[E93Dude]

I did not want to hijack another recent threat on oil changes, and take it off on a tangent, hence the separate post.

There has been A LOT of discussion about oil chage intervals, but I can't find any post that describes how the BMW's "CBS" (condition based service) actually determins when its time to change your oil. I don't think its trigged off mileage. Its it based solely on monitoring oil level? Does anyone know from a technical perspective how the CBS evaluates oil to determin when its degraded enough to warrant an oil change?

[Brian86]

The oil level sensor has two capacitive circuits.

One circuit measures the capacitance between electrodes A and B to determine the oil level. The other circuit measures the capacitance between electrodes B and C to determine how contaminated the oil is.

My understanding is that as the oil degrades the permittivity of the oil also changes, which can be read as a changing capacitance between the electrodes.

It's actually pretty old tech. Stationary industrial and rotary machinery has been using similar quality sensors since the 60s or 70s.

[Chappers 71]

All you need to know

https://www.google.com/url?q=https:/...u3ddFCNwzTE747

[Phil325i]

And this: BMW-OZS sensor.pdf

[E93Dude]

All of these replies are vey informative - thanks. I hate/love it when i *think* I've searched for something, and then others snap their fingers and come up with the answer.

I understand now how the vehicle senses oil level. I also understand now that capacitance changes in the oil fluid determine level of degredation. What I still don't understand, is why a change in capacitance = bad oil. What changes occur in oil that reduces efficacy, that can be evaluated by a delta in capacitance? I realize there were some really smart engineers and chemists in Germany who can vouch that this technique works...and I'll take it on faith that its good to go.

This is a really down-in-the-weeds sort of question (imagine being my parents when I was 9...). No one need answer if folks don't want to bother. I'll go spend time time digging through Bob the Oil Guy....

[Brian86]

Oil's behavior as a dielectric is an extremely well researched subject. Oil is used to insulate high voltage equipment. Every transformer can on every telephone pole on earth is full of transformer oil.

Somebody made the link 50 years ago that degradation in transformer oil changes it's voltage withstand and permissively. It's not a very big leap to start using sensitive capacitive circuits to measure oil quality.

As to what changes in the oil, I'm sure the change in the oil molecules themselves, as well as the ingress of fuel, and loss of the waxes that make motor oil multi-viscosity changes the indication, since each of those components will have different permissively and dielectric withstand.

Honestly it was probably more trial-and-error. They took some new oil and tested it, then took some known bad oil and tested it, and came up with the limits from that.

[Brian86]

This is all wasted knowledge on my part. I bought my car well past the 300,000km where the oil CBS system halts.

Now I just run it 10k between changes and top it up every 3,500is to replace what leaks and is consumed.

[E93Dude]

Originally Posted by Brian86

This is all wasted knowledge on my part. I bought my car well past the 300,000km where the oil CBS system halts.

Now I just run it 10k between changes and top it up every 3,500is to replace what leaks and is consumed.

To a large degree its wasted knowledge on me, too. I change the oil every 8-10K based on odometer readings. My CBS has never triggered since I got the car in 2012. This was more about me wanting to understand what - in BMW's mind - consitiutes degraded oil. I don't beat the hell out of my car on the track, so I've never really worried about oil condition. Fixing oil leaks, however, has been another problem entirely...

[Brian86]

As far as I can tell the CBS system works exceptionally well.

According to the BMW service records my N52 got the CBS based oil changes of 15-18k for the first 150k miles of it's life. It's just about to turn 300k now so the long CBS intervals don't seem to have degraded the engine in the slightest.