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[mleskovar]

Where is the pressure pickup for the TPMS sensor? The valve stem only holds the air valve and provide a mount for the sending unit and there are no holes in the sending unit so how does it sense pressure change? There is a small white plug stuck in the sealer that covers the back of the sending unit and I'm assuming that is for calibration....true? Anyone have knowledge of the TPMS operation detail?

[ORIGIN M.]

The receiving ends are inside of the bumper ends, behind the wheel-well of each wheel.

If you unplug one of the sensors, the system will trigger the light as if you are running low in one of the tires, even tho the air might be spot on. Kind of funny how it works. I wish it told you what tire was low.

[mleskovar]

Thanks, but I'm asking about the how the air pressure is sensed inside the tire, not how the system works. Here are front and back of the sensor....the two holes in the corners of the front are blind, probably for manufacturing.

[mleskovar]

Bump.....anyone know?

[jeremyc74]

Originally Posted by mleskovar

Bump.....anyone know?

I've got some in my desk in various stages of manufacture. I'll take a picture of them when I get back to work on Monday. I did a lot of the design work on the production equipment that's used to manufacture them for most of the vehicle built in the US.

There's a solid state pressure transducer mounted to a circuit board on one side, and a battery on the other. The white thing that looks like a plug is actually porous, and that allows the pressure to the transducer.

You should also be able to see a small metal pin that's molded into the housing. That's the antenna.

The calibration is performed wirelessly when they're built, if it's needed. The ones I worked with were accurate enough that they only required testing of every unit, not calibration.

[Justin(OKC)]

Originally Posted by #1

The receiving ends are inside of the bumper ends, behind the wheel-well of each wheel.

If you unplug one of the sensors, the system will trigger the light as if you are running low in one of the tires, even tho the air might be spot on. Kind of funny how it works. I wish it told you what tire was low.

I have iDrive, but it does tell me exactly which tire is low.

[mleskovar]

Originally Posted by jeremyc74

..There's a solid state pressure transducer mounted to a circuit board on one side, and a battery on the other. The white thing that looks like a plug is actually porous, and that allows the pressure to the transducer....

Thank You! How long is the battery expected to last? I'm inquiring about the pressure pickup point because I want to use a prophylactic tire sealant and I was wondering what effects it could have on the sensor. It seems obvious to me if the sealant I am considering is used in the correct quantity, and installed correctly, there should be no problem.

[jeremyc74]

Originally Posted by mleskovar

Thank You! How long is the battery expected to last? I'm inquiring about the pressure pickup point because I want to use a prophylactic tire sealant and I was wondering what effects it could have on the sensor. It seems obvious to me if the sealant I am considering is used in the correct quantity, and installed correctly, there should be no problem.

I wouldn't use any kind of sealant on these. Anything that will seal a hole in the tire will seal the sensor as well. I've seen some of the advertise that they're safe for TPMS systems, but I can't understand how that could be the case.

The battery life is supposed to be 5-7 years on most of the models I worked with, except Honda, and they're using rechargable batteries with a motion driven generator integrated into the unit to extend it. I'm not sure how long those are expected to last though.

[mleskovar]

Originally Posted by jeremyc74

...Anything that will seal a hole in the tire will seal the sensor as well. I've seen some of the advertise that they're safe for TPMS systems, but I can't understand how that could be the case...

You have to understand the dynamics of what makes the sealant work. First off, I am talking about proactive sealant. You put it in before you get the flat. The roadside 'repair a flat sealant' is reactive (and a gooey mess). The sealant enters through the valve stem at 6:00 (valve removed) and deposits directly on the inner casing of the tire (not the rim)...in a blob. Short blast of air to clear the valve stem and reinstall valve and fill with air. The blob doesn't go anywhere unless there's an opening to the atmosphere for the air to escape and drag the blob with it. The tire pressurizes and the blob stays there until the tire is rotated and then it deposits evenly around the inside of the casing, and stays there, through centrifugal force. Tire is punctured and the same force plus air forces the sealant outside the tire where it solidifies and plugs the hole. The sealant never comes in contact with the rim or the TPMS and since the pressure pickup is a blind hole (no air passing through it) and on the rim it's out of harms way. Tests show very little air is lost in the process and more likely than not you'd have a puncture and repair without even knowing it. Over and over. I don't believe my logic is flawed but I've been wrong before

[esses]

Subscribed. I always wondered how the damn things worked.

[jeremyc74]

Sorry for the delay guys. I completely forgot about this thread.

