Low Tire Pressure Alarm

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:

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---

"" wrote

Read Bob's post for how your vehicle works. On some other GM vehicles, there are actually sensors that are built into the valve stems (very special valve stems) and they are also small transmitters that will send a signal to the vehicles computer if a tire is low. Whenever you rotate tires on this type of system, you must relearn each tire to it's new location. Special tool for it, and wouldn't you know, it doesn't come with the owners manual.

Ian

Thats really "rocket-science" amazing ! Thanks so much for the explanation.

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:

to relearn the tire sensors on a vette, just use a regular magnet.. using the prompts on the dic, it works like a charm.... Bobo

"Bobo" wrote

Hey, good idea....Bobo. I realized that the tool is simply a magnet, but didn't pay much attention to the fact that probably any magnet will work.

Ian

wrote

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

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.

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---

"Bobo" wrote

Sounds a lot like all sorts of "special" tools that manage to walk off, or get destroyed in our tool room. I was looking for a special seal driver the other day, found it in a corner by sheer luck, went to use it and discovered that some silly ass had used it incorrectly and mangled the end that fit over the nose of the crankshaft. I fixed it and then promptly put in a "special" place....ie: where I can find it and use it properly.

Ian

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.

When my friend had the flat tire on his Corvette, it was not completely flat. It was only flat on one side.

---Bob Gross---

"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

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:

Most tires usually just get flat on the bottom, never heard of one gutting flat on the side ;)

mike hunt

Robertwgross 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---

I agree, the mechanism does not read airpressure but rather as explained very well below. It is important to note that if all tires are low in pressure then this mechanism will not detect low pressure. It works on relative information on all 4 tires. Also if tire replacement was done on a single tire than the the reading would be incorrect. The onboard computer interface allows for the user to set this function to bias for changes im the information sensing. You need to setup proper tire pressure then reset this feature in the computer, then the seetings are bias for all 4 tires.

I have this > >While reading the owners manual, I find that