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Post by slider09 on Mar 14, 2019 20:39:35 GMT
Hi
Is it possible to adjust the differential of when your TPMS light is activated? Usual pressure is 38 psi, TPMS light came on today. Checked tyres and they were down to 24.
Can this be changed so the light comes on at 33?
Thanks
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Post by vwjap on Mar 14, 2019 20:50:15 GMT
I thought it was how fast each wheel was spinning, as in if one is spinning faster than the other 3 then put the light on
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Post by slider09 on Mar 14, 2019 21:23:59 GMT
No, otherwise every time you went around a corner your TPMS light would come on.
Think it’s something to do with tyre circumference, which changes when pressure is low.
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Post by vwjap on Mar 14, 2019 23:21:39 GMT
Indirect TPMS works with your car’s Antilock Braking System’s (ABS) wheel speed sensors. If a tire’s pressure is low, it will roll at a different wheel speed than the other tires. This information is detected by your car’s computer system, which triggers the dashboard indicator light.
You can google yourself if you want
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Post by slider09 on Mar 15, 2019 21:51:38 GMT
Indirect TPMS works with your car’s Antilock Braking System’s (ABS) wheel speed sensors. If a tire’s pressure is low, it will roll at a different wheel speed than the other tires. This information is detected by your car’s computer system, which triggers the dashboard indicator light. You can google yourself if you want Regardless, that was not my question.
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Post by dv52 (Australia) on Mar 16, 2019 1:05:00 GMT
Indirect TPMS works with your car’s Antilock Braking System’s (ABS) wheel speed sensors. If a tire’s pressure is low, it will roll at a different wheel speed than the other tires. This information is detected by your car’s computer system, which triggers the dashboard indicator light. You can google yourself if you want Regardless, that was not my question. Slider: Alas- the answer to your question is NO (I think). As to your observation regarding "they were down to 24" - you have highlighted one of the shortfalls of TPMS - as distinct to those pressure monitoring system that VW uses that have actual pressure transducers in each separate wheel and a wireless challenge-response communication protocol to a central processing unit.
On the TPMS on a mk7, the car does not know actual pressures - instead it receives a series of pulses from the ABS module that TPMS uses as a surrogate for pressure (i.e the angular velocity at the circumference of the tire is the surrogate for tire pressure).
So, the alarm that is generated by TPMS uses a differential process (i.e. TPMS does not use the absolute value of the spinning circumference of the tire unless it's at a very low level, I suspect). This means for a TPMS alarm to occur in normal circumstances, the system must detect a difference in the spinning speed between wheels. Your point about cornering is quite valid, but I suspect that the TPMS calculation compensates for this (or the calculation is not done at all) when the steering angle transducer is beyond a preset value).
In terms of your observation (they were down to 24), if all 4 x tires lose pressure at about the same rate - which might happen over a long period of time - TPMS won't produce an alarm because the spinning speed of the tires will be about the same (until the tire pressure is very low). Conversely, TPMS is prone to false positive alarms when driving on loose gravel roads, or if one of the tires losses traction for some reason. These points notwithstanding, my experience with TPMS is that it is quite sensitive to a slow leak in an individual tire - the loss of pressure in the tire doesn't need to be high to generate the alarm. I recall that unbeknown to me, I picked-up a nail in a rear tire on my mk7 some time ago. I was unaware of the nail and I would repeatedly get TPMS alarms when the tire pressure dropped to about 29 PSI. I eventually fixed the problem with the tire - I run my tires @ 32 PSI so that's about a 9% drop in pressure - not bad IMO
Don
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Post by slider09 on Mar 17, 2019 18:34:52 GMT
Thanks for the explanation Don 👍
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