Heres the definitive answer to all who so wonder about their TPMS (Tire Pressure Monitoring System). First of all, your light coming on MEANS something. On page 121 in your manual, theres a discussion about it. There is also a section in there about RESETTING your TPMS light once you have gotten the right tire pressure. The only thing they dont discuss is HOW to reset the LIGHT properly, and why it has to be set this way.
Heres HOW.
Open your Drivers door, look for the Tire Pressure Sticker. Mine says COLD 26 Front, and 32 Rear. Its important to observe these recommended pressures, because you can really lose control of a little car on the interstate with WhacKed out tire pressures. Dont listen to the Dealer if they say, just put all your tires at 34 and forget it.....thats a big mistake. The Tire Pressure Monitor can be set, and Re Set to the CORRECT pressures easily. So....first, get a GOOD Dial type guage at AutoZone or wherever, and put the RIGHT pressure IN your tires after they have set ALL NITE. COLD. Use a portable pump and dont drive to the gas station, because that will heat up your tire. Set em right .....COLD.
Next. Now you have to let the TPMS system know what u have done. Heres how and why. Drive your SMART out on the open road for at least 20 minutes, at a consistent speed, like 60, so the tires are of uniform temperature. NOT 30, then stop, then 55, then 40, but at a CONSISTENT 20 -25 Min speed. Now everything is warmed up, and not fluctuating inside the tires. NOW. Push and HOLD the 3rd Button to the RIGHT of the Emergency flasher on your dash, PUSH AND HOLD FOR 15 Seconds, and watch the TPMS Light first Flash, 5 times, then go OUT. KEEP HOLDING until your 15 seconds is UP, then release. You should now be RESET at the Factory Monitoring position AT the pressures YOU put into the tires. NOW, STAY on the ROAD for at least 10 to 15 minutes longer at the CONSISTENT SPEED as before, and monitor your TPMS light. If it stays Unlit, then you are good to GO. If the Light comes ON again, it means you didnt have the TIRES warmed up consistently, and the tire pressure was not stable, probably still expanding and warming. So, Just RE SET again. Push the 3rd Button to the right of the Emergency Flasher, ( your TPMS reSet Button) , for 15 seconds AGAIN, WHILE youre driving, and HOLD it, and let the TPMS Monitor Light to the Left of the Speedometer Flash 5 times, the light stops, then All will be at peace.
These Monitors are very sensitive. After all, they are a Mercedes Benz product. As such, they will respond to changes in temperature, pressure, etc. BUT, the real reason they work accurately, is that ALL the tires are within the RANGE you set. The system monitors differential pressures, and warns if the pressures are out of balance. Setting the pressures on the tires to make the light GO OUT is the reverse of what you want here. YOU control how much pressure is SUPPOSED TO BE IN THE TIRES, then use the Monitor to tell you if and when it ever gets out of balance. Hope this helps.

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An interesting post, and quite timely! I got a TPMS warning on my way back from town a few days ago, and checked pressures to find that the RR tire was down to 30 psi. I used the onboard pump to get back to 36 (as per door post), and all was well.....except......

Next morning, I found RR tire flat! Careful inspection revealed a self-tapping screw driven straight into tread. Rats! Used pump again to pump up to 36, and set off for my local repair garage - about 25 miles away. TPMS light was out as soon as I got it back to 36 psi, and has stayed out since the repair - a plug - was put into tire, and re-inflation to 36 psi occurred.

It is not clear to me why any kind of reset is required. Can you explain please?

Well Dave, you are VERY right. Why would you want to reset this? ANSWER; YOU DONT. WHY ? the TPMS is Already SET at the pressures you want to maintain. GET IT? So... If you have a piece of metal in your tire, and it goes FLAT, or pressure decreases, the MONITOR lets you know. Since you repaired the tire, and put the original amount of air in the tire (36psi per your Label), and the TPMS is already set at that pressure, the light doesnt go on to tell you something is wrong !! So, Now You Know, and go out and PET THAT DASHBOARD, and say GOOD TPMS !!! HAHAhaha. Happy Smarting around !! DR ALEX
Alex, by your method when the tires cool down to a lower temp after the "20 minutes, at a consistent speed, like 60," the TPMS light will turn on! due to your setting the temp "mark" at the HOT temp and not the cold temp......
Well SATO, you have a point. I found that the TPMS seems to prefer you set the RESET at the HOT point, which, from my logic, (Pretty Frail Logic ) , would constitute the UPPER range. So, you inflate at COLD, set at HOT....and it works. From all my tinkering, it seems this is how the TPMS works. There has to be that RANGE. Hope that helps. If not, GO AHEAD and set it at cold, see if you have a light when it warms up outa range. Then , if that light going off bothers you, you can always do my method. Let me know !! Thanks for your reply. DR ALEX
Great information. My indicator light just came on and I just purchased a guage on the way home.

Well, Mr Bad, Physics cant be beat, and I hope you studied some, b/c Temperature and Pressure are related. Dont make a big deal of it, but the TPMS is like a pH meter. It has to have a slope to do its job. One setting LOW, (cold) and then the Second setting HIGH (hot). Im sure the factory doesnt set this, nor the dealer, so you better learn how it works. and apply your knowledge. Set your Tires COLD to the Placard in the driver's door. Front/Rear. This is your COLD setting. Next DRIVE your smart up to 20 min to get a consistent HIGH TEMP, (and just for you Mr Bad, the higher temp = a Higher pressure in the tire, and mash that TPMS button for 15 sec, and THAT is your high set point. Now....IN THAT RANGE, if the tires go OUT of range by 10% or whatever u say, then it trips the TIRE PRESS MONITOR to display. Hope that helps, and I didnt Inhale. HAhA !! Happy Motoring. And Hope that helps. Dr Alex

mr bad example said:
the factory doesn't set the TPMS like that! what kind of stuff are you smoking? every smart would come with a minimum of 35 miles on the odometer!

the TPMS goes off when a tire goes 10% below the "set value", going above the set value is normal during operation, setting the value when hot guarantees you'll get a display error for sure. the TPMS senses pressure and doesn't give a crap about temperatures.
the light turning off does not constitute a reset. it means that the "SET" pressure has been reached again and thus the warning light goes off. Alex, the sensors might be programmed to give warning when their "SET" pressure changes on the low side... and maybe not on the high side. I dont know for sure, but it would make more sense.
It is my understanding ( and I may be wrong ) the tpms only reg. low tire pressure.
I set mine per manual,& have never had a problem.
As for reason that we have them, it's a bit of protection since we don't have a spare. ( in my oppinion)
Theodore Emilio Montecillo said:
I didn't say the TPMS was reset by adding air. I simply stated that the light went off when I added air to the tire with low pressure which brings the tire pressure to a manufacturers desired threshold. Our cars have a very simple system. It simply tells us A tire has less or more than what the manufacturer recommends. The Corvettes system tells the owner which tire needs assistance. However all of the arguments I've read here seems to suggest that we can or should reset the system to pressures to levels other than what the manufacturer recommends. I go for following the manufacturers recommendations. Especially while the the car is still under warranty. Ted.

And with that in mind realize that smart has various conflicting pressure recommendations for the car. first off there are different tire and wheel sizes with tires that have different load ratings... than you also have the door sill psi label, the book, and the technicians manual... witch vary in what they recommend! We are trying to get at a general consensus to what is best but must compare apples to apples to do it right.

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