Tyre pressure warning(?)

[PoloGTInoob]

Hi guys

had my Polo GTI for 4 months now. I tend to pump up the tyres every ~2 months. Last time, the pressure had dropped from 40 to 34-36 in the tyres. 34psi is down 15% from 40psi, so im surprised the low pressure light didnt come on ever. Does anyone know when they are triggered, is it closer to -20%? (i know they work based on ABS system and not actual pressure, but still, seems a bit useless if it isnt telling the pressure is down from 40 to just 34....)

[SRGTD]

You’re correct - the Polo has an indirect tyre pressure monitoring system (TPMS) that uses the ABS sensors to measure differences in rotational speed of the wheels, so if one tyre has a lower pressure than the other three, then the wheel that tyre is fitted to will rotate at a different speed to the other three and the TPMS will trigger a warning.

I may well be wrong, but I think if all the tyres lose the same amount of pressure over time, the wheels will still all rotate at the same speed, so in that circumstance maybe a low pressure warning won’t be triggered? That’s why IMHO it’s important to still carry out regular manual tyre pressure checks, rather than rely solely on the TPMS. Also, manually checking the tyre pressures also gives a good opportunity to check the condition of the tyres for cuts, embedded objects and sidewall bulges / damage, and the tread depth.

[Harrihealey02]

An indirect TPMS system can still detect a gradual and equal pressure loss in all tires, but it operates differently than with individual tire pressure differences. In my experience it will detect low pressure if all wheels get to around 30-40% lower than their original baseline, which would be around 25 psi.

The system establishes a baseline for each tire's diameter based on their normal, properly inflated state. This data is stored in the computer when you first reset the tire pressure warning (this is why the screen asks you if all tires are set at their correct pressures).

Wheel speed sensors, used by the ABS system, track the rotational speed of each wheel. As tire pressure drops, the tire's overall diameter shrinks slightly. This means the tire needs to rotate more times to cover the same distance compared to a properly inflated tire.

The computer compares the rotation speeds of all four wheels. If all tires lose pressure equally, they will still spin slightly faster than their baseline due to the reduced diameter, even though the difference between them might be minimal.

The system analyses the combined data from wheel speed, baseline diameter, and vehicle speed. If the overall increase in rotation speed across all wheels exceeds a set threshold, a low tire pressure warning light will be triggered for each wheel that it detects is it of a pre set range than when it was first registered.

[Johntheo1]

What about cornering with this ABS based system, all wheels arn't turning at the same speed, that's what a differential is for?

[Johntheo1]

What about cornering with this ABS based system, all wheels arn't turning at the same speed, that's what a differential is for?

I found its a bit of a joke, see below. I took (and posted) these readimgs about 18 months ago I think.
The alarm doesn't annunciate until one tyre is ~ 18psi lower than the resetted readings.

Right Front: 18psi

Right Rear: 29psi

Left Front: 26psi

Left Rear: 30ps

Tyre alarm up when R.Front was 10.33psi (0.713bar) lower than the averaged other 3, or 18psi lower than the 32psi reset, 36.5% of the averaged other 3 (now) or by 43.8% of the original reset at 32psi.

So, better than nothing since its not actually monitoring the pressures?.

[Simonz]

It is not a perfect system. I have had odd alerts and checked the tyres with no obvious loss. Also a slow puncture which didn't show up. Best to keep checking the pressures manually. It's ok probably to show a rapid deflation, you would probably spot this from the car itself driving strangely. Good luck if then faced with the tyre mobility kit...