In this entry – let’s talk about oil quality sensor, using an example from a diagnostics session.


Patient: E9X, with N47 series diesel engine. When colder weather came (around -5 oC outside), a problem with starting the engine appeared. The essence of the problem – the battery looked “tired”. But in recent past following actions were performed:

a) replacement of the battery (because of the problem mentioned above);

b) repair of alternator performed, because after a moment the battery looked tired again;

c) oil quality sensor was replaced;

d) IBS sensor was replaced because the error messages regarding it appeared unexpectedly.


Unfortunately, all actions – without success.

Not taking into account repaired or replaced all elements of a BSD data network, the error message memory of the engine was full with error messages regarding all kind of problems of BSD network:


It says, there are problems with oil quality sensor, IBS, alternator. All repaired/replaced elements damaged (again)? That could be a possible conclusion of a “regular” diagnostics specialist when he would see the error messages. And the obligation of the customer – to pay for the replacement of details! But – I acted differently.


When connecting the multimeter, I found out:

in idle, with all large consumers turned off, the voltage on battery clips is around 13.90 V. Turning on the headlights, heating of rear window and seats, and also saloon vent, the voltage dropped to 13.70 V:

The conclusion – the alternator works, but in emergency mode or mode with a fixed voltage of 14.0 .. 14.4 V. It’s clear such low charge voltage is incorrect – it’s no surprise the battery fails. In case of working IBS system (if the temperature of the battery is below +8 oC) it would be 14.8 .. 15.0 V (additionally: the voltage would be maintained on clips of battery, not alternator). A voltage difference between 13.7 and 15.0 V is fundamental! In the first case (13.7 V) – and surrounding temperature around 0 oC the battery is not charged at all! Every time, when starting the car, energy in it will be reduced. Every minute, while the car is turned off – the battery continues to discharge. Every time, when the car will go to “sleep”, the remaining energy of the battery will reduce for additional 2 .. 3 Ah. In a time of one month even perfect (new and 100 % charged) battery will be discharged, not depending on the driving mode (even long and regular drives won’t help)!

To identify the state of the system before repair, IBS system data also were looked at:

As we see – the replacement of the battery is not registered, its SOH has dropped to 0. SOC, data of energetical balance haven’t been available, those, who are – are not correct (because IBS data are not read, see further).

Here, in this image I marked incorrect data with red:

Unfortunately, it comes out – all data are completely inappropriate!


Identification of IBS sensor was checked:

As we see, DDE hasn’t succeeded with connecting to IBS – its data are not available. Performing the test block of the alternator with ISTA D, the message regarding no alternator, which can be managed via BSD, was identified.

Connecting to BSD with an oscilloscope:

I had to identify the following situation:

As we see, some of the units (at least master) send data. So, BSD data channel has no complete short circuit with Ground or wire break. In the same time – “passive” level of the signal is not around 12 V, but only 2 V! It’s clear such BSD cannot perform!

Disconnecting the management signal of the alternator (assuming as a result of possible repair, the management unit of the incorrect model has been installed):

no positive changes observed.


In the next stage, the IBS sensor was disconnected. Also no positive change.

Such problem (with decreased “passive” level of BSD) can be caused:

a) the absence of power-up resistors;

b) damage of some “client” or even DDE/DME management module (for a short-circuiting transistor, which manages BSD signal).


To make sure the problem is not in power-up resistors (BMW don’t give any information, in which modules these resistors are placed), BSD data signal via 1K Ohm was connected to +12 V. Also this state didn’t give any results. However, I got 100 % confidence the problem is in the high load of BSD network.

Note: the architecture of LIN network provides, that power-up resistor should be also in Master unit (it means, DDE), but I assumed I can not trust any of units, connected to BSD.

Next step – disconnecting the oil quality sensor. It’s been replaced recently (a new one, additionally – Hella, not some cheap analog) – what could be happened?

After disconnecting the oil quality sensor:

Here, a correct signal of BSD!

Also the shape of its data package signal look’s fine – everything is in order!

And now also INPA recognizes IBS sensor:

Connecting back the alternator, onboard voltage reached 15.2 V!

Notice: in a menu, the voltage is depicted in mV.

Here, the confirmation of voltage, using multimeter:

So – the alternator however works? Wonderful! Test block with ISTA D confirmed – alternator, which can be managed via BSD, has been found, its voltage can be managed!


In the next step, the battery was registered, using ISTA D (it’s important not to use other tools, even INPA):

After registering we see:

a) the change of battery is confirmed;

b) data registers of SOH and test procedures, energetical balance and all statistics are deleted;

c) calculation of energetical balance and SOC is started.

Note: important – SOC-fit and Ladungzustand aktuelle are the same!


But, returning to the problem of oil quality sensor – what happened? Taking into account DDE had error messages regarding EGR, regarding flaps of inlet manifold (that was additionally suspicious), FUSE 02 (20 A) in the engine room was inspected.

This fuse ensures power for EGR, flaps and also oil quality sensor.

A fuse was in work order; when checking connector of oil quality sensor, I concluded – voltage of 12 V is supplied. Also, BSD data and Ground were connected correctly.


Checking the sensor itself, I had to conclude – according to its code, it been identified as suitable for E46! E46 didn’t have BSD for sure! A conclusion is simple: an inappropriate sensor (intended for vehicles of previous generations) has been installed!


Note: the sensor of the previous generation measured the only level of oil, and the signal was sent in an analog way.

Oil quality sensors of next generation (intended for BSD) are measuring:

a) temperature of oil;

b) level of oil;

c) quality of oil,

and data are sent in digital format, using BSD data exchange network. Although “old” and “new” sensors physically are identical (also plugs are identical), data, which they are able to send, are fundamentally different.


Unfortunately, this time (in part of oil quality sensor), the supplier of spare parts has failed – in its catalogs, this sensor has been identified as suitable for vehicles of E9X series. As we see, the problems can appear in most unexpected places, consequences – quite expensive. The customer replaced the battery once again (because it was unregistered), the repaired the alternator (which was in working order), and now he will have to buy one more oil quality sensor (and oil too). He also has bought IBS sensors (the error message regarding it appeared after replacing oil quality sensor), several times performed diagnostics by different specialists (unfortunately with no success). And in addition to all of that – the battery hasn’t been charged correctly for a while and is already worn out more than it would be if the battery would be registered.