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Tire pressure monitors

I saw a discussion of tire pressure monitors on an automotive news TV show yesterday. They are clever devices that use a wireless connection to the ECM. There is a hand held programmer module that tells the ECM where each tire resides so that the reading registers correctly. Apparently this system is standard equipment on all 2008 and newer cars and trucks. They did not tell us what happens when you rotate your tires. I suspect that the ECM needs to be reprogrammed. It might be as simple as pressing a button to let the ECM know that the front and back tires were switched, but rentals I have had with the TPM did not have this feature. Does anyone have more information on this dilemma?

There are two types of TPM systems. These are Indirect and Direct.

With the Indirect system, the ABS is used to measure if a tire is low on pressure. This is done with the wheel speed sensors. If a tire looses pressure, it will rotate at a different rate than a tire with the proper pressure. The wheel speed sensors detect this differential in tire rotation speed and causes the TPM system light to come on.

With a Direct system, each wheel has a sensor/transmitter mounted that measures the tire pressure. With this system the sensors have to be reprogrammed for their position if the tires are relocated on the vehicle.

So it could be the rental vehicles you drove had the Indirect TPM system.


My understanding is that the manufacturers found a cheaper way to satisfy the law, instead of the expensive RF devices.

This is to use the antilock brake systems, which measure wheel rotation. The supposition is that all the tires have the same diameter. So if one tire has low pressure, it then has a smaller diameter (?) and it’s rotation rate will be higher, and this can be sensed by the antilock brake system and reported as low pressure.

Remains to be seen how accurate they are. Also, if all 4 tires are low, it will not be reported (or anyway I don’t see how it could be).

I have an 06 Nissan Frontier with TPMS. I rotate my tires all the time and no problems with the light coming on or the system not working afterwards. The sensors are on the valve stems. If a tire goes below 32 psi, the light comes on.

Forgot to mention…the system and cabin light don’t tell me WHICH tire is low, just that it senses one or more tires low. Have to check all 4 when the light comes on.

My only comment is considering the low number of cars which currently have this feature, we have seen a lot of problems with it.

Misguided manufacturing of malfunctioning junk technology. Not much need for it.

But if you buy a new car, you will have it. This added information can help choose a car if you want to rotate your tires.

I they were going to mandate TPMS they should have mandated a system that provided the actual pressure for each tire. Like when they mandated “passive restraints” some cheesy car makes came up with motorized mice seat belts or those GM “spider web” door attached monstrosities. Now they have systems that can’t even tell you which tire is low or even what the pressure IS in each tire.

Fall out from the Ford Exploder / Firestone blow out roll over debacle, I guess.

One thing about the ABS based system, they do work and you can rotate your tires, the only issue is finding the low tire. Of course, if all 4 tires are the same amount low those systems will think everything is OK…


or if your tires have slightly different wear, this will indicated different pressure. How much of a difference is needed to trigger an alarm, I don’t know. Would need to know what the relationship is between a change in pressure and a change in diameter – there was an extensive post on this recently, but I don’t think any numbers came out of it – I doubt it’s linear or consistent from tire brand to brand.

One advantage to the ABS based types, they should be very reliable, as they are nothing but software and a dash light. How accurate they are is another question.

On my car, not much. I don’t know the exact number but I know that if I rotate the tires I need to reinitialize the system. It says so in the manual and I’ve found it to be true. I also got a warning light when I picked up a nail and the pressure drop would not have been visually obvious, even on my 45 series tires.