After about 8 yrs of owning this thing, I keep getting this message at random times. Most of the time a tire is actually low. But other times, all tires are good.

I have summer and winters on separate wheels, OEM are the winters, aftermarket for summer. I know there are no TPMS sensors in the summer wheels. But I rarely get an error in summer? I just came from outside because it went off again last night. New winter rubber put on the factory wheels about two weeks ago. I checked the pressure and all were at 40. Sticker said 38 rear 32 front, so I did that just to be sure.

Does it measure overpressure as well?? Why only after two weeks. And why don't my summers have an error all the time? How does the system actually work? Because it's only a "reminder" is it not a true TPMS? Just a random message that pops up?

 

Do you park your car near the winter tires?

 

The only false reading I had was on a roadtrip when 1 tire read zero… small panic but when I got out and read the pressures everything was ok. Like Max pointed out did you get a reading from your spare tires??? Back to my situation .. I carry a spare one in my glove box from a mishap at Sammy’s club .. another story for another day. So I figured on a start up it must have read that one… never did it again ?

 

Doesn't sound like your system is working correctly. Technically you should be getting an error if it can't communicate with the sensors, since you don't have any in your summer wheels. However, TPMS needs to be regularly reset by going into the service menu so it stores the current pressures as the reference values and discovers the new sensors for those who swap wheels.

Are you sure your TPMS system even uses sensors? There are technically two types of TPMS systems. They are referred to as direct TPMS and indirect TPMS. Direct uses sensors in the wheels to directly measure the tire pressure, and temperature in many cases. Indirect doesn't use sensors in the wheels. It uses the ABS sensors to detect wheel speed differences and more modern indirect systems also analyze oscillations in the wheel assembly that are directly tied to tire pressure. Then a signal processing algorithm can determine if one or more tires are low. Older systems could only detect if one tire was low, whereas the newer systems that analyze oscillations can detect if one or more tires are low. Indirect TPMS however can't measure the absolute pressure. It can only detect a 20-25% deviation from the learned base parameters. Therefore indirect TPMS must be reset every time a wheel is replaced or the tire pressure is adjusted. Direct TPMS only needs to be reset if the tire pressure is changed or the sensors are replaced, but it's good practice to reset it regularly anyway, to make sure the reference values are correct.

Mercedes moved to direct TPMS in 2014 based on what I can find via Google. Before that they were using indirect TPMS. However, some models may have gotten direct TPMS sooner than others, so I don't know which system the GL used, but the first step would be to find out. It could also be that somebody recoded it since you are not using sensors in your summer wheels to avoid the constant error messages that you should be getting complaining about the missing sensors.

 

no, they are stored at least 200 ft away from the vehicle.

Thanks for that info on TPMS, I must have indirect, as I do not have sensors iplace of the valve stems. I looked when I had the new summer rubber put onto the oem wheels. (I assumed I missed spotting the sensors and assumed they were glued on the inside of the wheel somewhere) This helps a lot of confusion.

THANKS, so I guess I have Indirect TPMS. Question answered.

 

An easy way of telling is direct TPMS uses metal valve stems and indirect uses rubber valve stems.