Low Tire Pressure Alarm

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While reading the owners manual, I find that My 2002 Buick Century has a "Low Tire" warning feature.
Seriously, how does it it determine this ?
<rj>
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Generally the wheels are equipped with a strain gage that sends a signal that activates some type of enunciator when the air pressure drops below a predetermined setting.
mike hunt
"" wrote:

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RJ wrote:

Contrary to the opinions of others, I can tell you the truth according to the Helm manual. In the Antilock Brake System, you have wheel sensors that can determine whether each wheel is turning, or locked, or whatever. The sensor does this by counting revolutions of the wheel. As long as all four wheel sensors count the same number of revolutions for a given distance traveled, then it is assumed that all four tires have the same circumference and hence the same tire pressure. You can get into a weird situation if you have mismatched tires and/or pressures. If the system sees (for example) 100 revolutions on three tires, and 99 revolutions on the fourth tire, then that passes. But if it sees 100 on three and only 90 on the fourth, then the system will display a warning. It is calibrated to warn by the time one tire is ten pounds low, as compared to the others. That is a lot. Typically, it will alert before one tire is five pounds low. Mine alerts at about three pounds low.
---Bob Gross---
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Thats really "rocket-science" amazing ! Thanks so much for the explanation.
<rj>
On 24 Oct 2003 19:01:55 GMT, snipped-for-privacy@cs.com (Robertwgross) wrote:

<rj>
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Think about what you are just said then think of how a differential works and what happens when your make a turn. once you have done that you will realize why your theory can not possible be correct. New regulations require all vehicle to have tire pressure sensors and not all vehicle have ABS, is another clue. The pressure detector that sends the signal is inside each wheel.
mike hunt
Robertwgross wrote:

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Phew....you better stop now, my good man. You are way out to lunch and really don't have a clue about what you are talking about. Just drop the subject and folks will forget about your stupidity in a week or two.
Ian
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I've changed more than a few tires in my time, and I can tell you for sure that many of the vehicles with a 'LOW TIRE' indicator do NOT have anything mounted inside the tire to directly measure pressure, they work of the ABS wheel speed sensors. Some systems DO use a pressure sensor with a transmitter inside the tire, but most do not.
From what I was able to glean about how they work, they "learn" the revolutions per mile of EACH tire, and when one varys from "normal" by a certain amount, it triggers the light. Which is why they tell you to reset the system any time you adjust pressures, or rotate tires, so the system can "relearn" the 'correct' settings of each tire.

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I've already gone through the description of how the low tire pressure is sensed in the 2002 Buicks, and there should be no intelligent argument on that.
Now, in contrast, there are other types of vehicles that have a rather individual sensor in each tire. A friend has a Corvette with that type of system. He got a flat tire, so he went to the tire shop (not the GM shop) to have it replaced. The tire shop did not know about the individual pressure alarm that was mounted on the rim. When they put it on the tire machine, they crunched the pressure alarm module, but they did not want to admit their ignorant mistake. My friend picked up the car and drove off. Within a few miles, the tire pressure alarm system warned him (cause one sensor was broken up). He took it back to the tire shop, and they scratched their heads before they finally admitted their mistake. Reluctantly, they agreed to replace the sensor, so they tried to order the part from GM. GM correctly informed them that the sensors are sold as a set of four, since each is slightly different (each transmits a different RF signal to a central module elsewhere on the car). When they got the set of four, they put the wrong one in to replace the broken one, so still the system reported a problem. It went on and on. Finally they got the GM Corvette guy to straighten it all out.
---Bob Gross---
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snipped-for-privacy@cs.com (Robertwgross) wrote in message

That's because the Vette uses "run-flat" tires, which don't change the outer diameter by much even with no pressure inside them. Therefore, the ABS sensors wouldn't be able to catch that reliably, thus the transmiting sensors to gauge the actual tire pressure.
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Neo wrote:

When my friend had the flat tire on his Corvette, it was not completely flat. It was only flat on one side.
---Bob Gross---
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Most tires usually just get flat on the bottom, never heard of one gutting flat on the side ;)
mike hunt
Robertwgross wrote:

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"Neo" wrote

Interesting theory, but the Cadillac's use the same system and they don't have "run-flat" tires. I suspect it's simply a much more accurate system. Which is why it's on the higher end cars.
Ian
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Yeah, it's definitely going to be more accurate, which is probably why the Cadillacs use it. If the vehicle has run-flat tires like the Corvette, though, it needs a system like that - otherwise you might not notice if you had a flat tire..
--
Robert Hancock Saskatoon, SK, Canada
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Just one more reason why one should go to a dealership for service. Goes to show why servicing your vehicle at a dealership is the least expensive usually way to go in the long run.
mike hunt
Robertwgross wrote:

<snip>
<snip>
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Mike wrote:

In my opinion, it depends on the nature of the service. For an oil change, I am simply not going to leave my car at the dealer all day. At another shop, I can get it changed and be on my way within the hour. If owners would really read the shop manual and learn the pitfalls, then when they take their car to a non-dealership, they can warn them. For example, the Corvette owner can warn them to be careful with the tire machine because of a pressure sensor on the rim.
---Bob Gross---
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You must live on the boonies if you have to leave you vehicle all day. Any dealership worth their salt today has a fast lub facility, that easily competes with the others, with more competent people working on your vehicle.
mike hunt
Robertwgross wrote:

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BenDover wrote:

No. I live 3.5 miles away from one dealership, but I live only three blocks from an oil change shop. There is nothing wrong with the dealership as long as you like their business practices and red tape.
---Bob Gross---
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Just about every dealer I've dealt with;
"Make an appointment" ( usually "next week" ) "Leave the car for the day"
And for warranty work, it might or might not get fixed. I think it was their way to discourage customer returns.
I still remember one Chevy dealer; ( my engine check light was lit ) I made the appointment, dropped off the car ( bummed a ride with my buddy ) bummed a ride to work the next day.... and the next day bummed a ride down to the dealers. ( they claimed they replaced ??? ) Picked up my car in the dealers lot On the way out, "ENGINE CHECK LIGHT" drove it back in to the bay...... "Leave the car" !!
GRRRRR...........
On Mon, 27 Oct 2003 14:54:39 GMT, snipped-for-privacy@lycos.com wrote:

<rj>
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Why in the world would any want to discourage warranty work? They get paid by the manufacture to do warranty that work.
"" wrote:

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I don't think warranty work pays as well as non-warranty work.
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