Can tire sensors be added later to used tires

I'm looking at a set of winter tires/rims but they don't have the tire pressure sensors.
Can I add that now? What would happend (other than an error message popping up) if I didn't get the sensors? How big of a deal is it? What would you do?

You can swap your sensors over to your winter tires then reset the TPM. or your can buy an extra set of sensors from your nearest BMW parts dept. if you dont install sensors you will get a relentless warning, that can get annoying... other than that, its no big deal and wont cause any other problems.

Get the extra set of sensors. All competent tire mechanics should be able to install them. It'll add $200 to $300 to do it, but removing and reinstalling a set over and over again will probably ruin them. Also, if you have two complete sets of tires & wheels, you can swap them yourself easily enough.

Pardon my ignorance, but where are these sensors exactly? Are they in the valve stem?

Originally Posted by BMW_GAL

Pardon my ignorance, but where are these sensors exactly? Are they in the valve stem?

More like the valve stem is part of them.

dj

Originally Posted by soledoc

I'm looking at a set of winter tires/rims but they don't have the tire pressure sensors.
Can I add that now? What would happend (other than an error message popping up) if I didn't get the sensors? How big of a deal is it? What would you do?

tire rack has the OE beru sensors the cheapest... 196 per set, shipping about 8 bucks.

figure you'll pay the cost of mounting and balancing all 4 wheels plus a little more to install sensors. the sensor adds a bit of weight right where the stem is, so you have to rebalance.
around me, it would be about 120 to mount and balance runflats... figure maybe another 20-40 to install the sensors. its not hard, the aluminum valve stem is attached to the rim, then the sensor is bolted onto the inside part of the stem. takes no more than 5 min per wheel. slight violation of workplace rules, but the shop i frequent let me mount the sensors on an old car that i retrofit tpms onto. they would unmount the rim, pass it to me, i secured the sensor and passed the rim back to them, they would remount and balance the tire. they didnt want to mess with the sensors at the time, this was back before they were more common, so they let me do it. really simple manual labor. only thing i could imagine going wrong would be either under or over tightening the sensor. a little more than hand tight is fine, i believe the torque spec for the hollow bolts holding the sensor to the stem is 3.5 - 4.5 nm


then the mechanic needs to be careful to avoid the sensor when remounting the tire.

Originally Posted by BMW_GAL

Pardon my ignorance, but where are these sensors exactly? Are they in the valve stem?

They are at the base of the valve stem inside the tire


(note the picture is just a generic TPMS, not necessarily a BMW)

Thanks for the picture, it all makes sense now! So they are easily interchangeable between all tires? You just push them out after the tire is remove and through the rim?

I believe so. I dont know if the BMWs have the ones like pictured though as they do not do actual PSI measurements like some TPMS systems do (sadly, as some can actually show you PSI of each wheel). Instead they use the rotational speed of the tire to detect a flat (which is why you have to do the reset when you inflate/deflate your tires).

I have seen conflicting reports on whether the tires even need sensors at all such as here:
http://www.e90post.com/forums/showthread.php?t=1376

Originally Posted by Grentz

I dont know if the BMWs have the ones like pictured though as they do not do actual PSI measurements like some TPMS systems do (sadly, as some can actually show you PSI of each wheel). Instead they use the rotational speed of the tire to detect a flat (which is why you have to do the reset when you inflate/deflate your tires).

Kind of a mix between two systems on BMWs here. The current manual is less than helpful, as it lists information for both the FTM (Flat Tire Monitor) and TPM (Tire Pressure Monitor) systems.

The pre-2007 models used differential wheel speed as a proxy for tire pressure differences. I believe this is the system (FTM) used everywhere other than the US at the current time.

Post-2007 US models use actual pressure measurements (TPM, option 2VB, standard on all US BMWs). They don't display the tire pressure, but they do use an active battery powered sensor for the measurement. When the pressure changes more than a tolerance level (I have heard 10 psi, but have NO idea if that is correct), it sends the warning. Doesn't tell you which tire. Thank the nanny state for this.

You do the reset on inflate / deflate to set the starting point for the system. When you drop more than X psi from that point, the warning triggers on the current systems.

More information than you ever wanted here: http://www.usa.epcos.com/Web/share/a...Automobile.pdf

Think it's fun now? Wait until these cars are 5 to 7 years old and the batteries on the sensors start to fail. It's going to be a couple of hundred $ to replace either the batteries or the entire sensors (with labor / mount / balance fees in the mix). But, hey, the US government knows take care of us in case of flats!

FWIW,
George

I see, thanks for the info.

Actually should not be too much of an issue on the batteries as I have heard they are made to last the life of the tire (7-10yrs) when you should usually be replacing tires anyways.

I do wish they showed the PSI measurements, my Aunt's Jeep has that and it is a pretty cool feature to be able to take instant readings whenever. I do know that they have to have some sort of TPMS with runflats because of law I believe...makes sense as you cannot always tell with RFTs when you have an issue.

7 - 10 years ? I wish !

I go through normal tires every 18 - 24 months (30k miles / yr). I've heard some ugly things about RFT life on the board, so I hope I still see that.

Still, having to lob another $50 or so (each!) onto the cost of replacement tires (the BMW sensors do not have replaceable batteries) when the batteries do go is just a pain.

I also believe the US requires TPMS on all cars now, not just run flats. Granted, considering my wife has driven 2 of her last 3 flat tires to death ("I didn't know it was flat"), this may not be a bad thing.

George

Man, this cost of getting winter tires is getting expensive...and confusing. Sure was easier when I ran a good set of conti extreme contact a/s tires on my volvo. I'll just have to suck it up. I'm probably going to end up just getting a whole new set of winter tires/wheels/sensors and be done with it...and heck I only live in central IN...not Fargo, ND. But I'm definitely not running my OEM summer tires.

Thanks for the info on the tire pressure sensors...makes sense now.

Originally Posted by Grentz

I see, thanks for the info.

Actually should not be too much of an issue on the batteries as I have heard they are made to last the life of the tire (7-10yrs) when you should usually be replacing tires anyways.

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I would not question your statement about battery life if those batteries were residing in my living room. However, the environmental conditions they are exposed to , ranging from possible below zero in the Winter, to quite high during summertime high speed driving may reduce battery life. I wonder if Big Brother has regulations qualifying the battery's life during such potential conditions. Big Brother, I know you are out there. What say you??

Originally Posted by jelliotlevy

I wonder if Big Brother has regulations qualifying the battery's life during such potential conditions. Big Brother, I know you are out there. What say you??

As in all cases regarding Big Brother (TPMS batteries, 40 mpg cars that people actually want to drive, etc.) the proof is left to the student as an exercise.

Far be it from him to actually figure out HOW to do it. That's left for those peasants .., er, ah, um, engineers, who can come up with some way to fulfill the vision he dares dream.



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Originally Posted by jelliotlevy

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I would not question your statement about battery life if those batteries were residing in my living room. However, the environmental conditions they are exposed to , ranging from possible below zero in the Winter, to quite high during summertime high speed driving may reduce battery life. I wonder if Big Brother has regulations qualifying the battery's life during such potential conditions. Big Brother, I know you are out there. What say you??

They supposedly test them in such conditions. Trust me, BMW, etc. do not want them to fail.