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Old 7th October 2017, 03:02 PM   #1
AlexZX
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Default How does the TPM system work?

When I drove my car home from the dealer after buying it the left rear tyre was down to about 10psi, but on the journey home the TPM didn't alert me to the extremely low pressure.

I am hoping they will replace the tyre when it goes in for the other issues the car has at the end of this month. In the mean time I inflated it to the correct pressure and put an ring inside the valve stem to stop it leaking which worked at first.

Today the tyre was flat again down to 10psi, but at no point during my ownership has the TPM ever given a warning that the tyre is losing pressure.

Is the system simply broken? If so what could it be? It will be another problem to add the unfortunately increasing list.
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Old 7th October 2017, 03:54 PM   #2
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Quote:
Originally Posted by AlexZX View Post
When I drove my car home from the dealer after buying it the left rear tyre was down to about 10psi, but on the journey home the TPM didn't alert me to the extremely low pressure.

I am hoping they will replace the tyre when it goes in for the other issues the car has at the end of this month. In the mean time I inflated it to the correct pressure and put an ring inside the valve stem to stop it leaking which worked at first.

Today the tyre was flat again down to 10psi, but at no point during my ownership has the TPM ever given a warning that the tyre is losing pressure.

Is the system simply broken? If so what could it be? It will be another problem to add the unfortunately increasing list.
Assume youíre resetting it after itís got the correct pressure?
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Old 7th October 2017, 03:58 PM   #3
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I've found the TPM very sensitive because when I had a slow puncture earlier this year and as soon as the offending tyre lost a few pounds below the other three, up pinged the red warning light.

I take it you've reset the TPM system through the i drive?

I think the tyres work in conjunction with the speed sensors. Once a wheel slows down, and we're talking fractions here, the sensor picks it up and you get the warning lights indicating there's a fault.

If you've reset it and the tyre is still losing pressure without you getting the warning lights, give it the dealer you bought it from and get them to sort it.
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Old 7th October 2017, 04:05 PM   #4
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Yes I reset the system once the tyre had been inflated. I just don't understand how the tyre can get to 10psi without the system picking it up, so I assume it is faulty.
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Old 7th October 2017, 04:48 PM   #5
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Just a quirky thought........ are all the speed sensors in place. i.e there's not one missing?
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Old 7th October 2017, 07:21 PM   #6
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No idea, how do I tell? Is the TPM an option or is it standard? Can you have the TPM elements in the software but not actually have any of the physical components that make it work?
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Old 7th October 2017, 07:56 PM   #7
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The sensors consist of a black wire with grey outer core and a plug fixed to each end. There will be 4 of them, one for each wheel. The sensor is about 6cm's (2ft) long. The plugs at each end are different from each other and I think one end fixes into the rear of the hubs. The other end will be attached to another part of the car.
You should be able to see them once the car is up on the ramp.
Hope this helps.
Good luck.
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Old 9th October 2017, 01:01 AM   #8
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On the E92, the tpm system uses the ABS/wheel speed sensors to monitor the "tyre pressures" by means of wheel speed/rotation. A deflated tyre will have a smaller rolling circumference and hence rotate at a slightly faster speed than its set point. There is no actual TPS sensor as per the F8x series cars.
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Old 9th October 2017, 09:18 AM   #9
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Quote:
Originally Posted by Slarty View Post
On the E92, the tpm system uses the ABS/wheel speed sensors to monitor the "tyre pressures" by means of wheel speed/rotation. A deflated tyre will have a smaller rolling circumference and hence rotate at a slightly faster speed than its set point. There is no actual TPS sensor as per the F8x series cars.
Thatís what my 2007 car had, itís deflation detection rather than active tyre pressure monitoring
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Old 9th October 2017, 09:54 AM   #10
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If you have a missing/broken/empty battery TPM valve you will get a idrive warning.
Recent cars (designed after 2012 or something) have to have a working TPM system to pass the MOT.
Maybe that rear wheel has a jammed TPM sensor, so the iDrive thinks its ok?

EDIT: sorry, didn't notice its the E9x sub forum. The mention of TPM made me think you had an F8x series M car.

Last edited by MaximumBob; 9th October 2017 at 10:13 AM.
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Old 9th October 2017, 10:07 AM   #11
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Not trying to be funny but these suggestions seem fanciful to me.

If you had a duff or missing speed sensor the dash would be a christmas tree as it would knock out ABS, TC, DSC and all the other systems that rely on it.

More probable is that the the deviation is not enough to trip the sensor. different brand of runflats?

I would suggest that you reset it once fully inflated again
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Old 9th October 2017, 06:57 PM   #12
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I seem to recall it takes a while after resetting for the system to learn what "normal" is. I think it may be as much as driving a half hour because it is measuring quite a small difference and has to compensate for difference in tyre sizes (new / old) and cornering - a new new tyre is a few mm different to an old tyre. If the tyre is losing pressure while it's learning then that i guess will be "normal".
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Old 9th October 2017, 08:51 PM   #13
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I inflate the tyres to the pressures it says on the car, so would a drop from 35psi to 10psi be enough to trigger the system? I assume if it only goes by wheel speed it has to be pretty much flat then before it can detect the loss of pressure?
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Old 9th October 2017, 09:52 PM   #14
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I had a bit of search and in the US TPMs must show a 25% loss, so yes it "should" show that. What i don't think it would show is 25% loss on all tyres.

I try to check the pressures once per month, i like to think i would notice a 5 psi difference as i tend to run at 33 psi mostly. I had the pressures at 45 psi for a day then reduced them back to 33 and that did not trigger the TPM.
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Old 9th October 2017, 11:51 PM   #15
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The system will only detect a difference in one tyre relative to the rest, so if they're all overinflated, you won't see an alarm.

That the OP has a puncture that isn't being detected could be that it drops before the system can learn what normal is?
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Old 16th October 2017, 04:27 PM   #16
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Is it on run flats? It shouldn't be. That would defeat the system.
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Old 16th October 2017, 09:06 PM   #17
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Originally Posted by Barnzzz View Post
Is it on run flats? It shouldn't be. That would defeat the system.
I'm not sure, how do I tell?

Not sure if it is relevant, but the tyre is not actually puntured; it is the valve stem that is faulty and that's where the air is leaking out.
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Old 16th October 2017, 09:11 PM   #18
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There will be a marking on the side... Depending on the brand of tyre....

Check this site then check your tyres.

https://www.oponeo.co.uk/tyre-articl...tyres-markings
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