Tyre Pressure I think.
same here, tyre pressure
Dumb question how does it work?
E36 TDS (15-09-2015)
On my e90 I think it monitors a difference in wheel speed on the same axle.
The ABS sensors constantly monitor the rotational speed of all 4 wheels. As a tyre pressure decreases, the rolling radius of the wheel also decreases & therefore it rotates faster.
When the tyre pressure system is initialised (when your tyres are at the correct pressure & you reset it), it uses that initial reading of rotational speed as a base line & when it detects that any of the wheels are rotating by a set percentage faster (the rolling radius has decreased) it activates the warning that the pressure in that tyre has decreased by that set percentage.