Initial Inspection and Diagnostic Overview

General information about fault-tracing in the CAN network

General
The CAN network is a pure communication network and contains no fuses or sensors. The network is divided into two parts: the average speed network (MS-CAN, High 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/powertrain and MS-CAN to the passenger compartment and the cargo compartment.
There are two network resistors installed in two of the control modules in both the average speed and high speed networks. The control modules in which the resistors are installed may differ on different car models. The resistors both have a resistance of 120 ohms. If you measure the resistance between the two CAN cables, a normal value should be about 60 ohms on condition that the network, between the two control modules equipped with the resistors, is complete and that the network is connected. The value can differ from vehicle to vehicle. Permitted values are approximately 55-65 ohms.

Note! To troubleshoot each CAN-net, see troubleshooting for CAN-related fault codes for Central electronic module (CEM). For example, choose CEM-U000100/CEM-U000111/CEM-U000112/CEM-U000188 (HS CAN) or CEM-U001000/CEM-U001011/CEM-U001012/CEM-U001088 (MS-CAN).

Diagnostic trouble code (DTC) types
There are three different categories of diagnostic trouble codes (DTC), which indicate problems with CAN communications. Two of them describe whether there is a fault on the network or on one of the connected control modules, while one of the categories indicates that a node cannot communicate on the network due to an internal configuration fault. Thus, the latter does not imply a fault on the CAN bus itself.

Communication fault
The diagnostic trouble codes (DTC) can be stored on all control modules connected to the CAN network. A diagnostic trouble code (DTC) is triggered on a control module when it cannot communicate on the network. The control module then closes down its communication and generates the diagnostic trouble code (DTC). This functionality is required to minimize the risk of a control module, which does not receive a response, resending this at such a high rate that it blocks all other communication on the network. In theory this means all control modules should generate diagnostic trouble codes (DTCs) as soon as another control module does this, as communications disturbed by the faulty control module affect the entire network and in doing so results in no control modules receiving a response to the sent messages. However, in practice, not all modules detect the fault and trigger a diagnostic trouble code (DTC) before the faulty control module stops disturbing the network and the problem ceases. This is usually rectified by temporarily cutting the power supply to the faulty control module.
In addition to internal problems in a control module, the fault can also be a result of a short-circuit on a CAN cable.
We recommend measuring the voltage between CAN-H and CAN-L in ignition position II to localize what is disturbing communication on the network. The difference between the cables should be approximately 0.5 - 0.6 Volt on both the high speed side (HS-CAN) and the average speed side (MS-CAN). If a control module is disturbing the network this difference will be on a higher level. To measure this difference while de-energizing the control modules one at a time is a reliable procedure to localize the faulty control module. This is known as successive elimination. When the voltage drops to the normal value you have localized the faulty control module. If any of the cables are short-circuited to ground, the voltage difference between the cables will be higher than expected and if the voltage is closer to 0 Volt, the cables are short-circuited to each other.

Interrupted communication
These diagnostic trouble codes (DTC) can be stored by all control modules. A control module that generates this diagnostic trouble code (DTC) has discovered some type of communication interruption with another, specific control module. The diagnostic trouble code (DTC) does not identify a direct source of the fault. Examples of faults include: internal faults, problems with the voltage supply, connector play or a backed pin for one of the CAN cables in a connector.
The central electronic module (CEM) is the only control module that checks all other control modules on the CAN network. In general other control units only check the central electronic module (CEM) and possibly another control module of extreme importance to it. The check is made by control module (A) listening for a specific message from the control module (B) being checked. If the message does not reach A within a specific time, A interprets this as B not existing on the CAN network and the diagnostic trouble code (DTC) that B has lost communication is then generated by A. See below for an example of fault tracing interrupted communication.

Configuration fault
Diagnostic trouble codes (DTC) can be stored by all control modules on both HS-CAN and MS-CAN with the exception of the Central electronic module (CEM). The cause of the fault is incorrect software configuration in the control module generating the diagnostic trouble code (DTC), which means it cannot interpret the messages on the network correctly. Consequently, the fault was not caused by an internal problem on the CAN network, but by the control module expecting one key from the central electronic module (CEM) to decode the CAN communication but receiving another.

