Voltage Coding of Coolant Level/Windshield Washer Fluid Level

General
The circuit in the figure is used to determine the coolant and windshield washer fluid level. The coolant level switch (S41) and windshield washer fluid level switch (S42) are connected in series. The information on both levels reaches the instrument cluster (A1) voltage-coded via only one cable.

Voltage Coding
The resistances r(suffix)1 and r(suffix)2 are connected in parallel with the switches s1 and s2 for voltage coding. (The values of r(suffix)1 and r(suffix)2 must differ). When the switches are open, these resistances, together with r3 of the instrument cluster form a voltage divider, which is supplied with a constant voltage U(suffix)const.
The input voltage U(suffix)e therefore depends on the positions of the switches and the values of the resistances.

When the switches are open, the following applies to the voltage Ue:

U(suffix)e = U(suffix)const * (r(suffix)1 + r(suffix)2)/(r(suffix)1 + r(suffix)2 + r(suffix)3)

If the following values are inserted:
r(suffix)1 = 110 Ohms, r(suffix)2 = 174 Ohms, r(suffix)3 = 357 Ohms, U(suffix)const = 8 V
U(suffix)e can assume the following values depending on the switch positions:

- Both levels not in order:
i.e. : Both switches are closed and therefore r(suffix)1 and r(suffix)2 are bridged (r(suffix)1 = r(suffix)2 = 0 Ohms)
The input voltage U(suffix)e is therefore approx. 0 volts.
- Both levels in order:
i.e.: Both switches are open, all resistances are effective. The input voltage U(suffix)e is therefore approx. 3.5 V.
- Only coolant level in order:
i.e.: s(suffix)2 is closed, s(suffix)1 is open and therefore only r(suffix)1 is effective.
The input voltage Ue is therefore approx. 1.9 V.
- Only windshield washer fluid level in order:
i.e.: s(suffix)1 is closed, s(suffix)2 is open and therefore only r(suffix)2 is effective.
The input voltage U(suffix)e is therefore approx. 2.6 V.
- Open-circuit:
If there is an open-circuit at a point in the outer circuit, the input voltage Ue is equal to U(suffix)const, 8 V in this example.
- Other defect in system:
If the voltages are outside the tolerance ranges of the above values, there is a defect in the system.

The ideal case has been considered in these calculation examples. In reality values may deviate from the calculated values due to resistance tolerances and dirt resistances which have not been taken into account.

Evaluation
The input voltage Ue reaches the input of the analog-to-digital converter (ADC). The analog/digital converter converts the analog voltage value into a digital value of 0-255 (8 bit converter). This is necessary as the computer can only work with digital values. The computer compares the values supplied with those threshold values stored in a memory module and outputs the appropriate message to the multifunction display (A1p13).
The digital value of the analog/digital converter can be determined with the following formula:

ADW = 255 U(suffix)e/5 V


NOTE: A warning always appears only when the condition exists for at least 60 seconds.







NOTE: There is no message in the multifunction display (A1p13) in the event of a cable open-circuit or broken sensor. However a fault is stored in the fault memory.