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      11-09-2012, 12:10 PM   #1
MysticblueX3
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Tire Pressure Monitor

So after you fill your tires with air and reset the TPM, how much does the air pressure have to drop before it will throw a warning?

I checked my tire pressure this morning and it dropped from 36 front/ 38 rear to 32 front/ 35 rear. Wasn't sure if that was enough to trigger the TPM.
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      11-09-2012, 12:16 PM   #2
631twentyeighteye
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i believe its 20% below the minimum(what the inside the door says) pressure.
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      11-09-2012, 12:19 PM   #3
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Not sure. At least 3 or 4 psi. Don't worry, that frikken thing will be going off every Winter all Winter. IMO that TPMS is a PAIN IN THE ASS. it's forever going off when the Temp. drops. wish the car never had it. Just another idiot warning for .... (you fill in the blank)
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      11-09-2012, 03:32 PM   #4
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About a 25% drop from the baseline pressures will trigger the wrning. The baseline pressures are the pressures at the time of the most recent system reset - which may or may not be the same as the door placard recommendation.

Tom
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      11-09-2012, 03:32 PM   #5
JimD
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I am pretty sure that you can select how much it has to drop. I believe that was one of the many things we set during delivery back in 2009. I carry a Viair 70P compressor in the trunk so I just pull it out if I get the warning. Only takes a couple minutes and I like having it around in case I ever get a flat.

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      11-09-2012, 08:21 PM   #6
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Not sure the %. I am guessing it's just that, a comparison of wherever it is at a given moment compared to the last reset. I doubt it has any idea of an actual pressure, just that there is or isn't a big change.

When winter hits it does go off. Not such a bad thing though, because your pressure is in fact lower and you should fill the tires back up. It's annoying, but it's "right" for lack of a better word.
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      11-09-2012, 08:27 PM   #7
Stohlen
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2008 BMW 135i  [2.83]
Quote:
Originally Posted by tracer bullet View Post
Not sure the %. I am guessing it's just that, a comparison of wherever it is at a given moment compared to the last reset. I doubt it has any idea of an actual pressure, just that there is or isn't a big change.
Not true, you'll notice that if your tires are too low, aka 25psi or lower for example, that the TPMS will not accept the reset and will still tell you that your tires are low. Most oems use TPMS sensors that provide an actual readout. Not sure what BMW uses but I wouldn't be surprised if its sensors are accurate within 1 psi or so.
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      11-10-2012, 06:14 AM   #8
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For the TPMS system to work properly you HAVE to set the target pressures from time to time. You do this when the car is running but STOPPED and you use the iDrive or BC stalk.

I also think the TPMS light goes off around 20% low. I think it also will trigger when a sudden loss of air pressure is detected. Its a dual system so to speak.

Why not just set your target pressure and then let air out of one tire until the TPMS light come on. Then you know for a FACT when it will trigger.
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