Dear Car Talk:
How does a digital odometer keep its reading when you disconnect the battery? No, I don't work for a used car dealer. – Steve
RAY: It uses something called a PROM, Steve. And it's not the thing that you rented a tuxedo for in high school.
A PROM is a type of computer chip. The letters stand for Programmable Read-Only Memory. There’s a PROM in the odometer itself, which is constantly updated with the new odometer reading.
Each time a new reading is recorded, it permanently replaces the old reading. And it doesn’t require a battery. It stores whatever information was last written on it indefinitely. Think of it like an etching. Except not the kind you tried to invite your prom date up to your room to see.
The odometer’s PROM cannot be tinkered with. At least not by us mere mortals.
When someone comes into our shop with a bad speedometer head, we can’t simply order a new one and install it and set the odometer’s mileage. We have to send it to an authorized facility, where they transfer the old mileage reading to the new odometer and send it back to us.
We have neither the equipment nor the authorization to set a PROM reading in an odometer. Otherwise, like you, Steve, we’d go into the used car business.
The disappearing tire trick
Dear Car Talk:
My 2010 Chevy Impala keeps telling me that I don’t have a left-rear tire! But I can see it right there on the car. And it’s even correctly inflated to 32 psi. It happened soon after I took the car to the tire guy to get my winter tires put on. Two days later the tire sensor says “POOF,” my left-rear tire is not even recognized. Gone.
I take it back to the tire guy and the sensor light goes off as we drive into tire guys’ parking lot.
The tire sensor light is playing games and goes off and on when the weather fluctuates. Can I take this car on a road trip later this month, or is it not safe? Should I use the Car Talk black tape solution? – Mindy
RAY: My brother used to be confused by the Check Engine light. He'd check the engine, see it was still there, shrug, and get back in the car.
I think you’ve got a bad tire pressure sensor, Mindy. There’s a tire pressure sensor in each tire. And each one communicates wirelessly with the car’s computer.
They originally tried wiring them, but after the test drivers wrapped about 50,000 feet of wire around each axle, they gave up on that approach.
Anyway, if your sensor is broken, or if it’s incompatible with your car, and the car’s computer cannot connect to it, it’ll trigger a warning that tells you it’s not recognized.
It’s possible that your tire guy damaged the sensor when he swapped out your tires and didn’t know it. Or it’s possible that he knew he broke it and replaced it with an aftermarket sensor that doesn’t work well with your car. Or it’s possible that the sensor’s time was just up, or that its battery is dead after 10 years, and the visit to the tire store was just a coincidence.
In any case, a trip back to the tire store is in order. Explain that within two days of your visit, the warning light came on and is now coming on intermittently. The two days gives him plausible deniability. But if he’s a good guy, and he thinks it’s likely he damaged the sensor, he’ll replace it. If he’s a good guy and he doesn’t think he damaged it, he may at least replace it without charging you for labor. If he’s not a good guy, he’ll still replace it, he’ll just charge you for it. It’ll cost you between $50 and $100.
But should get it replaced. And ask him to use the original equipment (OEM) General Motors sensor for your Impala. The OEM sensors tend to pair up most easily and work most reliably, in our experience.
While we sometimes recommend the “black tape solution” (putting a piece of electrical tape over an offending warning light) for problems with no safety consequence, this is not one of those, Mindy.
Even though you’ve confirmed not only the presence of your rear tire, but it’s proper inflation pressure, that could change on a road trip. And that’s exactly the purpose of the tire pressure monitoring system; to let you know if you lose air and are in danger of a blowout. So get it fixed and enjoy your trip.
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