Note! The software in a control module cannot "break". This means that these diagnostic trouble codes (DTCs) can only be detected if the customer or workshop replaces a control module with one that has previously been installed in another vehicle without loading software after replacement. If such replacement has been carried out, the fault can be rectified by downloading new software to the control module.

Design and Function, CAN network S80 (07-)
- The network The Network
- Construction of the network Construction of the Network
- The controller area network (CAN) The Controller Area Network (CAN)
- Error management in the Controller area network (CAN) Error Management In the Controller Area Network (CAN)

Diagnostic functions in the central electronic module (CEM)
The central electronic module (CEM) sits as a hub between the two networks. It acts as a switchboard, distributing traffic between the high speed network (HS-CAN) and average speed network (MS-CAN) and vice versa. There is, however, no electrical connection between the two parts of the CAN network. Thus, a fault in one part of the network cannot disrupt the other part of the network. The central electronic module (CEM) also has diagnostic functions to monitor the traffic on the CAN network and the voltage levels on the CAN cables.

Fault tracing example:
The following are examples of cases that could lead to a quicker determination as to which fault occurred and where the fault could be.

Diagnostic trouble codes in pairs between the central electronic module (CEM) and a single control module
If the central electronic module (CEM) has generated a diagnostic trouble code (DTC) for interrupted communication with a control module and this control module has also generated a diagnostic trouble code (DTC) for interrupted communication, start fault tracing with the control module in question. The diagnostic trouble codes (DTC) indicate that an open circuit occurred at some point somewhere between this control module and the central electronic module (CEM).

Note! If the diagnostic trouble code can be read out from the control module in question, the fault is probably intermittent. Check the junctions and connectors along the wiring harness from the central electronic module (CEM) and to the control module that generated the code. Check the connectors especially for recessed pins or oxidation.

Individual diagnostic trouble codes in the central electronic module (CEM)
If the central electronic module (CEM) has generated diagnostic trouble code (DTC) CEM-DEXX for an interruption in communication with a control module and there is no diagnostic trouble code (DTC) stored in the corresponding control module, it indicates an intermittent fault in the power supply to the indicated control module. The diagnostic trouble code (DTC) cannot be generated if the control module which could not communicate with the engine module (CEM) is not powered.

General procedure
- Check the power supply 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:
- Driver door module (DDM)/passenger door module (PDM): Check if the power window in each door can be operated "locally.
- Power seat module (PSM): Check whether the seat can be moved manually
- Multimedia module (MMM): Check if a disc can be inserted or ejected.
- 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 average speed network (MS-CAN)
Measure resistance between the CAN cables in the power seat module (PSM) connector if such is present (easy to access and it sits in the middle of the network). Otherwise, connect the breakout box at the central electronic module (CEM). The advantage of the breakout box is that you can check the functionality of the entire network with all control modules connected. The negative battery cable must be removed during measurement. Otherwise, readings could be off since some control modules are also supplied power when the ignition is off.
This resistance measurement only controls main paths in MS-CAN, not the branching paths to certain control modules. However, a resistance measurement between the two CAN-lines can show if there is a short-circuit between the two lines, even on these branching paths. A normal value, measured somewhere in the middle of the net, is between 55 ohms and 65 ohms when the net is connected (both terminating resistors connected, 120 ohms in parallel). Twist and bend the cable harness at suitable points to detect any intermittent malfunction sources.
Also measure the resistance between each CAN-lead and voltage feed and ground. Twist and bend the cable harness in suitable places to detect any intermittent fault sources. The resistance should be over 1 kohms. If resistance is less than 1 kohms, this indicates short-circuiting between the CAN-lead and ground/voltage.

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 over 1 kohms. If the resistance is less than 1 kohms, this indicates short-circuiting between the CAN-lead and ground/voltage.

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 instead.

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.

Abbreviations and terms
- CAN Controller Area Network
- MS-CAN (High Speed CAN). Average speed network in the car. In practice this is restricted to the passenger compartment and the cargo compartment
- HS-CAN (High Speed 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 4 V when there is traffic on the CAN network.
- CAN-L CAN cable with the lower voltage. The voltage oscillates between 1 V and approximately 2.5 V when there is traffic on the CAN network.