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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legacygt

I drove my Mazda CX-9 with the TPMS light on for the last 5 years that I owned it. It was maddening but I had squandered several hundred dollars on new sensors and just decided to live with it. A guy at my local shop estimates that about 70% of the cars that come in have the TPMS warning light on. I can't speak to which technology is the biggest culprit but we have an example of a government mandate gone horribly wrong. We assume our cars will tell us if there's a problem and then we learn to ignore the warning message when we see it. Meaning that, in some cases, we'll be ignoring a legitimate warning.

kirkhilles1

Wow, that's a poor design by Mazda. Surprising from a company that prides itself on reliability.

thecardoc3

Honda is also still using indirect TPMS systems. As far as what might have occurred, the system cannot distinguish a tire going low from a tire heating up and increasing pressure. Either condition will cause a variance in relative tire speeds and could result in a warning.

longtimelurker

The indirect system on my 2011 GTI works great...the one time I had a leak, one front tire had gotten down to 34 psi from the recommended 38...and I got the indicator light. I am not thrilled with direct-reading ones...if you were to go to an auto-supply store, get 4 identical pressure gauges, then mark them for a given wheel position and use them only on the given wheel, what are the odds that they are all calibrated identically, and that if they all say 35 psi, that the tires are all ACTUALLY at 35 psi? That's what you're getting with direct-reading TPMS.

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.

thecardoc3

Anytime someone tries to simplify an explanation they risk decreasing the accuracy, but I'll do it anyway.

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.

longtimelurker

That is all true...and the fact that the system picked up what was almost certainly an extremely small variance in rotational speed here certainly puts the lie to the idea that indirect-reading systems are insensitive or don't work.

djd352

I like the indirect systems as you do not have to worry about the sensors breaking or buying new ones for your winter tires etc. I have found the direct system on my Hyundai more likely to go wrong and also more costly when I go to install my winter tires ever year. No thank you. Now that I know that Mazda uses an indirect system, I might need to consider a Mazda as my next car. :P

desmolicious

I feel your pain Dan. The TPMS came on in my Wrangler this Sunday. During the deluge (4 inches of rain fell that day in Long Beach) and I was stuck on the 110 Fwy which was closed due to flooding! Talk about worst case scenario to have a flat!
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.

actualsize

thecardoc3 said:

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. 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.

OK. two things: I rotated both sides front to back, so the diagonal pairs were not broken up. They were moved to the opposite diagonal, though. Also, the TP-138 test procedure that goes with FMVSS 138 mandates that a flat should be detected within 20 minutes. This was not a flat, but if the rotation was responsible, then five hours is a little "excessive." That amount of time does seem to suggest the cause was something other than tire rotation.

thecardoc3 said:

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.

Sounds plausible, but I'm not sure how one tire would heat up more than the others. Sure, the sun is probably warming one side of the car more than the other when driving straight across country, but that happens all the time. If indirect TPMS was that sensitive to differential heating there'd be false TPMS warnings all the time.

thecardoc3

This is where the information that we can get our hands on for training often has a lot of holes in it that we have to figure this stuff out one piece at a time. Tire temperatures are very sensitive to brake temperatures and with the front brakes doing 70-80% of the stopping, combined with road friction during normal driving its reasonable to expect to see the front tires get to be 40f to 60f above ambient temperature. When you allow for 1psi tire pressure change for each 10f that means tires inflated to the spec "cold" would check between 4-6 psi higher when they are hot. That is why when servicing a car with a regular TPMS system we have to compensate the tire pressures by measuring the tire temperatures at the time of servicing and compensate to the relative cold temperature.

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.

scott31

My 2010 Mini Cooper has direct TPMS, yet I have a reset function. When I switched rims and tires (with their own TPMS sensors), I had to reset the system. Many Mini owners switch between rims and tires during the winter. The winter set has to have the sensors, and you have to perform a reset when you switch. Also whenever I have the tires rotated I have to perform a reset. So I think just having a reset button/function doesn't blanket-ly mean you have an indirect TPMS system.

isbelldl

You may not have realized it, but you likely did have a pressure gauge with you. The factory tire pump that's in the trunk has a built in gauge.