Signal Missing
Signal missing
Checking the function of the audio module (AUM)
Check the power supply for the control module by switching on the radio or playing a CD.
There must be sound from the radio or CD.
Note! Radio reception may be poor indoors. On certain cars, the radio reception may deteriorate if the trunk lid is open.
Is there sound from the radio?
Yes - Checking communication with the audio module (AUM)
No - Checking the power supply and ground for the audio module (AUM)
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Checking the power supply and ground for the audio module (AUM)
Check the fuse in the passenger compartment fusebox for the main unit/audio module (AUM).
Note! If the fuse has blown, check the circuit after the fuse before replacing the fuse. Check for a short-circuit to ground. Then replace the fuse. For further information about the circuit after the fuse, refer to the appropriate wiring diagram.
Check the connector for the main unit/audio module (AUM). Check for contact resistance and oxidation.
Check the power supply cable for the main unit/audio module (AUM). Check for an open-circuit.
Check the ground cable between the main unit/audio module (AUM) and the ground terminal. Check for an open-circuit. Also check the ground terminal. Check for contact resistance and oxidation.
Remedy as necessary.
Other information:
- To replace the fuses, see Fusebox, passenger compartment, replacing Fusebox, Passenger Compartment, Replacing
- To replace the main unit/audio module (AUM), see Radio cassette player, replacing Radio/Cassette Player, Replacing
- To connect the breakout box, see Connecting the breakout box, Audio control module Connecting the Breakout Box, Audio Control Module
- For information about signals and connectors, see Signal specification, audio control module Signal Specification
- For information about checking wiring and terminals see Checking wiring and terminals Checking Wiring And Terminals.
Was a fault detected?
Yes - Checking the function of the audio module (AUM)
No - Signal missing
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Checking communication with the audio module (AUM)
Hint: The CAN communication with the control module is probably OK if the control module ID can be read off. In that case the fault is probably intermittent and will then be more difficult to trace.
- Try to communicate with the audio module (AUM) by reading off the control module ID
- Click on the symbol for VCT2000 to begin reading off
Continue - Signal missing
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Signal missing
General information
Hint: For general information about fault-tracing the control area network (CAN), see the linked document below.
In order to check the control area network (CAN) cables, the resistance in the network must be measured. The resistance between the two CAN cables, with the network connected, must be 60ohms in both the high speed network and the low speed network.
Note! Resistance will only be 60 ohms if the main artery of the CAN network is whole and the network is connected. If the main artery of the CAN network is split somewhere and a reading is taken between the CAN cables, resistance will be 120 ohms. For further information, see the wiring diagram.
If diagnostic trouble codes (DTCs) can be read off from the relevant control module, the fault is probably intermittent. The locations that must be checked in the event of a fault in the low speed network are 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 fault in the current supply to the actual control module is indicated by the DTC 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 without power.
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 network must not be powered when taking resistance readings. Otherwise an incorrect value may be obtained.
- 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 readings taken from there. Readings can even be taken in the connector for the Power seat module (PSM). The advantage of the adapter wiring to the Central electronic module (CEM) is that all control module can be connected in the network when taking readings and access can be gained to both the high speed and low speed networks.
The resistance reading only measures the main arteries in the low speed network, not the junctions that exist for different control modules. For further information, see the wiring diagram.
When taking readings on the high speed network
- When taking readings on the high speed network the adapter wiring should be connected to the Engine control module (ECM) and readings taken from there. The Engine control module (ECM) must be connected. This resistance reading only measures the main arteries in the high speed network. For further information, see the wiring diagram.
Connect an ohmmeter to the terminals for CAN-L and CAN-H on the relevant network. Turn the contacts and turn the cable harness. Resistance must be approximately 60 ohms.
Note! Resistance will only be 60 ohms if the main artery of the CAN network is whole and the network is connected. If the main artery of the CAN network is split somewhere and a reading is taken between the CAN cables, resistance will be 120 ohms. For further information, see the wiring diagram.
Other information
- Battery, disconnecting Battery Disconnecting
- Connecting the breakout box Connecting the Breakout Box, Central Electronic Module (CEM)
- 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 respective CAN cable (CAN-L and CAN-H) and voltage supply and between the CAN cables and ground. Turn the contacts and turn the cable harness. The resistance must be infinite or very high (more than 1 M Ohms).
Hint: Readings can be taken from one and the same connector if the CAN wiring, voltage supply and ground is prerouted. If possible, measure from the rear of the affected connectors in order to minimize any damage and future loose connections. If this is not possible, use the adapter wiring or loose terminal pin for 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.
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. The load can be checked by reading the average value of the voltage on the CAN cables in relation to ground.
The following applies for model year -2004:
Connect a voltmeter between CAN-L and ground. The voltage shall be approx. 2.3 V.
