Drove a late 90’s Buick Park Avenue the other day “loaded” (the car, not me).
Checking out the electronic gauges. Usual stuff—but then I see “tire pressure normal”.
Ok think I —how does it know the tire pressure is normal on a car with rotating tires?
I check the owner’s manual. Somehow the car’s computer can tell whether one tire is low on pressure or not by comparing it to the other tires. (if 2 tires are low, you are screwed, because it can’t tell anything that way) Owner’s manual said that it somehow compares the rotation speed of the 4 tires and can tell if one is off compared to the others===low on air.
I am lost completely on how they can do this. The wheel and tire are rotating- no way to have a fixed sensor–how can any sensor pick up anything at all? I could even sort of understand a height of the car sensor on all 4 corners which could actually work somehow. But doing it by checking the rotation comparison of the 4 wheels?
However it may work, sounds like a very expensive system for very little benefit. (I have always been able to tell a tire was low by how the car rides).
There are at least a couple different ways of monitoring tire pressure. (The link has two articles explaining the two ways described below.)
One way is with a sensor inside the tire and an RF transmitter to send the data from the rotating tire to the vehicle. (A direct coupling at the wheel hub would be possible too; I don’t know if anyone does that. The Humvee has a system that even allows the tires to be inflated while moving, using a rotating pneumatic coupling. Google “Humvee CTIS” for more details.)
But this car is using a different system, measuring the “effective” tire radius (the distance from the hub to the pavement) by measuring the wheel rotation. Suppose one of the tires is completely flat so that you’re riding on the wheel rims. The effective “radius” of that tire is now the radius of the wheel (the rubber isn’t doing anything). So that wheel will have to rotate faster than the fully-inflated tire to keep the car on a straight course. For a partially-flat tire the difference won’t be as large, but since the distance from the wheel hub to the pavement is lower for a flat tire, the effective radius is smaller and the flat still has to rotate more quickly.
This only works if the car is traveling a straight course. If you’re going around a curve the wheels on the outside of the curve rotate faster even with both tires inflated properly. Probably the measurements are only done when the steering wheel is set for a straight course (otherwise you could get a warning by driving in circles for a while).
Tires inflated differently rotate at different speeds. The lower inflated will rotate faster so it is just telling you the tires are inflated differently, that’s all. It has no way of knowing which one is right or wrong, just that they are different.
Wheel speed sensors are not all that amazing, as modern electronics go. They’ve been a key component of ABS brakes for a number of years.
From http://members.aol.com/carleyware/library/abs1.htm (partly paraphrased), here’s how they do it:
The wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor ring (sometimes called a “tone” ring). The [pickup] may be mounted in the steering knuckle, brake backing plate, or other non-rotating part…The ring may be mounted on the axle hub behind the brake rotor, on the brake rotor itself, or other rotating part…The sensor pickup has a magnetic core surrounded by coil windings. As the wheel turns, teeth on the sensor ring move through the pickup’s magnetic field. This reverses the polarity of the magnetic field and induces an alternating current (AC) voltage in the pickup’s windings. The number of voltage pulses per second that are induced in the pickup changes in direct proportion to wheel speed. So as speed increases, the frequency and amplitude of the wheel speed sensor goes up.
The tone ring and sensor pickup have to be close enough for the former to affect the latter’s magnetic field, but they certainly don’t have to touch.