Here's a couple of pictures of the guts:

The hole right next to the valve stem that's blocked is a small metal antenna.

On these two examples the pressure sensing hole doesn't have the white material in it, but these are early examples and they're all I've got laying around. The square one is complete out of production now as far as I know.

I used to have a couple of the boards, but the last time I moved my cube I think I tossed them. The board gets pressed into the housing. There's a small rubber gasket that gets placed on top of the pressure sensing chip (a small 20 pin surface mount IIRC), so that it's the only part that's exposed to the internal tire pressure.

The white thing laying behind them is the cover that gets laser welded on, then the whole unit is pressure tested to make sure the seal around the chip is good, and the cover weld is airtight.

[jeremyc74]

Originally Posted by mleskovar

You have to understand the dynamics of what makes the sealant work. First off, I am talking about proactive sealant. You put it in before you get the flat. The roadside 'repair a flat sealant' is reactive (and a gooey mess). The sealant enters through the valve stem at 6:00 (valve removed) and deposits directly on the inner casing of the tire (not the rim)...in a blob. Short blast of air to clear the valve stem and reinstall valve and fill with air. The blob doesn't go anywhere unless there's an opening to the atmosphere for the air to escape and drag the blob with it. The tire pressurizes and the blob stays there until the tire is rotated and then it deposits evenly around the inside of the casing, and stays there, through centrifugal force. Tire is punctured and the same force plus air forces the sealant outside the tire where it solidifies and plugs the hole. The sealant never comes in contact with the rim or the TPMS and since the pressure pickup is a blind hole (no air passing through it) and on the rim it's out of harms way. Tests show very little air is lost in the process and more likely than not you'd have a puncture and repair without even knowing it. Over and over. I don't believe my logic is flawed but I've been wrong before

The fact that there's no air movement into the pressure sensing hole might prevent this stuff from clogging it up, but I've seen inside wheels where this stuff was used, and it does contact the rim. It's not a chance I'd take personally.

[creepy coupe]

We don't get that here in the UK, our system just relies on the rolling circumference changing due to lost pressure.

How long do the batteries last on your systems?

[jeremyc74]

Originally Posted by creepy coupe

We don't get that here in the UK, our system just relies on the rolling circumference changing due to lost pressure.

How long do the batteries last on your systems?

I'm not sure on the BMW system, but the ones I posted are supposed to be good for 5-7 years.

[mleskovar]

I'm curious about the pressure testing algorithm. Does it trip when any wheel drops X lbs pressure? If they all drop/raise the same amount does it care? When you initialize them in the wheels do they care about what pressure it is? Could it be atmosphere when initialized?

[mleskovar]

Originally Posted by jeremyc74

.. I've seen inside wheels where this stuff was used, and it does contact the rim...

You probably saw the aftermath of a "fix a flat", one size fits all, pressurized can. Once again, that's reactive (after the flat), very messy and it goes everywhere. I'm thinking of using a product that is poured/pumped in (not pressurized) in very specific amounts for the size of the tire. It remains for the life of the tire and you don't need to ever remove the tire from the rim unless you get a major hole (>9mm), sidewall puncture, or blowout.

Originally Posted by jeremyc74

...It's not a chance I'd take personally.

You're not alone Everyone thinks of the "fix a flat" concept but it's a different concept. I'll try it out and let the forum know what happens. All I'll probably be able to report though is if it buggers up the TPMS because I'll probably never know if it repairs a hole

[jeremyc74]

Originally Posted by mleskovar

I'm curious about the pressure testing algorithm. Does it trip when any wheel drops X lbs pressure? If they all drop/raise the same amount does it care? When you initialize them in the wheels do they care about what pressure it is? Could it be atmosphere when initialized?

The systems work differently for various manufacturers. BMW's system allows you to set the "normal" pressure, and then gives you an alarm when the pressure deviates from that.

On GM's systems the computer displays the individual tire pressure for each corner for you to see, as well as warning you when it gets low.

With the BMW system I'm guessing there's a minimum pressure that can be used as the "normal" setpoint, which would prevent you from setting it while you had an extremely low tire, but I can't honestly say for sure. The TPMS sensors report actual pressure and temperature to the ECU, and the programmers decide what to do with the information, so it's hard to tell exactly what's going on in there.

[jeremyc74]

Originally Posted by mleskovar

I'll try it out and let the forum know what happens. All I'll probably be able to report though is if it buggers up the TPMS because I'll probably never know if it repairs a hole

Good luck. Let us know how it turns out.