False TPMS Warning and a Flat Tire Scare - 2016 Mazda MX-5 Miata Convertible Long-Term Road Test

2016 Mazda MX-5 Miata: False TPMS Warning and a Flat Tire Scare

by Dan Edmunds, Director of Vehicle Testing on January 23, 2017

2016 Mazda MX-5 Miata

I was in the middle of nowhere in the Arizona desert on the first day of a cross-country trip to Texas in our 2016 Mazda MX-5 Miata when the tire pressure monitoring system's (TPMS) low-tire warning light came on.

Oh, great.

My initial concern turned to confusion after I stopped and inspected the tires. None of them looked low or even different from one another. And I found no embedded debris that might suggest a slow leak.

But I couldn't be certain I was in the clear because I had neglected to bring my tire gauge. Instead I pulled over and tried the trucker's "thump test" and heard no discernible tonal difference. I felt comfortable enough to press on as far as the next inhabited area.

2016 Mazda MX-5 Miata

A half-hour later I came upon a small-town gas station, which had an air compressor. The hose's built-in gauge confirmed that none of the tires were low and all were at nearly the same pressure. With the scare over, I relaxed and resumed my trip. But the TPMS warning light continued to burn steadily, mocking me.

I couldn't understand it. Was there a faulty sensor? If that was true, the TPMS light should have been blinking — the standard way such systems alert the driver that the system itself (or an individual sensor) isn't working properly.

It turned out that nothing was amiss. I had made a big assumption that turned out to be wrong.

I assumed the Miata had direct TPMS, the kind with pressure measurement sensors and transmitters inside each wheel. The Miata's lack of a pressure readout screen does not preclude this. There are many examples of cars that monitor pressure for the purpose of issuing warnings but lack the capability to display the pressures to the driver.

I thought indirect TPMS — the kind that uses the ABS' wheel speed sensors to monitor and detect changes in wheel rotation speed to predict a leaking tire — was dead and buried, for two reasons. First, indirect TPMS dates back before there was a rule requiring such systems be fitted to all cars, and the early systems I had come in contact with were never very dependable.

Second, I had conducted TPMS compliance tests at my last job at Hyundai Proving Grounds. The law requiring TPMS was beginning to phase in, and Hyundai was fitting direct TPMS across the board. The federal test procedure defined numerous scenarios in which it must issue a warning, and I tested them all. One of the scenarios was all four tires leaking at once, which seemed impossible for indirect TPMS since all four tires would still rotate at the same speed relative to one another. The other engineers I knew seemed to agree that even though the law was "technology neutral" and did not seem to require direct TPMS, the test procedure seemed impossible to pass reliably without it.

This worldview began to unravel when I scanned the Miata's owner's manual. It didn't come out and specify whether it had direct or indirect TPMS, but it pointed out the location of a TPMS reset button that cars with direct TPMS generally do not need. A call to a Mazda engineer I know confirmed that the 2016 Miata indeed has indirect TPMS.

I pushed the button and the light went out. After that, it all fell into place.

2016 Mazda MX-5 Miata

Before I left town I had rotated the tires in my driveway, checked the pressures and torqued the wheels. But I hadn't pressed the TPMS reset button because I didn't know I had to.

This matters because the computer in an indirect system monitors and learns the rotation speed of each tire. Gradual changes due to tire wear are accounted for, even if the rear tires wear at a quicker rate than the fronts, as they generally do on this car. I rotated the tires front to back to make sure they lasted to Texas and back, but in so doing I created a step-change in the tire rotation speed at each corner. A TPMS reset would have told the computer not to be concerned, but I hadn't done it.

This would make perfect sense if not for one more fact: The warning didn't come on until I was some seven hours and 450 miles into my trip. That's a long time for a TPMS to respond. The federal test procedure I'd used back in the day allowed no more than 20 minutes. I might have put two and two together if it had happened soon after I left home.

So now I'm still confused, but for an entirely different reason. I know why it happened, but I still don't know why it took so long for the TPMS to respond. And indirect tire pressure monitoring systems are still out there? I thought they'd done the honorable thing and died out.

Dan Edmunds, Director of Vehicle Testing @ 15,150 miles

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