Tire inflation dashboard warning....

There can be more than one way to sense tire inflation.

One of the commonly used techniques is to sense the rotation of the tire which is related to inflation.

There are others.

There are a couple methods. One uses sensors in each wheel (valve stem or attached to the rim center usually) That one tells you actual pressure on some vehicles as well. The other method uses the ABS wheel speed sensors to count tire revolutions. The computer looks at the difference between tires and if it reaches a particular level for a set time it turns on the light.

Depends on what model car it is.

There's a third one: it checks the rate of rotation of the tires and sees if it's the same. This will effectively detect ONE underinflated tire, but not two.

Since you don't mention what model car it is, or what the tire pressures really are, my guess is that you have one or more underinflated tires.

--scott

"Proctologically Violated" wrote in news:4925fa7e$0$20299$ snipped-for-privacy@cv.net:

Yep. Your friendly federal NHTSA has dictated that your dashboard should become alarmed when your tire pressures aren't just-so, and warn you thusly.

There are two monitoring systems that are (currently) legal:

1) ABS-based. This has rubber tire valve stems. This system compares rotations of each tire relative to each other via the car's existing ABS system. A discrepancy beyond an expected limit activates the warning system. 2) Transponder-based. This system uses steel valve stems. Each tire (sometimes including the spare) independently communicates wirelessly with the car's receiver, which determines from its several inputs whether a tire (or more) is out of spec. A discrepancy beyond an expected limit activates the warning system.

You haven't got your own tire gauge? Are you relying on the gas station's highly-inaccurate pump gauge? Why wouldn't you check all the tires at once?

How much variation in rotational speed would you expect from under- or over-inflation by a couple of PSI? Is such a difference distinguishable from the effects of differential speed whenever you turn a corner? To avoid false-triggering it may be designed to take an average over a long period of time, but that would prevent the system detecting deflation at the earliest possible opportunity.

Isn't it just as likely - or perhaps even more likely - that your own gauge could be inaccurate? At least gauges in garages have to be checked and recalibrated at frequent intervals. When I discovered that my tyres were about 5 PSI too high after filling up at one garage and then noticing a rough ride, I mentioned it to the man at the garage who turned pale, hastily wrote out an "out of order" sign and stuck it on the machine. I think he was afraid I was going to report him to Trading Standards. So garages do take calibration of their gauges seriously - at least here in the UK.

The main thing is to make sure that all the tyres (or at least those on the same axle) are the same pressure. If they are all systematically a couple of PSI too high or low, that's less serious than if one is wrong and the other is correct, especially on the front wheels where it can cause the car to pull to one side.

If I suspect that a tyre is low while I'm on a journey, I will take a reading of the pressure on the other tyre on that axle and pump the low one to that reading, even if it's a couple of PSI higher than the correct pressure, to allow for pressures being higher when the tyres are hot. Then I'll re-check once the tyres are cold, and make any adjustment.

Actually, it's not my car -- loaned someone my truck, took his car. Just noticed one really low tire, was in a rush.

I've got my own shop compressor, cain't bear the thought of paying for air -- still remember the good ol days. Not to mention that filling tires from a "real" compressor with a tank goes much much faster.

But, my observation that the warning lite did not occur until I had filled the one tire pretty much confirms the ABS-based system described. The car is an Impala, about 2000 give or take, 185,000 miles.

Which means *both* front tires were low! Perty inneresting.

Good info.

Tegger,

You always seem to have good info over the years... I just picked up some steel wheels and snow tires for my 08 Altima. I had to go across the border because of the current QC tire crisis. Anyway. the tire guy told me that he wouldn't mount my rims because it would bypass the low pressure warning system. He said that it is illegal to mount a steel rim without the pressure sensing sensor on a vehcile designed to run with this system. My question... Is it possible to swap this gizmo on to a steel rim or to bypass the light??

Sorry to hijack the thread but it seemed at least at little on topic...

Thanks, Andrew.

"Andrew" wrote in news:v51Wk.56$ snipped-for-privacy@read1.cgocable.net:

Don't know a lot about Nissans.

Do you have rubber valve stems or steel? If rubber, there's no reason you can't install new wheels, as there's nothing inside them to do with the TPMS.

If you do have transponders (steel stems), will they not fit the new wheels? Did the tire guy even try it? I've known people who have swapped the transponders to their new wheels without trouble.

I don't know where you live, so I don't know if your jurisdiction has a mandatory safety check or not. If there is a safety check, you will probably fail that check if you disable the pressure warning system, if the inspection covers that system.

Federally, in Canada, tire pressure warning systems are not legally required until Septemper 1, 2009. If you have no provincial safety inspection where you live, nobody would know you disabled the TMPS if you didn't tell them.

Tegger wrote in news:Xns9B5F4CDF837B0tegger@208.90.168.18:

Of course, even if the transponders do fit the new wheels, there then wouldn't be much point in getting new wheels in the first place; you'd just need to swap the transponders back again in the spring.

In that case you might as well just put your snows on the original wheels, unless you can find used transponders at the wreckers. New ones are pretty expensive, I understand.