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[tccox]

Coldest night in 20 years last night in Charlotte, got down to 10 degrees f. My 128 had to spend the night outside of its garage for reasons beyond my control. Got up this morning go to work started my car and immediately got low tire pressure warning. Warning showed all four tires is being low. Tires looked fine and car drove fine so I proceeded cautiously to work a short distance away. When I got home this afternoon I let tires cool off and check tire pressures. All were perfect right on what they were supposed to be. I did the reset thing warning lights went off and all appears to be ok now. Question, is this normal behavior ?is this system that sensitive ?

[bmw1racer]

You should've checked the tires pressures before you drove off in the morning just to see where they were at.

If I recall, the warning goes off if the tires are off by about 20% or so... And if it got that cold, it's no surprise that the tires shriveled up.

[tccox]

Originally Posted by bmw1racer

You should've checked the tires pressures before you drove off in the morning just to see where they were at.

If I recall, the warning goes off if the tires are off by about 20% or so... And if it got that cold, it's no surprise that the tires shriveled up.

But here's the point. When I got home I checked the tire pressures, they were perfect. I did not need to add any air. I did the reset, the lights went off all was well. The only difference was it was 10 degrees in the morning and 27 when I got home.

[Fume]

PV = nRT http://en.wikipedia.org/wiki/Ideal_gas_law

When temperature gets lower, pressure gets lower. In the morning with the cold temperature, pressure in the tires were probably low enough to set off the warnings. Most likely in the afternoon, temperatures were warmer so pressure got back to regular operating range.

[bmw1racer]

Originally Posted by Fume

PV = nRT http://en.wikipedia.org/wiki/Ideal_gas_law

When temperature gets lower, pressure gets lower. In the morning with the cold temperature, pressure in the tires were probably low enough to set off the warnings. Most likely in the afternoon, temperatures were warmer so pressure got back to regular operating range.

Spot on.

I thought about bringing up the gas law, but I didn't want to come off as being a geek.

[CAVU]

Originally Posted by tccox

But here's the point. When I got home I checked the tire pressures, they were perfect. I did not need to add any air. I did the reset, the lights went off all was well. The only difference was it was 10 degrees in the morning and 27 when I got home.

I've heard the TPMS warning is triggered when a sensor detects a pressure value that is greater than 20% less than the pressure value the last time your TPMS system was reset, so that could be the variable in play. The difference in pressure for 10 F. vs. 27 F. is small… perhaps 3 psi, so outside air temp by itself is not the entire answer to your problem. Probably also has to do with the pressure values stored by the car from the last time you reset the TPMS system. Checking your tire pressure when the tires are warm after driving is probably worth another 3 psi.

[Qunadry]

Originally Posted by Fume

PV = nRT http://en.wikipedia.org/wiki/Ideal_gas_law

When temperature gets lower, pressure gets lower. In the morning with the cold temperature, pressure in the tires were probably low enough to set off the warnings. Most likely in the afternoon, temperatures were warmer so pressure got back to regular operating range.

I have not seen the ideal gas law since P Chem in college in 1969.

[Mike135is]

Originally Posted by tccox

Coldest night in 20 years last night in Charlotte, got down to 10 degrees f. My 128 had to spend the night outside of its garage for reasons beyond my control. Got up this morning go to work started my car and immediately got low tire pressure warning. Warning showed all four tires is being low. Tires looked fine and car drove fine so I proceeded cautiously to work a short distance away. When I got home this afternoon I let tires cool off and check tire pressures. All were perfect right on what they were supposed to be. I did the reset thing warning lights went off and all appears to be ok now. Question, is this normal behavior ?is this system that sensitive ?

That's perfectly normal don't worry about it if u get nitrous in ur tires the temperature in the tire doesn't get hot or cold it's the best for when u live in cold climates