"Harry Krause" wrote in message
...
Out of curiosity, might someone who actually knows share how those "tire
pressure low" indicators work?

I don't mean the ones you put on your tire valves; I mean the ones that
come stock on some new vehicles, and set off a warning light on the dash
when a tire is low.

Oh, and I'm thinking of getting a set of valve pressure indicators for my
boat trailer tires. Any recommendations?

Thanks!



Most are mini transmitter devices. My wife's car still has the blue
stickers on the rims with the transmitting frequency printed on them. (BMW
750)

However, not all are. Are you ready for this? Here's how it's done on my
BMW M5:

You, or the dealer inflates the tires to the proper pressure.

You enable the flat tire monitor (on BMW's this is done through a computer
interface called "I-Drive". This calibrates the system.

As you drive around, a sensor on each wheel records the wheel rotation and
compares it to the other three wheels. Each sensor compares to the other
sensors, in other words. If one tire starts going flat, it's number of
rotations per unit distance will be different than the other three wheels.
If this difference goes out of a predetermined window (allows for turns,
etc.) the flat tire alarm will notify the driver.

I had to read this three times in the manual before I'd believe it. German
engineers have a reputation for getting carried away, but this is
ridiculous.

RCE