Electrical - Overview Of On-Board Electrical Systems

Date: June 2006

Order No.: P-B-54.10/172

Supersedes:

Group: 54

SUBJECT:
MODEL 221

Brief Overview on on-board Electrical Systems

Due to the high current requirement when the ignition and telematics are on, the power consumption may be as high as 30 A. For this reason, the battery should always be connected to a charger during a workshop visit (as per AR54.10-P-11275X).

The charging capacity of the charger must be at least 30 A.

The display of the on-board electrical system battery voltage can be read out in the instrument cluster (IC) as follows:

1. Set the display in the IC to "Trip" and change to the "Trip counter" mode. Confirm with "OK" in the left multifunction steering wheel button group (S110). "Reset odometer" appears in the display (picture 1).

2. Press and hold down the "OK" button in the left multifunction steering wheel button group (S110) again (picture 2) while also pressing the "Pick up phone" button in the right multifunction steering wheel button group (S011). Hold down both buttons for 5 seconds. "Vehicle data" appears in the display.

3. Confirm the "Vehicle data" menu by pressing the "OK" button in the left multifunction steering wheel button group (S110). The voltage of the on-board electrical system battery UB is shown first in the display (picture 3).

On-board electrical network line pyrofuse (N82/1f1)

In the vehicle power supply control unit (N82/1) there is a pyrofuse (picture 4) to protect the on-board electrical network line to the prefuse box (F32).

The on-board electrical network line pyrofuse (N82/1f1) is triggered under the following conditions:

1. Temperature of on-board electrical network line higher than 140°C.

^ As of software number 221 442 03 55, the pyrofuse is no longer triggered on the basis of temperature (as of 08/05). Only crash signal 2 is relevant.

2. Crash signal 2 from airbag control unit (ARCADE).

Brief instructions, quiescent current fault diagnosis

Procedure for vehicles with discharged on-board electrical system battery:

1. Using Star Diagnosis (DAS), read out the last and next to last entry for bus keepawake event detection. Compare both entries with each other:

- If fewer than 4 entries exist, check the appropriate control units. Inspect and replace, if necessary.

- If there are more than 4 entries, the CAN system is extremely unstable and the information is unusable.

- With gateway control units, always check the associated CAN system too.

2. Prepare the vehicle for the quiescent current test:

- Close the latches of the doors, trunk lid and engine hood.

- The charging converter must be deactivated.

- After switching off the engine, briefly turn the key in the EIS to position 2, then back to position 0, and then remove.

- The quiescent current switch must not be allowed to open during the quiescent current test. Both batteries must therefore have a voltage of mm. 12.2 V.

3. Perform the quiescent current test on the ground cable of the on-board electrical system battery.

- Use the current clamp or non-interrupted looped-in multimeter.

- Isolate the active CAN system.

- If the quiescent current is higher than 2.5 - 3.0 A, there is a "bus keepawake event" in the on-board electrical system.

- Identify the bus keepawake event control unit.

- Identification by means of voltage measurements at the fuses and disconnection of the intermediate gateways.

- Disconnect the control units one by one from the CAN voltage distributor and reconnect if the quiescent current is unchanged.

- If the quiescent current consumption is lower than 2.5 - 3.0 A without CAN bus or MOST ring activity, there is a problem in a local control unit. The control unit in question can be determined as follows: [Down Arrow]

- Check the voltage drop at all circuit 30 and circuit 30g fuses at the front SAM, rear SAM, left interior fuse box and right interior fuse box (see S-B-54.10/162 for further details).

- Detection of bus keepawake events in the CAN systems:

Bus keepawake event detection in the central gateway (CGW) (N93) determines control units which keep the data bus "awake" during an anticipated bus idle phase, i.e. registration of the control units which do not send any "sleep indication bit" signals.

- Control units which are detected as bus keepawake events are stored in a non-volatile memory with the following data:

- Date, time of entry

- Control unit identifier

- CAN bus system assignment

- Additional advantage for diagnosis: If, for example, a vehicle is parked for longer than 75 minutes with the low beam switched on, the front SAM control unit is entered as the bus keepawake event (executing control unit). In addition, "Low beam active" is entered to make analysis easier.

Disadvantage:

- Control units which wake the data bus cyclically are not detected by the bus keepawake event detection function because continuous activity (CAN trace) is not evaluated.

- The bus keepawake detection is factory-set to 75 minutes. For diagnosis work carried out at the workshop, this setting can be reduced to 7 minutes since a bus idle phase can be expected after this time. This can be set through the CGW, then Actual Values, then Configuration of the Bus Keepawake Event Recognition.

- Following repair work, the default time for bus keepawake event detection must be reset to 75 minutes.

- Operating principle of prefuse box:

- The front prefuse box together with the vehicle power supply control unit, the main energy line and the on-board electrical system battery forms the basic structure of the on-board electrical system.

- The alternator and the following high-power circuit 30 consumers are connected to the prefuse box:

- suction fan

- glow time output stage (diesel only)

- front SAM/SRS-FP (N10/1)

The prefuse box contains the coupling/cold-start relay for parallel connection of both batteries during emergency operation of the on-board electrical system. The positive charging point for the on-board electrical system battery and the screw-on point for the alternator line (M1) are also in the prefuse box (F32).