Signal to Noise: Bosch Oil Sensor

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In some cars, the oil level, oil temperature, and oil quality are all analyzed using a single oil sensor. The sensor is installed in the engine's oil pan.

In a Mercedes-Benz running Bosch motor electronic (ME) 2.8 , the oil level measuring range is 80 mm, which is the normal level plus or minus 40 mm. The measurement accuracy is plus or minus 3 mm.

Oil quality is determined by the electrical capacitance of the engine oil (engine oil can store an electrical charge). The charge level increases as the oil quality deteriorates. By measuring that capacity, the engine control unit (ECU) can determine the quality of the oil

All three checks are performed independent of each other. The information is continuously transmitted using a pulse-width modulation (PWM) signal from the sensor to the ECU. From there, the information is sent via the CAN to the instrument cluster.

Each information block consists of three successive square-wave signals, followed by a short pause. This is followed by another short pause. A measured variable is assigned to each square-wave signal (A, B, C). Using these signals, the ECU can inform the driver if the oil level is too low, or when the next oil change should be performed. In some applications (this is well known of BMW V10's), Bosch ME reduces power until the oil is up to operating temperature.

Faults at the oil sensor are detected by the engine control unit and entered in the fault memory.

The current for the oil sensor (about 5V) is supplied by the ECU.

These are the signal assignments from the oil sensor:

On/off ratio signals OK
A: Oil temperature 20 to 80%
B: Oil level 20 to 80%
C: Oil quality 20 to 80%
On/off ratio signals is greater than 80%
A: Oil temperature is greater than +160°C
B: Oil level is greater than 80 mm
C: Oil quality good
On/off ratio signals is less than 20%
A: Oil temperature is less than -40°C
B: Oil level is less than 0 mm
C: Oil quality poor

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