Part 2
Too high signal/signal missing/too low signal. (Continued)
Faulty communication
Faulty communication
General information
In order to check the control area network (CAN) cables, the resistance in the network must be measured. The resistance between the two controller area network (CAN) wiring, with the network connected 60 ohms. The resistance can differ depending on the configuration of the car. Permitted values are 55-65 ohms. This applies to both the high speed side network and the low speed side network.
Hint: Use Wiring diagram to provide support when taking readings. Take a copy of the data communication side for each model. Note which diagnostic trouble codes (DTCs) are stored in each control module. This facilitates fault-tracing considerably.
Check if the diagnostic trouble codes (DTCs) for failed communication are stored in the central electronic module (CEM). If this is the case, first fault-trace according to these diagnostic trouble codes (DTCs).
Other information
- For general information about fault-tracing the control area network (CAN), see
Continue - Checking the resistance of the control area network (CAN) cables
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Checking the resistance of the control area network (CAN) cables
- Disconnect the battery negative lead. The power to the network must be cut when taking resistance readings. The reading may otherwise be incorrect.
Note! Also read: Battery, disconnecting Battery Disconnecting
- Connect the adapter wiring to the central electronic module (CEM).
Hint: Where a breakout box cannot be used, take readings from the rear of the affected connectors if possible. This prevents terminal pins and socket housings being damaged.
When taking readings on the low speed network
- When taking readings on the low speed network, the adapter wiring should be connected to the central electronic module (CEM) and the readings taken from there. The readings can also be taken in the connector for the power seat module (PSM). The advantage of using the adapter wiring connected to the central electronic module (CEM) is that all control modules can be connected to the network when taking readings and there is access to both the high speed and the low speed networks.
This resistance reading only measures main arteries in the low speed network, not the junctions for the road traffic information module (RTI), accessory electronic module (AEM), power seat module (PSM), phone module (PHM), audio module (AUM), driver's door module (DDM) and passenger door module (PDM). See the Wiring diagram, Data communication.
When taking readings on the high speed network
- When taking readings on the high speed network, the readings should be taken at the engine control module (ECM) using adapter wiring. The engine control module (ECM) must be connected. This resistance reading only takes readings from the main arteries in the high speed network. See the Wiring diagram, Data communication.
Connect the ohmmeter to the terminals for CAN-L and CAN-H on the relevant network. Work the connectors and twist the cable harness. The resistance must be approximately 60 ohms. Permitted values are 55-65 ohms.
Other information
- Checking wiring and terminals Checking Wiring and Terminals
Is the value OK?
Yes - Checking a short-circuit in CAN cables
No - Identifying fault causes in the CAN network
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Checking a short-circuit in CAN cables
Connect an ohmmeter between the relevant CAN cable (CAN-L and CAN-H) and the power supply, and between the CAN cable and ground. Twist the connectors and turn the cable harness to different positions at each reading to determine any intermittent short-circuits.
Take the following readings:
- The resistance between CAN-L and ground
- The resistance between CAN-L and voltage
- The resistance between CAN-H and ground
- The resistance between CAN-H and voltage.
The resistance must be infinite or very high in all cases (greater than 1 Mohms).
Hint: The reading can be taken from 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.
Was a fault detected?
Yes - Identifying fault causes in the CAN network
No - Checking the traffic load on the CAN cables
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Checking the traffic load on the CAN cables
- Connect the battery
- Ignition on
Also read: Battery, disconnecting Battery Disconnecting
Faults in the CAN network may be due to a control module sending faulty messages that disrupt "normal" communication. When messages are sent incorrectly, this results in a considerable increase in the amount of traffic on the CAN network. Check the load by measuring the average voltage value on the CAN cables in relation to ground.
Connect a voltmeter between CAN-L and ground. The voltage must be approximately 2.3 V.
Then connect a voltmeter between CAN-H and ground. The voltage must be approximately 2.8 V.
Hint: If possible measure from the rear of the affected connectors so that they are not damaged.
Is the value OK?
Yes - Fault-tracing without result
No - Identifying a control module in the CAN network
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Identifying fault causes in the CAN network
- Disconnect a control module that disconnects the CAN network (a control module that has four terminals to the CAN network).
Hint: For the low speed network, suitable places for disconnecting the network are at the steering wheel module (SWM), the driver information module (DIM) or the climate control module (CCM).
In the event of a short-circuit between one of the CAN cables and voltage or ground, the fault can be located by disconnecting a control module that disconnects the CAN network. Readings can then be taken to determine on which side of the disconnected control module the fault is located.
