Wiper and Washer Systems: Description and Operation
GF54.30-P-3006A Voltage Coding Of Engine Coolant/windshield Washer Fluid Level, Function
General
The switch shown in the figure is used to determine the level of engine coolant and windshield washer fluid. The coolant level switch (S41) and windshield washer fluid level switch (S42) are connected in series. The information from both is transmitted via a single line to the instrument cluster (A1) and is voltage-encoded.
Voltage coding
Switches S1 and S2 are connected in parallel to resistors r2 and r3 to allow for voltage encoding. (The values of r2 and r3 must be different).
When the switches are open, these resistors, together with r1 of the instrument cluster, form a voltage divider which is supplied with a constant voltage u(const).
The voltage at point p therefore depends on the position of the switches and the values of the resistors.
When the switches are open the voltage at point p is given by:
u(p)=U(const) * (r2 + r3) / (r1 + r2 + r3)
Using the following values:
r1=357 Ohms, r2=110 Ohms, r3=174 Ohms, u(const)=8V
u(p) can take on the following values according to the position of the switches:
- Both coolant and washer fluid levels not OK:
In other words, both switches are closed; r2 and r3 are therefore
short circuited (r2=r3=0 Ohms)
Thus, the voltage at point p is about 0 Volt.
- Both coolant and washer fluid levels OK:
In other words, both switches are open, all resistors are effective.
Thus, the voltage at point p is about 3.5 V.
- Only coolant level OK:
In other words, s2 is closed, s1 is open and therefore only r2 is effective.
Thus, the voltage at point p is about 1.9 V.
- Only washer fluid level OK:
In other words, s1 is closed, s2 is open and therefore only r3 is effective.
Thus, the voltage at point p is about 2.6 V.
- Interruption:
If the outer circuit is interrupted, the voltage at point p is the same as u(const), in this case 8 V.
- Other fault in system:
If the voltage is outside the tolerance zones of the aforementioned values, there is a fault in the system.
The previous examples looked at the ideal case scenario. In reality, however, the values may deviate from the calculated values due to resistance tolerances and the influence of dirt on the circuit not taken into account here.
Evaluation
The voltage at point p reaches the input of the analog-to-digital converter a. The analog-to-digital converter (ADC) converts the analog voltage into a digital value between 0 and 255 (8-bit converter). This is necessary since the computer can only work with digital values. The computer compares these values with the threshold values stored in a memory chip (EEPROM) and then activates the corresponding indicator lamp(s) (A1e11) or (A1e13) accordingly. The digital value can be determined using the following formula:
As a precautionary measure, values which cannot be assigned to an ambiguous state (due to a fault in the system) are taken to indicate low coolant level and the low ECL indicator lamp (A1211) is actuated. A warning appears if this state is detected for at least 60 seconds.
For vehicles with electric suction-type fan (engine/AAC) (M4/3) only, e.g. M111 engine
The low ECL indicator lamp (A1e11) also comes on if a fault occurs with the electric suction-type fan (engine/AAC) (M4/3). In this case, the lamp is actuated via the CAN data bus and the engine control module.
Diagnosis with HHT
If diagnosis is carried out using a HHT, the digital value (from 0 - 255) is displayed. In addition, the HHT also indicates the possible error by means of value tables.