Voltage Coding of Coolant Level/Windshield Washer Fluid Level

The circuit in the figure is used to determine the coolant level and windshield washer fluid level.
The coolant level switch (S41) and windshield washer fluid level switch (S42) are switched in series. Both switches are read in by the right front SAM control module (N10/7) via two cables.


NOTE: The figure shows the status of the switches for the correct fluid levels.


Voltage coding
The resistors r(suffix)1 and r(suffix)2 are connected in parallel with the switches s(suffix)1 and s(suffix)2 (The values of r(suffix)1 and r(suffix)2 must differ). Together with r(suffix)3 of the right front SAM control module (N10/7), these resistances form a voltage divider which is supplied with constant voltage U(suffix)const when the switches are open.

Assignment; Switch States/Warning Message:

The voltage U(suffix)e at the input of the right front SAM control module (N10/7) therefore depends on the positions of the switches and the values of the resistances.
With the two switches there are a total of 4 combination options of switch states (refer to table).
Each of these combinations produces another voltage value U(suffix)e at the input of the right front SAM control module (N10/7).

Evaluation
The voltage U(suffix)e is applied at the input of the analog-to-digital converter (ADC) of the right front SAM control module (N10/7). The analog-to-digital converter (ADC) converts the analog voltage value into a digital value. This is required as the processor of the right front SAM control module (N10/7) can only work with digital values. The processor compares the applied values with the threshold values stored in a memory and, if necessary, places the corresponding message (engine coolant level and/or windshield washer fluid level too low) as a message on the CAN (interior). The instrument cluster (A1) reads this message and actuates the multifunction display (A1p13) accordingly.


NOTE: From the known resistance values the known voltage U(suffix)const, U(suffix)e can be calculated using the following formula:
U(suffix)e = U(suffix)const * (r(suffix)1 + r(suffix)2) / (r(suffix)1 + r(suffix)2 + r(suffix)3)


Assignment: Switch states/warning message
These messages can be acknowledged with the reset button on the instrument cluster (A1)


NOTE: If there is a cable open-circuit or sensor break, no message appears on the multifunction display (A1p13). However, a fault is stored in the fault memory of the SAM R.