| Vehicles with data BUS (CAN BUS) |
| As the control units are interlinked over a data BUS line, always start fault finding by interrogating the contents of the fault memories of all the control units fitted to the vehicle. |
| The interrogation of the fault memory is performed with the key function 00 - „Automatic test sequence“. |
| When doing so check whether a possibly stored fault might affect the ABS system. |
| The 25-pin ABS control unit -J104- together with the hydraulic unit forms a compact unit. The unit is located in the left of the engine compartment. The control unit is equipped with a fault memory. The connection for self-diagnosis is located in the storage compartment on the driver's side. |
| Self-diagnosis relates to the electrical/electronic part of the ABS, i. e. faults are detected only over the electrical connection to the control unit (e.g. open circuit of a wheel speed sensor). |
| The control unit detects faults during the operation of the vehicle and stores them in a permanent memory, whose information is retained even if the battery voltage is disconnected. |
| Faults which occur sporadically (isolated) are likewise detected and stored. If this fault no longer occurs during 50 vehicle starts and driving-off procedures, it is erased from the fault memory (fault delete counter), with the exception of the fault „Control unit defective“. |
| After the ignition is switched on, the ABS warning light -K47- and the red warning light for the hand brake/brake fluid level -K14/33- light up for approx. 2 seconds. |
| During this period a test sequence (self-check) is performed in the control unit with the following functions: |
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| Test of supply voltage, min. 10.0 V |
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| Test of control unit including valve coils. |
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| Check of control unit coding |
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| Static test of the wheel speed sensor (without speed signal) |
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| After a start-up, should it appear that the speed signal is not O.K as of approx. 20 km/h, the ABS warning light lights up again -K47-. |
| Systematically initiate self-diagnosis at the start of fault finding and retrieve the stored information using the |
| vehicle system tester -V.A.G 1552- or fault read-out scan tool -V.A.G 1551- |
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| The following description only relates to the vehicle system tester -V.A.G 1552- using the current program card. |
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| The use of the fault reader -V.A.G 1551- with integrated printer is similar. A minor deviation on the display read-out is possible. |
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| To end the self-diagnosis or to switch to another address word select function 06 „End output“. |
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| Identify the faults displayed by referring to the fault table and subsequently carry out the fault elimination. |
| On vehicles with the ABS system, the functions „Reading measured value block“, „Coding control unit“, „Basic setting“ (only necessary for vehicles with EDL) and the „Final control diagnosis“ are added to the self-diagnosis. |
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