Another tire pressure question
 

Just bought a used 2010 C300 4matic, love the car so far. But my question is what activates the tire sensors, is it rotation of tires that trigger something like a mercury switch in the sensor or the cars on board computer that activate the sensors then monitors the pressure? I was told by service tech the sensors have a sleep mode to save battery life but could not get a answer about what activates the tire sensor. Thanks

I have a 2011 C300 Sport and wonder just how accurate the sensors are!

Please, Mercedes Guru correct me if I am wrong....I did some research about it and found there are two type of TPMS (tire pressure monitoring system). The one I got on my C350 2010 (Canadian car) got no sensor on the wheel itself so I cannot see the exact pressure inside each wheels. What happen is if the ESP (electronic stability program) detect one wheel not going at the same speed as the other for a certain period of time and in certain condition it will trigger a signal for tire low pressure (very low pressure tire will not turn same speed as the other) .... But if you got the other type, the one that indicate tire pressure, the pressure is simply transmitted through radio wave 315 mhz via a small transmitter install in each wheels...The battery is suppose to last 5 to 7 years...

You're correct, it's the TPMS. I don't know about the rest.

Mine shows the pressure of each tire, but I suspect accuracy....

[QUOTE=Johnathanswift;5476108]Mine shows the pressure of each tire, but I suspect accuracy....[/QUOTE

Easy to test, just compare with a tire gauge!

No, you can't do it easily: the on-board system does not show pressure until the tires are warm, so you can't get the cold pressure. Now, I have found the warm pressure to be within a pound or two, so I reckon that's good enough. So, no, you can't directly compare cold pressures, and can only speculate that the relation with warm pressure is linear.

I have the same system on my 03 BMW M3 like your Canadian car, my U.S benz has the four sensors. Thinking outside the box (Can't take credit or blame) a guy built a small cylinder (Home Depot- pvc pipe with caps) with a tire valve attached and put his original 4 sensors matched to his car in the cylinder and sealed it up and inflated to his car tire pressure. He then just put the small cylinder in his back seat. If you swap out tires alot or don't want to buy extra sensors for snow tires this might work. But then you would not have protection and would just have to go back and check pressure manually like the good old days. How much does Benz want for a sensor? You might also have the ability to replace a battery with this method since you would not have to mount it back on tire and could disassemble unit and install new battery and put back in cylinder. They say 5 to 7 years mine went bad in 2 years because of mileage and living in northern climate killed battery.

I didn't know you wanted to be so precise....I don't have this system on my Mercedes but I have it on my Porsche and it read all the time??? Strange set-up.....

Well, for whatever reason, MB engineers wanted the owner to have warm pressure, only. I'm no engineer, so I express no opinion....

Mercedes product is new to me and I am always happy when I'm learning something ...Thanks for this clarification!

That's really funny, but a good way to get the annoying tire pressure message in front of the driver off!

No warning lights as the computer is sensing proper air pressure from the sensors in the pressurized cylinder. The computer does not know that the sensors are not in the tire. I would think if you removed each sensor from your rim and did this you should label each sensor as to what wheel it came off so it would be easier to trouble shoot as each sensor gives a psi display for each tire. Example would be if left front sensor went bad and gave a message to that fact you could identify the sensor that was already labeled left front instead of guessing.

Sorry Elto, I just added OFF, mean warning OFF, sorry.

Now, a related question: why do you need to "reset" the tire pressure function? I mean, doesn't it read in absolute terms?

I was curious about your question and found in the owner's manual "when" you should reset the system.

Yes, but why?

I speculate that it is because changing the inflation of one or more tires, up or down, may trigger the alarm since it works on pressure differential.

Agree.

I just found out the Canadian system is completely different (why ???) and it is called "tire pressure loss warning system" which work with rotational speed. And USA got a system called "Advance tire pressure monitoring" and got sensors in each wheels....

Well, apparently a statute or regulation mandates "them" in the U.S., and very likely the regulation provides the technical specification, too.

My C starts the reading in about 30 seconds, about one small block from my driveway. I think the tires are still pretty "cold" at that time.

Well, mine seems to take a bit longer: my cold pressures, with a very accurate dial-type instrument (which has a pressure relief valve, too), are 30/35, and when the on-board gauge comes on, they are already 2 pounds up, and usually wind up between 3 and 4 pounds up, so I'm a bit suspicious and will consider them as "accurate" only for the pressure differential which triggers the alarm.

would appreciate some input if possible

C300 4 matic 2009 with the tpms that doesn't show individual tire pressure on the dials

Now using correct pressures at 30 front and 35 rear, as it's got colder over the last few days the tpm warning lights have come on..more so as the tire warms up and in the evening

When we bought the car a few months ago all 4 tires were at 35. I check the tire pressures at cold and no punctures or leaks

an odd thing..this has only started to appear since fitting hids, could the canbus ballasts be interfering with the tpm signal?

Any suggestions?