Repair-It

31,000 miles later, BAM, ABS inop. Near 100% chance this is a wheel speed sensor which is supposed to be a common issue on the W221.

So I'll put this here. I will scan next opportunity.
https://www.fcpeuro.com/Mercedes~Benz-p ... ic&b=Sedan

I would price out Mercedes but these are affordable.

That thing I said. I guess it's one of the rears. Could just do both.

It hasn't happened again. Would be nice to catch it in the act but I may still just do both rear wheel speed sensors at next oil change.

Still nothing.

Haven't been driving it a ton but still has not recurred. It is due for an oil change so I have to decide if I want to preventatively replace that wheel speed sensor or let it fail for real.

The S550 is looking for attention so this happened. Left rear. Both rear sensors are A2215400117.
https://www.mercedesbenzpartsshop.com/o ... 2219057700

Core charge? Weird.

https://www.fcpeuro.com/products/merced ... 2219057300

Ordered the ATE which says 1/7 with free shipping.

Received my sensor today. Since I was home I decided to tackle this.

Initial inspection of new part 27F outside so I decided to warm the garage. An upside of an attached, normal size garage is I can heat it up really quickly. Working my way to sensor removal. Not too difficult. Removed and comparing original to the Ate and they do both say Ate. Close-up of the E10 head bolt I dabbed some silicone dielectric grease on the knuckle before installing the new sensor. Also dabbed some blue loctite on the bolt. I torqued to 8 Nm per the included instructions. Felt right. Looks like it took about an hour. Reset codes and road tested. No issues. 113,350 miles or something close.

It's been error free. So that's good. If the code came back my next step would have been to blow out the reluctor. After that either wiring or probably the module itself. But hopefully no need to deal with any of that.