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Discussion Starter #1
I've purchased a new full-size wheel to use as a spare and am gathering the rest of the assembly (metal stem, pressure sensor, tire, jack, wrench, emblem, tote, etc.). My question is: will the spare be 'read' by the TPMS? I don't know exactly how it all works. Is the sensor a proximity switch or a radio transmitter or what? If it functions as a prox switch it won't affect anything but if it sends or reflects some type of signal to a nearby transceiver I think it might.
 

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GODSPD
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In my experience, all that's been required is that you 'reset' the TPMS after installing the TPMS-equipped spare. This will recalibrate the system to accept the signal from the new wheel.
 

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Bimmerdex 7.4!
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On a BMW, each wheel well has a receiver which mates to the strongest signal when the 'Reset' procedure is done. The TPMS monitor has an extremely low antenna signal strength and it is highly unlikely that the spare tire, surrounded by sheet metal, will broadcast a stronger signal than the tire adjacent to the receiver in the wheel well.
 

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Yeah, Is it looking for any and all signals, or just 4 since that's what usually on a car? Are there any other TPMS out there from BMW or other Manufacturers that monitor the spare?
 

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I found this info in my Owners Manual for my '08 535i. I do have the compact spare but according to this it has no electronics. If I had the full size spare, apparently it is not recognized until it is mounted for driving on. :)

TPM Tire Pressure Monitor*

The concept

TPM checks the tire inflation pressure in the
four mounted tires
. The system reports when
the tire inflation pressure has dropped considerably
in one or several tires.

Resetting finishes during driving, which can be
interrupted at any time. When driving resumes,
resetting is continued automatically. Do not
reset the system when driving with a compact
wheel*.


In contrast to a compact wheel, a spare wheel is
equipped with the necessary TPM electronics
and also monitored after mounting and after
resetting the system.


dj
 

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Bimmerdex 7.4!
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Right deej (if Boltjames is beej, then...). You don't want to try and do a reset with the non-TPMS spare on because it will show an error light. It will think there is supposed to be a signal from all four wheels and will only get three.
Some mfrs' monitors keep track of the air pressure in the spare. They even cost less than BMWs. I guess BMW spent that money on putting "Joy" somewhere in the car.
 

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Discussion Starter #9
So the compact spares don't have sensors because the system extrapolates pressure from RPMs. Since the the compact spare will always spin faster (at greater RPMs) due to its smaller circumference, it would send a 'flat' signal to the receiver? And that's why the full-size spare for the 5-series mentioned above does have the sensor (because it is the correct size). That also supports the previous post about the receiver getting a signal from the nearest sensor and disregarding weaker, extraneous signals coming from the spare or other tires. So getting a sensor for my full-size spare tire will not screw up anything in the TPMS. Thanks everyone for the input! :thumbup:
 

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So the compact spares don't have sensors because the system extrapolates pressure from RPMs. Since the the compact spare will always spin faster (at greater RPMs) due to its smaller circumference, it would send a 'flat' signal to the receiver? And that's why the full-size spare for the 5-series mentioned above does have the sensor (because it is the correct size). That also supports the previous post about the receiver getting a signal from the nearest sensor and disregarding weaker, extraneous signals coming from the spare or other tires. So getting a sensor for my full-size spare tire will not screw up anything in the TPMS. Thanks everyone for the input! :thumbup:
That is correct for the FTM system, not the TPMS. The TPMS just wouldn't reset since only 3 wheels would be detected.

dj
 

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Bimmerdex 7.4!
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So the compact spares don't have sensors because the system extrapolates pressure from RPMs. Since the the compact spare will always spin faster (at greater RPMs) due to its smaller circumference, it would send a 'flat' signal to the receiver? And that's why the full-size spare for the 5-series mentioned above does have the sensor (because it is the correct size). That also supports the previous post about the receiver getting a signal from the nearest sensor and disregarding weaker, extraneous signals coming from the spare or other tires. So getting a sensor for my full-size spare tire will not screw up anything in the TPMS. Thanks everyone for the input! :thumbup:
So the compact spares don't have sensors because the system extrapolates pressure from RPMs
No, the pressure in each tire is read directly from a pressure transducer inside each wheel. You are thinking of old indirect type pressure monitoring systems which are now obsolete.
 
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