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Checking convenience data bus
Function of bus systems
Bus:
Bus is the term used to describe a data transfer and distribution system.
The Audi A4 features different bus systems. These systems operate on different levels. The control unit in the dash panel insert forms the interface (gateway) between the various bus systems.
CAN:
The Controller Area Network is a two-wire bus system. The wires are referred to as bus wires. Data are transferred serially to the connected control units by way of the bus wires.
These control units communicate, i.e. exchange data, by way of the bus systems.
=> Self-Study Programme No. 186
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◆ The convenience data bus
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‒ Features a transmission rate of 100 kbaud (100000 data units per second)
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‒ Continues to operate even if there is a fault in one of the two wires, however in emergency operation mode (e.g. on account of open circuit in wiring)
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‒ Examples of connected control units:
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‒ Control unit with display in dash panel insert -J285 (interface with drive train data bus)
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‒ Heating system operating and display unit for air conditioner/Climatronic -E87 (heating)
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‒ Additional heater control unit -J364 (vehicles with genuine Audi auxiliary/additional heater only)
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‒ Steering column electronics control unit -J527
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‒ For other control units connected to convenience data bus, refer to
=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder
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◆ The drive train data bus
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‒ Features a transmission rate of 500 kbaud (500000 data units per second)
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‒ Only operates if data can be transmitted on both wires (safety circuit)
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‒ Examples of connected control units:
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‒ Control unit with display in dash panel insert -J285 (interface with convenience data bus)
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‒ Applicable engine control unit
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‒ For other control units connected to drive train data bus, refer to
=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder
Notes:
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◆ "CAN bus" is the term used to describe a data transfer and distribution system.
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◆ Depending on vehicle equipment, various control units are linked to the convenience or drive train data bus.
=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder
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◆ In the bus systems, data are simultaneously transmitted via the "CAN-Hi" and "CAN-Lo" wires (two-wire bus systems). Depending on bus system design, these wires form twisted pairs to minimise interference from other systems; they may also be screened with respect to other wires.
=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder
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◆ The wires between the control units on which data are transmitted are referred to as signal wires.
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◆ Data are transmitted serially by way of these signal wires to the connected control units and cannot be checked using workshop equipment.
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◆ Data are transmitted and received by the various control units at regular intervals in the form of telegrams. The interval between two telegrams transmitted by a control unit depends on the priority rating of the data transmitted and is between 0.01 and 0.2 s.
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◆ A fault is stored in the fault memory if a connected control unit does not receive any information within a specified time or if the data telegrams received cannot be evaluated.
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◆ For the heating system, for example, reception of data is displayed in measured value block, display groups 125 and 126=>Page 01-178.
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◆ If a control unit cannot provide the data requested (or only a certain item of information) due to a component fault, transmission of the data telegram is accompanied by a fault message requesting readout of fault memory in this control unit.
Example:
Ambient temperature sensor -G17 is not connected; control unit with display in dash panel insert -J285 cannot therefore transmit sensor measured value in data telegram.
Control unit in dash panel insert stores this fault and inserts fault message in data telegram in place of measured value.
Operating and display unit for air conditioner/Climatronic -E87 (heating) recognises fault message on evaluating telegram and stores the following information in the fault memory "Control unit in dash panel insert - read out fault memory".
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