Functional description

The oil condition sensor is attached to the oil sump and is accessible from below. The oil dipstick has been deleted from all of the new engine series (except diesel engines). Electronic oil-level monitoring is now a feature of all engines.

The oil condition sensor consists of 2 cylindrical condensers. The condensers are mounted above one another. 2 metal tubes are inserted one into the other to serve as electrodes. The engine oil is used as a dielectric medium between the electrodes.

Fig 1: Identifying Temperature Sensor And Plug Connection

Courtesy of BMW OF NORTH AMERICA, INC.

A dielectric is defined as a non-conductive material in an electrical field. An insulator penetrates the electrical field. Permittivity (lat.: permittere = permit, let through) is also referred to as the dielectric conductance. The permittivity specifies the degree to which materials allow electrical fields to pass. The factor indicates by how much the voltage at a capacitor drops when a dielectric, non-conducting material is arranged between the capacitor plates.

The temperature sensor is seated on the housing of the oil condition sensor. The housing of the oil condition sensor contains an electronic evaluation unit. The electronic evaluation unit has self-diagnosis. A fault in the oil condition sensor is entered in the fault memory of the engine control system.

The electrical material properties of the engine oil change as the engine oil wears and ages. The changed electrical properties of the engine oil (dielectrics) cause the capacity of the capacitor to change.

The electronic evaluation unit converts the measured capacity into a digital signal. The digital sensor signal is sent to the engine control system. The engine control system uses the signal for internal calculations (e.g. condensate in the engine oil).

The oil level is measured for the electronic oil level check. The 2nd capacitor in the upper part of the oil condition sensor registers the oil level with the engine running. The capacitor is at the same level as the oil level in the oil sump. As the oil level changes, the capacitance of the capacitor changes. The electronic evaluation unit creates a digital signal from this. The engine-management system employs this as the basis for calculating the engine's oil level. The electronic oil level check is displayed on the Central Information Display (CID) as well as in the instrument panel. On vehicles without a CID Central Information Display the oil level is only shown in the instrument cluster.