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 house two network resistors in two of the control modules. Each resistor has resistance of 120 ohms. If resistance between the two CAN cables is measured, a normal value is approximately 60 ohms. The value may vary between vehicles. Permissible 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 DFXX (electrical fault)
These diagnostic trouble codes can only be stored in the central electronic module (CEM) and are generated if the central electronic module (CEM) detects an electrical fault in the CAN cables, such as a short-circuit between CAN cables or a short-circuit between a CAN cable and supply voltage or ground. These trouble codes are not generated by an open-circuit on a CAN cable. In addition, these trouble codes do not in any way indicate a fault in a control module.

Diagnostic trouble codes 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 for communication faults
- CEM-E000 (CEM in HS-CAN only)
- XXX-U000100 (other control modules in HS-CAN)
- XXX-E001 (all control modules in LS-CAN)
The central electronic module (CEM) uses the "old" coverage while other control modules in the HS-CAN use the "new" Generic Global Diagnostic (GGD). A control module that has stored this diagnostic trouble code has detected that there has been some type of disruption in communication. The trouble code does not directly indicate the source of the fault. Possible causes include intermittent short circuits between CAN cables, intermittent short circuits on one of the CAN cables to supply voltage/ground or a control module that has sent an incorrect message on the CAN network.

Note! The diagnostic trouble codes for communication fault can thus be stored in one or more control modules due to the same causes as trouble codes DFXX in the central electronic module (CEM), that is to say the "short circuit" of one or both CAN cables to supply voltage or ground. It is only the central electronic module (CEM), however, that can store the trouble code as a direct result of the short circuit.

Diagnostic trouble codes for configuration fault/interruption of communication
- XXX-E003 (LS-CAN)
- XXX-U030000 (HS-CAN GGD)
This type of diagnostic trouble code can be stored by any control module in either the HS-CAN or LS-CAN except the central electronic module (CEM). The control modules on the CAN network listen for regularly transmitted messages from the central electronic module (CEM). Each message contains an ID number that should be the same for all control modules in the CAN network. This makes it possible to detect if the wrong software has been downloaded to a control module in the vehicle. The diagnostic trouble code indicates that a control module has not received the expected communication from the central electronic module (CEM) or that the ID number in the control module does not match that sent by the central electronic module (CEM). Possible causes include incorrect software configuration in the control module that generated the trouble code or an open-circuit on the CAN cables to the control module that generated the trouble code. A loose connection or retracted pin in a connector of any of the CAN cables can also generate this diagnostic trouble code.

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 Wiring diagram.

Design and Function, CAN network

Note! The following document applies in general to all variants of S60/S80/V70/XC70/XC90. The information also applies in general to XC90 with V8 engine. Certain diagnostic trouble code names and the like do not apply for XC90 with V8 engine. The methods and other descriptions, however, apply to XC90 with V8 engine as well.

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 in pairs between the central electronic module (CEM) and an individual control module (CEM-1AXX and XXX-E003/XXX-U030000 (HS-CAN GGD))
If the central electronic module (CEM) has generated diagnostic trouble code CEM-1A5X or CEM-1A6X for an interruption in communication between a control module and communication that cannot be established with a corresponding control module or if the control module has generated XXX-E003/XXX-U030000, start fault tracing with the control module in question.

Note! If the diagnostic trouble code can be read out from the control module in question, the fault is probably intermittent. Spots to specifically pay attention to when there is a fault in the low speed network are junctions behind the central electronic module (CEM) and the audio module (AUM)/infotainment control module (ICM) and the cable harness behind the audio module (AUM)/infotainment control module (ICM).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 power supply to the relevant control module.
- Check the ground connection to the control module.
- Check the CAN cables to 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 illuminates or if the sunroof is operable with the ignition 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 resistance between the CAN cables in the contact of the power seat module (PSM) if the vehicle is equipped with such (easily accessible). Otherwise, connect a breakout box at the central electronic module (CEM) or at the driver information module (DIM). The advantage of using a breakout box is that one can check the functionality of the entire network with all control modules connected.
This resistance measurement only checks the main arteries of the LS-CAN, not the branches (such as to the road traffic information module (RTI), accessory electronic module (AEM), power seat module (PSM), phone module (PHM), audio module (AUM), driver door module (DDM), or passenger door module (PDM). For further information see Wiring Diagram, Data Communication). Measuring resistance between the two CAN cables can, however, indicate if there is a short-circuit between the two cables, including along a branch. A normal value measured near the middle of the network lies between 55 and 65 ohms with all control modules connected (120 ohms connected in parallel). Twist and bend the cable harness at suitable spots to detect any intermittent fault sources.
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).

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.

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 higher voltage; the voltage fluctuates between 2.5 and approx. 3.5 V when there is traffic on the CAN network.
- CAN-L. CAN network with lower voltage; the voltage fluctuates between 1.5 and 2.5 V when there is traffic on the CAN network.