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False TPMS Warning and a Flat Tire Scare - 2016 Mazda MX-5 Miata Convertible Long-Term Road Test
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False TPMS Warning and a Flat Tire Scare - 2016 Mazda MX-5 Miata Convertible Long-Term Road Test
Panic ensued when our 2016 Mazda MX-5 Miata issued a low tire-pressure warning on a remote road in Arizona, but it turned out to be a false alarm.
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Also, your Miata is supposed to have 29 psi in all 4 tires...if they are all at 29, then the rotation should not have triggered a light unless you have severely uneven treadwear from not rotating them enough.
No matter how closely you match four tires they will be slightly different in circumference and rotate at different speeds even when going in a straight line. The system uses that to determine if the inflation of a tire has changed.
The system measures the wheel speeds of all four tires and combines them for a total speed. Then divides that by four to get an average wheel speed. It also cross measures the left front with the right rear, and the right front with the left rear. There are several computations done with these measurements that allow the system to account for varying wheel speeds due to turns. When this is all completed each wheel has its own average rotational speed in relationship to the others.
When you rotate the tires without retraining, the system doesn't know that it has to reset these calculations. That often times sets the warning all by itself within few miles. The fact that this Mazda took a long time to set is more likely to have been caused by a tire heating up raising its pressure and making it rotate slower which makes its cross companion appear to rotate faster and that is what the system looks for to decide if a tire has gone low.
After a few minutes the warning light went out and when I eventually was able to check the tyres, nothing was amiss. I wish my Jeep had the system where it actually shows the pressure for each tyre so at least I would know where to look first.
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The fact that we got into the habit of measuring the tire temperatures exposed the flaw in the way servicing used to be done with regular TPMS but at the same time opened the door for some less predictable behavior with the indirect systems. Consider that the tires on the front before rotating might have been about 40f warmer than the tires on the rear. When you rotate the tires and set the pressures, the tires going onto the front get the pressure reset while they are close to the current ambient temperature. The tires going onto the rear while set at the same pressure at that moment in time, cool off and end up 4psi under inflated. That alone isn't enough to trigger the system, but when you then take a trip and they stay cool, while the front get warm from the brakes, the pressure variation can then turn around and exceed the warning threshold. That would be annoying all on its own but then we get to throw in the variable that we really don't know what the tire pressures were the last time the system was commanded to relearn and we also don't know what the actual circumference of each tire really is.
Even if you had retrained the system and the tire pressures were compensated for temperature, what exactly would that do to the relative wheel speeds?
When you really think about it, its a wonder there aren't more false warnings than there are, but then again the system is only supposed to alert when a tire is grossly over/under inflated. It doesn't replace regular checking and adjustment of the tire settings.