I think I know, but what I see defies my logic. This is on a 2010 GL 550. I thought about posting on the X164 forum, but decided that tires/wheel might suit it better. If it needs to be moved, admins, apologies for making work for you.

I picked up a set of used OEM wheels for winter tires (w/ tires mounted, but are not in good shape so I am replacing them). The person I bought them from said they had sensors in them. Two of the wheels were leaking at the stems and I asked my mechanic to replace the stems, expecting to see sensors mounted on the bottom of the stem. We (I was there when he broke the bead seam) did not find TMPS sensors in those two wheels. Chances are the other two don't have any either.

I realize there are different types of sensors, but I am rather sure none of these types were in the tire.

The wheels were mounted on the car Monday afternoon. I did not reset the monitoring system (I know how, but I wanted the car to do the work. It seems like it can and should). I have driven the car about 50 km with the winter wheels and have not seen any complaints from the TPMS (yet). I have read that the system might take some time to register the new sensors, but I thought that it should start complaining about the lack of, quickly. Unless the system does not rely on TPMS sensors in the wheels, but the wheel speed sensors (ABS). I can make an argument about why it should and shouldn't use those, but if MB engineers built a car which knows when my pants are getting too warn and dials down the heated seat, surely they knew what they were doing with the TPMS as well.

So which is it? Or is the system working in a third and different way?

The dilemma I am facing is that when I take the car in for the new rubber, since there aren't sensors in the wheels now and the car needs them to communicate properly with the wheels, I can take a set with me (not sure if the tire shop will have a correct type on hand, although a phone call would solve that). In any case, I'd interested to learn what you have to say about the TPMS.

 

I think I know the answers now.... if you don't have the TPMS sensors installed in the tires, the display will not show individual tire pressure values (makes sense), but will still have a message indicating the tire monitoring system is active (somewhat makes sense). From what I gather, Canadian spec vehicles may not have the individual readings for tire pressure even if sensors are present in the wheels, but I am not entirely sure about this. I did not have them with my OE 21" wheels installed, but that does not mean much as I am not sure those have TPMS senders.

 

I believe you have TPWS (tire pressure warning system), also known as "indirect" TPMS. It does not use sensors in the wheels. It uses tire rotation data from the ABS system to detect if any wheel is rotating at a different speed than the others, indicating its rolling radius has changed. It was cheaper to implement but is now not allowed.

 

I think you are correct. I am alright with the lack of individual tire pressure readings. I usually have a good feel for my cars and tend to notice them doing things outside of their norms so I don't necessarily rely on some of the electronics. I was somewhat worried that I might have issues with TPMS. I will forego the sensors and disappoint the tire shop for not spending $200+ on top of the tires.