the controller area network (CAN) wait for messages from the central electronic module (CEM) which are sent at regular intervals. This message contains the identity number which must be the same for all control modules in the controller area network (CAN). This is so that any faulty software that may have been downloaded to a control module can be detected. The diagnostic trouble code (DTC) indicates that a control module has been unable to receive the expected communication from the central electronic module (CEM) or that the identity number in the control module is not the same as the identity number transmitted by the central electronic module (CEM). Examples of fault causes could be incorrect software configuration in the control module which stored the diagnostic trouble code (DTC) or an open-circuit in the controller area network (CAN) wiring to the control module which stored the diagnostic trouble code (DTC). This diagnostic trouble code (DTC) can also be stored if there is play in the connectors or bent terminal pins in a connector on one of the CAN cables.

Note! Software in one control module cannot "malfunction". If the control module that stored the diagnostic trouble code (DTC) functioned in the vehicle at one point, the fault cannot be remedied by downloading software. The only instance when downloading software to a control module is of use is if the customer or workshop replaced a control module with a control module from another vehicle without downloading software after replacement.

For more information about the function and structure of the CAN network, further documents are available in addition to Service Manual Wiring diagram.

Information about the controller area network (CAN), construction and function (TNN 37-22)
- Information about the CAN network, structure and function Information About The CAN Network, Structure And Function

Supplementary information about faults in the CAN network (TNN 37-25)
- Supplementary information about faults in the CAN network Supplementary Information About Faults In The CAN Network

Information about incorrect replacement (TNN 37-24)
- Information about incorrect replacement Information About Incorrect Replacement

Diagnostic functions in the central electronic module (CEM)
The central electronic module (CEM) acts as a hub between the two networks. This operates as an exchange and can distribute traffic between the high speed network (HS-CAN) and the low speed network (LS-CAN) and vice versa. The central electronic module (CEM) also contains diagnostic functions for monitoring the traffic on the CAN network and the voltage levels in the CAN cables.

Note! The central electronic module (CEM) contains more functions for monitoring the CAN network than other control modules. For example, the central electronic module (CEM) is the only control module that can store diagnostic trouble codes (DTCs) for short-circuits between the CAN cables, between a CAN cable and ground and between a CAN cable and supply voltage. The fact that the central electronic module (CEM) can do this does not mean that the central electronic module (CEM) is the cause of all faults in the CAN network. On the other hand, other control modules can store diagnostic trouble codes (DTCs) as a result of the short-circuit. This is a small, but very important difference.

Diagnostic trouble codes (DTCs) in pairs between the central electronic module (CEM) and a particular control module (CEM-1AXX and XXX-E003)
Fault-tracing of the relevant control module begins if the central electronic module (CEM) has stored diagnostic trouble code (DTC) CEM-1A5X or CEM-1A6X for an open-circuit in communication with a control module and communication cannot be established with the corresponding control module or if the control module stored XXX-E003.

Note! If diagnostic trouble codes (DTC) can be read off from the relevant control module, the fault is probably intermittent. In the event of a fault in the low speed network, it is particular important to investigate the junctions behind the central electronic module (CEM) and the audio module (AUM) as well as the cable harness behind the audio module (AUM).Intermittent faults in the power supply to a control module are indicated by a diagnostic trouble code (DTC) for a break in communication being stored in the central electronic module (CEM) (CEM-1A5X or CEM-1A6X). There is probably no diagnostic trouble code (DTC) in the relevant control module as the control module cannot store a diagnostic trouble code (DTC) when it is not supplied with power.

General procedure
- Check the supply voltage to the relevant control module
- Check the ground terminal for the control module
- Check the CAN cables for the control module.

