| Basic information on fault finding and on repairs |
| As the control units are interlinked over two data BUS lines, 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 vehicle diagnosis, measurement and information system -VAS- → Chapter |
| When doing so check whether a possibly stored fault might affect the ABS system. |
| The ABS control unit -J104- together with the hydraulic unit for ABS -N55- forms a compact unit. This unit is located in the right of the engine compartment. The control unit is equipped with a fault memory. The connection for the self-diagnosis is located below the left steering wheel from the fuse carrier. |
| 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 to a wheel speed sensor). |
| The control unit detects faults while the vehicle is being operated 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 40 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-, the red dual circuit and hand brake system warning light -K7- and the traction control system warning light -K86- or the ESP and TCS warning light -K155- 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. 11.0 Volts |
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| Test of control unit including valve coils. |
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| Electrical test of wheel speed sensors; not fully completed until vehicle reaches 20 km/h. |
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| Check of control unit coding |
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| Test of electric motor for return flow pump. |
| Systematically initiate self-diagnosis at the start of fault finding and retrieve the stored information using vehicle system tester -VAS- → Chapter |
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