Connect an ohmmeter between the CAN cable and supply voltage or ground. Take readings at both ends of the CAN network where the network is disconnected. Twist and bend the cable harness to locate any intermittent short-circuits. The resistance must be infinite or very high (greater than 1Mohms).
If a fault can be localized in one direction, disconnect the network at a point further down on the CAN network in the direction in which the fault was discovered and repeat the reading. In this way, try to identify where in the network the fault might be located.
Hint: A short-circuit between the CAN cables and supply voltage or ground may be in either the wiring or in a control module.
Rectify the wiring as necessary. Replace the control module if the fault has been localized to a control module.
Continue - Verification
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Identifying a control module in the CAN network
- Connect the battery
Also read: Battery, disconnecting Battery Disconnecting
- Ignition on
If the load on the CAN network is too high, this may be due to a control module sending faulty messages on the CAN network (known as a disruptive control module). The fault may also be due to an intermittent short-circuit to supply voltage or ground in one of the CAN cables. This results in the control modules on the CAN network starting to send faulty messages. The faults can occur without a diagnostic trouble code (DTC) being stored by the control modules. The voltage can be measured between the CAN cables in order to check the load on the CAN network.
Connect a voltmeter between the CAN cables. If the communication on the network is correct, the average voltage value should be approximately 0.5 V (0.4 V to 0.6 V). If there is a fault in the communication, the voltage reading will be approximately 0.7 V or higher.
In order to identify from which control module the faulty communication is being transmitted, cut the supply voltage to one control module at a time and check the voltage in the CAN cables again. The easiest way to cut the supply voltage is to remove a fuse. Repeat until the voltage between the CAN cables drops to approximately 0.5 V.
Hint: When the probable faulty control module has been identified, connect and disconnect the supply voltage to the control module a number of times to verify that the fault appears and disappears.
Note! Do not cut the power to the central electronic module (CEM) unless it is the last control module left to check.
The control module that is communicating incorrectly must be replaced. Remedy as necessary.
Note! Following a fault involving "disruptive" control modules, other modules may remain in "Limp-Home mode" even after switching the ignition off and on a number of times. To remedy this, disconnect and reconnect the battery negative terminal to restore the control modules to normal function.
Continue - Verification
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Fault-tracing without result
Information
Fault-tracing information
Verification shows that the fault is no longer present. The diagnostic trouble code (DTC) was probably caused by an intermittent fault.
Fault-tracing for this diagnostic trouble code (DTC) is not followed by a verification.
You can view the information again or exit fault-tracing for this diagnostic trouble code (DTC).
Do you want to view information again?
Yes - Attempt New Test
No - No Fault Found
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Attempt New Test - Checking the resistance of the control area network (CAN) cables
No Fault Found - No Fault Found
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Identifying fault causes in the CAN network
- Disconnect a control module that disconnects the CAN network (a control module that has four terminals to the CAN network).
Hint: For the low speed network, suitable places for disconnecting the network are at the steering wheel module (SWM), the driver information module (DIM) or the climate control module (CCM).
In the event of an open-circuit or a short-circuit between the CAN cables, the fault can be localized by disconnecting a control module that disconnects the CAN network. Readings can then be taken to determine on which side of the disconnected control module the fault is located.
Connect an ohmmeter between the CAN cables. Take readings at both ends of the CAN network where the network is disconnected. Twist and bend the cable harness to locate any intermittent short-circuits. The resistance must be 120 ohm.
If a fault can be localized in one direction, disconnect the network at a point further down on the CAN network in the direction in which the fault was discovered and repeat the reading. In this way, try to identify where in the network the fault might be located.
Remedy the cables as necessary.
Continue - Verification
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Verification
Hint: After carrying out the repair, check that the fault has been remedied.
- Reinstall the connectors, components, etc.
- Ignition on
- Erase diagnostic trouble codes by clicking the VCT2000 symbol
- Start the car. Allow the car to run for 10 minutes.
- Read off diagnostic trouble codes (DTCs).
Check if a diagnostic trouble codes (DTCs) for communication fault recurs in the central electronic module (CEM).
Does the diagnostic trouble code (DTC) recur?
Yes - Fault-tracing information
No - FAULT FOUND
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Fault-tracing information
Information
Fault-tracing information
The fault should have been detected and remedied. As this is not the case fault-tracing has failed.
Exit fault-tracing for this diagnostic trouble code (DTC) or make another attempt.
Do you want to exit fault-tracing?
Yes - Exit fault-tracing
No - Attempt New Test
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Attempt New Test - Checking the resistance of the control area network (CAN) cables
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No Fault Found - No Fault Found
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