Checking the power supply
A simple way to check the supply voltage and ground is to check if there is any functionality remaining in the control module. Examples of corrective action:
- Audio module (AUM): Check whether the radio plays
- Upper electronic module (UEM): Check if the seat belt reminder lights or if the sun roof works when the ignition is on
- Power seat module (PSM): Check whether the seat can be moved manually
- Road traffic information module (RTI): Check whether the display for the road traffic information module (RTI) can be folded up or whether a disc can be inserted and fed out
- Climate control module (CCM): Check the function. The blower fan must blow. However, the air conditioning (A/C) will not work in the event of interrupted CAN communication.
In the event of difficulty, check as follows: Measure the supply voltage and ground terminal in the connector directly adjacent to the control module that has stored the diagnostic trouble code (DTC).

Note! The terminals could be damaged when if readings are taken using measurement cables directly in the connector. If possible, take readings on the rear of the connector. If this is not possible, use a loose terminal strip to insert in the connector.

Diagnostic testing of main arteries in the low speed network (LS-CAN)
Measure the resistance between the controller area network (CAN) cables in the connector for the power seat module (PSM) if applicable (these are easy to access). Otherwise connect the breakout box at the central electronic module (CEM) or at the driver information module (DIM). The advantage of using the breakout box is that it is possible to check the functionality of the entire network with all control modules connected.
The resistance reading only checks the main path in the LS-CAN. The resistance reading does not check the divergence paths such as the road traffic information module (RTI), accessory electronic module (AEM), power seat module (PSM), phone module (PHM), audio module (AUM), driver door module (DDM) and passenger door module (PDM). See the Service Manual, Wiring diagram, Data communication for clarification. However, resistance readings between the two CAN cables can show if there is a short-circuit between the two cables (or their junctions). A normal value, measured somewhere in the middle of the network, is between 55 and 65 ohms when all control modules are connected (120 ohms parallel connected). Twist and bend the cable harness at suitable points to detect any intermittent fault causes.

Note! Resistance 60 ohms can only be measured if the main artery is intact and connected in the part of the CAN network in which the reading is taken. If there is an open circuit anywhere along the main artery or if any control module through which the main artery passes (a control module with four CAN connections) is not connected, resistance will be 120 ohms when the reading is taken. For more information, see the wiring diagram.

Also take resistance readings between each CAN cable and the supply voltage and ground. Twist and bend the cable harness at suitable points to detect any intermittent fault causes. The resistance must be infinite or very high (higher than 1 Mohms).

Hint: Both readings can be taken in the same connector if the CAN cables, power supply and ground are exposed. If possible, take readings on the rear of the connector to prevent damage to the terminals in the connector. If this is not possible, use the adapter wiring or loose terminal pins to obtain a good contact. The resistance must be infinite or very high (higher than 1 Mohms).

If the main artery is OK (i.e. approximately 60 ohms measured in the power seat module (PSM), for example), measure the resistance in the CAN cables from the relevant control module.

Diagnostic testing of main arteries in the high speed network (HS-CAN)
In principle it is only possible to measure the resistance in the high speed network (HS-CAN) using adapter wiring. It is easiest to connect the adapter wiring at the engine control module (ECM). Connect the engine control module (ECM) again to the adapter wiring to obtain the correct values when taking readings. The resistance must be approximately 60 ohms.

Note! Resistance 60 ohms can only be measured if the main artery is intact and connected in the part of the CAN network in which the reading is taken. If there is an open circuit anywhere along the main artery or if any control module through which the main artery passes (a control module with four CAN connections) is not connected, resistance will be 120 ohms when the reading is taken. For more information, see the wiring diagram.

Abbreviations and terms
- LS-CAN. Low speed network in the car. In practice this is restricted to the passenger compartment and the cargo compartment
- HS-CAN. High speed network in the car. In practice this is restricted to the engine compartment
- CAN-H. CAN cable with the higher voltage. The voltage oscillates between 2.5 V and approximately 3.5 V when there is traffic on the CAN network
- CAN-L. CAN cable with the lower voltage. The voltage oscillates between 2.5 V and approximately 1.5 V when there is traffic on the CAN network.