After re-inflating an underinflated spare tire, why did I have to drive a few yards before the under-pressure indicator on my dashboard shut off?
The car is 2009 Toyota RAV4 with the spare mounted on the back gate.
I can fabricate reasons I would have to drive any of the mounted wheels a little distance before they signaled the detector to shut off. But how does it work that for a re-inflated, and not spinning, spare the car has to move?
My first thought is that for some reason, the engineers developing this system decided that they wanted to only read pressure when the wheels are spinning. And, when they were coding up the program, they did something like
if(wheel motion > 0)
{
UpdatePressureSensors();
}
And then they put all the pressure sensors, including the one for the spare, in the function UpdatePressureSensors(). Oopsie.
This is not true for all vehicles. I had my low pressure indicator on all week. I topped off all 4 tires and when I restarted my engine, the indicator did not light.
Was your engine running while you re-inflating the offending tire?
Are you sure you had to drive a few yards or is it possible that it was still on from before and it took that long before the computer checked to see if it was still under pressure.
IOW, maybe you didn’t have to drive, you just had to wait.
The owners manual on my Challenger cautions/advises a TPS reset will not turn off the warning light right away. I believe it’s the same on my wife’s Ford Edge. As to why, no can help.
I believe some tire pressure monitoring systems are tied to the anti-lock braking system. Basically the car senses a wheel is locked up when its rotational speed is out of whack (highly technical term there) with respect to the other wheels. The same sensors can be used to determine that the tire pressure is low (because the rotational speed will be out of whack.
And the other common technique is to use a sensor in the wheel with an RF transmitter. The transmitter doesn’t transmit constantly. This article says:
“When the vehicle is in motion, the sensor will transmit more frequently (typically once per minute) than when not in use.”
So it’s possible that the sensor hadn’t had a chance to transmit the updated pressure after the tire was inflated, until the motion of the car triggered it.
[
“When the vehicle is in motion, the sensor will transmit more frequently (typically once per minute) than when not in use.”
So it’s possible that the sensor hadn’t had a chance to transmit the updated pressure after the tire was inflated, until the motion of the car triggered it."
But, again, this was the spare which does not rotate when the car moves…kinda hard (not impossible) to picture that the spare can detect any motion of the car.
I don’t drive that car. But the embedded electronics system I work on at my day job has a fast cycle, a 2 second cycle, a six minute cycle and a 24 hour cycle. Stuff that isn’t critical gets shuffled off to the 6 minute cycle, to leave more time in the fast cycles for stuff that is critical.
I’m guessing it’s to preserve battery life in the sensor. If the system is programmed only to update at intervals while in motion, the spare will be updated when the rest are.
A very common method of sensing “motion” of a vehicle is to detect vibration. And for a sensor that’s mounted on a wheel, that seems to be the most logical choice.
Fascinating. No seriously, if that works as well as they think it does, it’s ingenious. I remember hearing something about how modern cars have something like 35 antennaes in them, hidden and embedded all over the place. There’s an antennae inside each wheel (including the spare) to send the air pressure back to whatever receiver picks it up. That ‘one strange trick’ eliminates 4 antennaes and saves the manufacturer a bunch of money.
I wonder how well it works. I have thoughts on why it shouldn’t work, but I have to assume their engineers thought of at least the easy things that I came up with and figured out how to deal with them.
It is an older system,we encounter them in the shop I work at. The reason it is not as good is because if the tires are of different tread depths, it throws off the rotation count from the anti lock sensor.
From what I read in the information we are provided.
The others reset like you guys said, time, rolling and sending signals. Some you have to reset with the tool.
I’ve never seen the TPS system on spare tires. The cars I deal with all have space saver spares.
In fact I just aired up my daughter-in-law’s Hyundai’s spare tire. It was down to 20 psi from the 60 psi listed. It had an old fashion valve stem.
I air up spares on my cars every 6 months (first time for DIL’s car). People often have a flat tire only to find out that they really have two flat tires when they mount the spare! The spare on my wife’s new car was down 15 psi on delivery.
The RAV4 has a full-sized spare tire, and it has a TPS on the spare. I know, because my wife drives one, and when the TPS goes off it has been the spare that is the cause more often than not.
ETA: At least, the 2008 RAV4. I don’t know about other years.
I drive a 2009 Toyota Venza. The summer tires are on the original wheels with the sensors. The winter tires are on after-market wheels without the sensors.
In winter, I can drive about 20-25 minutes before the warning light come on. When I get back home and park the car in the garage with the summer tires stacked in the corner, the light usually resets itself.
However, every now and then, maybe if I drive off more quickly than usual, it fails to reset.
I think that makes it obvious that the monitoring is not continuous.