A day without a tire-pressure warning is like a day without sunshine. At least for me lately.
Actually, with this week's weather in Southern California I've seen more tire pressure warnings than sunshine. The most recent came courtesy of our long-term 2009 Hyundai Genesis yesterday morning. I was up early and probably a good mile from the house before I looked down and noticed the "Low Tire Pressure!" message in the Hyundai's information screen.
After a few colorful phrases I looked closely and saw that both rear tires were flashing in the icon.
Even with my recent tire luck it was hard to believe both rear tires were dameaged and losing air. More likely the cold (for Southern California) temps had bunched all the atoms in the tire's air together, reducing psi and setting off the Hyundai's Tire Pressure Warning System.
I pulled into a gas station and confirmed both rear tires at 28 psi. This was after driving for a couple miles, so they were already hot and no doubt holding more pressure than when I first started the Genesis at my house. I'd guess the cold pressure was probably closer to 24-25 psi. The sticker in the door jam calls for 33 psi. I pumped both rear tires up to an indicated 36 psi to account for the heat-induced pressure in the reading (we all know you should check psi levels when tires are cold, right?).
The warning light went out almost immediately and hasn't returned after 24 hours, but it raises a good point. Most TPMS won't trigger unless a car's tire(s) is at least 5 or more psi low. It took some rare mid-30s temperatures to reach this trigger level, but the Genesis' tires were clearly low before the cold snap hit; likely around 26-28 psi when the ambient temperature was a more L.A.-typical 55-70 degrees. Still far below the recommended level, but not enough to set off lights and bells inside the car.
Check your tire pressures regularly people. Thank you.
Karl Brauer, Edmunds.com Editor in Chief @ 21,445 miles
chavis10 says:
10:51 AM, 12/ 9/09
I too always wondered why the threshold of the TPMS is so liberal. My car is supposed to run 32psi on all four tires yet anytime my warning goes off, the culprit is around 25psi or lower. So theoretically, I could've been riding around with low pressure for quite some time as pressure gradually decreases (unless it's due to a puncture) as the outside temp drops. This is why systems with an ACTUAL real time pressure reading should be mandatory on all cars with TPMS. All cars have displays capable of reading off a few alphanumeric characters so automakers, get with the program. My car has a tell-tale light and a chime and that's it! It's up to me to get down and find out which tire is low which is hard to do with low profile tires unless the tire is damn near flat.
wobbly_ears says:
11:10 AM, 12/ 9/09
I seem to recall having read somewhere that the TPMS is supposed to be triggered anytime the air pressure falls below 25-30% of optimal pressure.
wobbly_ears says:
11:26 AM, 12/ 9/09
On an unrelated comment Karl, did you get a chance to check the keyless system in Genesis? Is it also as stupidly designed as in your BMW 7 LT? (Regardless, it doesn't change an iota of my belief that these keyless things are an expensive gimmick that are better off dead)
jeepsrt says:
11:32 AM, 12/ 9/09
My Jeep grand Cherokee gives me an instant readout of all four tires, which is nice to tell how low a tire is at anytime. Lately though it is showing low as we have been below 0 degrees for the past couple of nights.
subaru123 says:
11:51 AM, 12/ 9/09
Mine usually goes of when they reach 30-31 PSI. Recommended pressure is 33 front 32 rear
prince34 says:
12:28 PM, 12/ 9/09
I've never actually had my TPMS warnings go off, but as mine are easy to check in the DIC, I do this at least twice a week, if not every time I get it the card. Just a few presses of one button, and I've checked all the vitals. If they need a little bit, but I'm in a rush, I make a note and do it the next morning.
yellowbal says:
02:57 PM, 12/ 9/09
Wow, all these fancy tools. I use a tire gauge and a foot pump.
ken428 says:
04:40 PM, 12/ 9/09
@ chavis10, They are designed that way so you don't get false readings. Cold temperatures always drop the pressure around 2 psi until the tires warm up; so if its too sensitive, your tire pressure light always comes on. They will go off immediately on a rapid decline in pressure or when they drop below a predetermined value. TPMS is not a replacement for checking your pressure regularly. Also, TPMS is not perfect since it uses 315MHz as its transmit frequency and they usually have a hard time compensating for altitude offsets. You get a large amount of interference from key fobs, other TPMS systems, etc., so relying on it for accurate data is not recommended.
actualsize says:
08:07 AM, 12/10/09
Here's the deal about TPMS trigger thresholds: The federal law that regulates such systems (FMVSS138, I know it from memory) states that TPMS systems must turn on the warning lamp by the time the tire pressure drops 25% below the tire pressure listed on the standardized door jamb placard.
The Genesis' placard lists the tire pressure as 33 psi, and that makes the minimum trigger point 24.75 psi. At this pressure, the tire doesn't yet look obviously low.
I say minimum because a carmaker can elect to turn on the TPMS warning lamp sooner, say at 15% or 20% underinflated, but doing so increases the chance that temperature swings could cause a lot more semi-false alarms.
Say they ran this car at 20%--that would make the trigger point 26 psi. Temperature swings can make for huge pressure changes (5 psi or more) in places like Minnesota where you might set your "cold" pressures in your heated garage, then drive the car out into sub-freezing temperatures and park it outside at work, where it's really cold. It's easy to imagine a 50 or 60 degree temperature difference, or more.
Ok, so you don't live in the Great White North. Maybe your tires are more-or-less harmlessly down 2 or 3 psi. It could be you last set them a month ago, or maybe your $1.99 Pep Boys tire gauge is off by that much. Maybe you set your tires--when cold (undriven) as we all suggest--at 11:00 am on a Sunday morning, but the temperature is 20 degrees lower when you usually leave for work at 6:00 am on Monday morning. All of this stuff happens, sometimes simultaneously.
A 15% or 20% trigger threshold in these circumstances might leave you only 1 or 2 psi of headroom before you get an alarm, and that's enough for the overnight temperature variation to set the lamp on a cold (or what we call cold) southern california morning until you drive the tires and get them warm.
Karl's guess of 24 to 25 psi at the crack of dawn seems plausible, especially if you accept that his 28 psi warm reading could be off a psi or two because consumer tire gauges aren't terribly accurate. And that puts us right at the 25% deflation point for a tire that should have 33 psi in it to begin with.
Not also that Karl is griping about how often he sees low pressure warnings. If the trigger point were 20% or 15%, this would happen significantly more often. I happen to agree with the FMVSS 138 on this point: 25% seems to strike the right balance between safety, convenience and nanny-ism. Carmakers that have high complaint rates for TPMS false alarms may also be the ones that opted for an earlier trigger point.
That both tires were down simply reinforces that theory that these tires haven't had their pressure checked often enough. As @ken248 says, TPMS is not a replacement for checking your tires regularly. We are generally good at this, but this example suggests we need to be even more vigilant--it's not the fault of the car or the TPMS system.
But keeping track is admittedly tough in an environment where when no individual owns the car and drives it every single day. I predict an inter-office memo in the near future.
colorado kid says:
02:12 PM, 12/10/09
On our 2004 Suburban we call this the low tire tempurature warning light - it indicates that one or more tires is down to 32 PSI from the recomended 35 PSI, which happens any time the car sits for more than an hour with the air temp below freezing. It goes out within 1 mile of drving even at minus 15 degrees. It does this until I get irritated enough to leave the Suburban in the driveway overnight and then inflate the tires to 35-36 PSI in the cold. One time in 5.5 years and 130,000 miles the TPS has tipped me off about an actual (very slow) leak. The other 200 or so times it has illuminated to tell me the tires were cold.