Then connect a voltmeter between CAN-H and ground. The voltage shall be approx. 2.8 V.
The following applies for model year 2004-:
Connect a voltmeter between CAN-L and ground. The voltage should be approx:1,8-2,2V
Connect a voltmeter between CAN-H and ground. The voltage should be approx:2,8-3,2V
Note! For vehicles of later model years, normal voltage values lie within a specific interval due to increased traffic between the control modules on the CAN network upon update of the electrical system.
Hint: If possible, measure from the rear of the affected connectors so that they are not damaged.
Other information
- Battery, disconnecting Battery Disconnecting
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 an short-circuit between on of the CAN cables and voltage or ground, the fault can be localized disconnecting a control module that splits the CAN network. Readings can then be taken to determine on which side the fault is on the disconnected control module.
Connect an ohmmeter between the CAN cable and voltage supply respectively ground. Take readings from both ends of the CAN networks at the network junction. Twist and bend the cable harness to find any intermittent short-circuits. The resistance must be infinite or very high (more than 1 M Ohms).
If a fault can be localized in one direction, split the network at a point further on in the CAN network in the direction that the fault was discovered and take a new reading. Try to locate the fault in this way.
Hint: A short-circuit between the CAN cables and supply voltage or ground may be in either the wiring or in a control module.
Remedy the cables as necessary. If a fault is found on one control module, replace the control module.
Continue.
Continue - Verification
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Identifying a control module in the CAN network
- Connect the battery.
- Ignition on.
If the load on the CAN network is too high, this may be due to a control module transmitting faulty messages on the CAN network. This is called 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 fault may occur without any of the control modules storing a diagnostic trouble code (DTC). The voltage can be measured between the CAN cables in order to check the load on the CAN network.
The following applies for model year -2004:
Connect a voltmeter between the CAN cables. In the event of correct communication on the network, the average value of the voltage must be approximately 0.5 V (0.4 V to 0.6 V).
The following applies for model year 2004-: The following applies for model year 2004-:
Connect a voltmeter between the CAN cables. During correct CAN network communication, voltage should be about 0.6-1.4 V.
Note! For vehicles of later model years, the normal voltage value is slightly higher due to increased traffic between the control modules on the CAN network upon update of the electrical system.
In the event of a communication fault the voltage is significantly higher than the above specified values. In order to identify which control module has faulty communication, cut the power supply to one control module at a time. Check the voltage in the CAN cables again. The easiest way to cut the power supply is to remove the fuse(s) for the relevant control module. Repeat until the voltage between the CAN wiring drops to the normal level.
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! After a fault with "interfering" control modules, other control modules may remain in "Limp-Home mode" even when the ignition has been switched off and on several times. To remedy this, disconnect and reconnect the battery negative terminal to restore the control modules to normal function.
Other information
- Battery, disconnecting Battery Disconnecting
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 on, or short-circuit between the CAN cables, the fault can be localized by disconnecting a control module that splits the CAN network. Readings can then be taken to determine on which side the fault is on the disconnected control module.
Connect an ohmmeter between the CAN cables. Take readings from both ends of the CAN networks at the network junction. Twist and bend the cable harness to find any intermittent short-circuits. Resistance should be 120 ohms.
If a fault can be localized in one direction, split the network at a point further on in the CAN network in the direction that the fault was discovered and take a new reading. Try to locate the fault in this way.
Remedy the cables as necessary.
Continue.
Continue - Verification
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Verification
Hint: After rectifying a fault, a verification should be carried out.
- Reinstall the connectors, components etc.
- Ignition on.
- Erase diagnostic trouble codes by clicking the VCT2000 symbol.
- Start the car. Allow the vehicle to run for 10 minutes.
- Read off diagnostic trouble codes (DTCs).
Does the diagnostic trouble code (DTC) for a communication fault recur (CEM-1A5X or CEM-1A6X)?
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 - FAULT-TRACING FAILED
No - Attempt New Test
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FAULT-TRACING FAILED - FAULT-TRACING FAILED
Attempt New Test - Checking the resistance of the control area network (CAN) cables
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FAULT FOUND - Fault Found
FAULT-TRACING FAILED - FAULT-TRACING FAILED
No Fault Found - No Fault Found
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Checking the function of the audio module (AUM)
Hint: After carrying out the repair, check that the fault has been remedied.
- Ignition off
- Reconnect the connectors. Reinstall the components
- Ignition on.
Check the power supply for the control module by switching on the radio or playing a CD.
There must be sound from the radio or CD.
Note! Radio reception may be poor indoors. On certain cars, the radio reception may deteriorate if the trunk lid is open.
Is there sound from the radio?
Yes - Fault Found
No - Fault-tracing information
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Fault-tracing information
Information
FAULT-TRACING FAILED - FAULT-TRACING FAILED
Attempt New Test - Checking the function of the audio module (AUM)
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