How does computer know spare tire is low?

Dear Tom and Ray:

I have enjoyed your column for years and have always admired your bottomless knowledge and advice for car owners, abusers, know-nothings and BYMs (backyard mechanics). I have but one question I need you to resolve, and then I can drive happily ever after. The “Low Tire Pressure” warning light on my dash came on this morning. So I looked at all four tires, and they appeared OK. Just to be safe, I fired up my air compressor (with apologies to my sleeping neighbors) and topped off all four tires. The tire warning light returned! I eventually checked the manual of my Toyota Tundra, and it told me to check the spare, too. I did, and sure enough, it was low. My question to you is: How did it know? How does the computer system know that the air pressure is low in my spare tire? Weight? Sonar? Internet? Seismic vibrations? Make me happy and tell me the answer, boys. — Kim

TOM: It works via a wireless transmitter inside the tire, Kim.

RAY: It was first tried with wires, but they kept getting all wrapped up in the axles.

TOM: There’s a small pressure gauge and wireless transmitter that are part of the valve stem (where you put the air in) in most tires these days. When the pressure drops below a predetermined level, it signals the car’s computer wirelessly, and the indicator on your dashboard lights up. Pretty neat, huh?

RAY: On some higher-end cars, the computer even differentiates between the tires. So the computer can tell you, for instance, that your “left front tire is low.” Of course, that means you have to reprogram the system every time you rotate your tires.

Send your comments or questions for Tom and Ray to: Car Talk Plaza, P.O. Box 3500 Harvard Square, Cambridge, MA 02238. Listen to them Saturdays at 10 a.m. on 91.3 FM or 88.5 FM. Visit them on the Internet at www.cartalk.com.

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Ray & Tom Magliozzi