Initial Inspection and Diagnostic Overview

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 network house two network resistors in tow 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 as measured from about the center of the network. The value may vary between vehicles. Permissible values are approximately 55-65 ohms.

Note! Troubleshoot each CAN-net, see troubleshooting for CAN-related fault codes for Central electronic module (CEM). For example, choose one of CEM-DF01 - DF17 depending on CAN-net.

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:

Electrical fault
These diagnostic trouble codes (DTCs) can only be stored in the central electronic module (CEM). This type of diagnostic trouble code (DTC) is stored when the central electronic module (CEM) has detected an electrical fault in the controller area network (CAN) wiring. For example a short-circuit between the controller area network (CAN) wiring or a short-circuit between the controller area network (CAN) wiring and voltage or ground. The fault may be due to an open-circuit in the CAN-H on the network. These diagnostic trouble codes (DTCs) do not normally indicate that a fault has occurred in any control module.

Communication error, CEM
These diagnostic trouble codes (DTCs) can only be stored in the central electronic module (CEM). The diagnostic trouble code (DTC) indicates that a control module, with which the central electronic module (CEM) expects to communicate, is not responding for one reason or another. This can be due to an open-circuit on one or both the controller area network (CAN) cables to one control module or the control module not being supplied with power, for example.

Communication error, control module
These diagnostic trouble codes (DTCs) can be stored by all control modules. A control module that has stored this diagnostic trouble code (DTC) has detected 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 CAN cables, intermittent short-circuits on a CAN cable to supply voltage/ground or a control module that transmits incorrect messages on the CAN network.

Note! The diagnostic trouble codes (DTC) for communication errors can thus be stored in one or more control modules and stem from the same causes as the diagnostic trouble codes for electrical faults in the Central Electronic Module (CEM), that is to say "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 a diagnostic trouble code (DTC) that is generated due to the short-circuit itself.

Configuration fault
The diagnostic trouble code (DTC) can be stored by all control modules in both the HS-CAN and LS-CAN except the central electronic module (CEM). The diagnostic trouble code (DTC) indicates that a control module could not receive anticipated communication from the central electronic module (CEM). Examples of fault causes could be incorrect software configuration in the control module that generated the diagnostic trouble code (DTC) or an open-circuit on the CAN cables to the control module that generated 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 "break". If the control module that generated the diagnostic trouble code once has functioned in the vehicle, the malfunction cannot be remedied by downloading new software. The only time when downloading new software to a control module is of help is if the customer or workshop has replaced a control module with a control module from another vehicle without downloading software after replacement.

Design and Function, Controller area network (CAN) S40 (04-)/V50
- The network The Network
- Construction of the network Construction of the Network
- 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) 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.

Example:

Diagnostic trouble codes in pairs between the central electronic module (CEM) and a single control module
If central electronic module (CEM) has generated a diagnostic trouble code for open circuit in communication with a control module and this control module has generated a diagnostic trouble code configuration error, start troubleshooting at the indicated control module. An open circuit has occurred at some point somewhere between the central electronic module (CEM) and this control module.

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 retracting pins or oxidation.

Individual diagnostic trouble codes in the central electronic module (CEM)
If the central electronic module (CEM) has generated diagnostic trouble code for an open circuit in communication with a control module and there is no diagnostic trouble code (DTC) stored in the corresponding control module, it indicates an suspected malfunction in the voltage 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 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:
- Driver door module (DDM)/passenger door module (PDM): Check that the power window of each door can be operated "locally.
- 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 in the connectors of the power seat module (PSM) if such is fitted (easy to access and sits in about the middle of the network). Otherwise connect a breakout box at the central electronic module (CEM). The advantage of using a breakout box is that the functionality of the entire network can be controlled without disconnecting control modules.
This resistance measurement only controls main paths in LS-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 measurements can be taken in the same connector if the CAN-cables, voltage feed, and ground are exposed. If possible, take measurements 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.

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
- LS-CAN (Low Speed CAN). Low 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 2.5 V and approximately 1 V when there is traffic on the CAN network.