Can you trust the TPM

[JohnV6]

I noticed on the ismart that it said tyre pressures were normal but opening up the header they were 2.6 bar. The manuals says 2.8 so I checked they were at 2.55 according to my pump. I took them up to 2.8 bar but ismart still shows 2.6 as does the screen display.

I haven't driven the car since but wondered if this was normal?

 

[Lovemyev]

It will update after a short drive.

 

[Data]

I've noticed you have to drive the car for the tpm to update. It takes a few minutes to happen. Or...maybe you can just start the car.

 

[JohnV6]

Have just driven it and it works now. Thanks

 

[Lovemyev]

Have just driven it and it works now. Thanks

Some TPMS systems ( VAG for one ) use the ABS system via a sensor to monitor the rotational speed of each wheel and compare.
So of course, this needs the wheels to be turning to evaluate the speed.
In this case, the tyre pressures shown in the car, is the reading from the previous trip.
When the car takes its next trip, it starts the evaluation process again ( hope that makes sense ).
The TPMS system on the original ZS EV model, uses a sensor inside the rim of each wheel, that is powered by a coil cell battery.
The position of each wheel is coded into the TPMS module of the car.
In this case, it remembers the position of that wheel .
So if the wheels are rotated, it still thinks the wheel in the original position, so recoding is necessary.
The VAG / BMW systems are clever, they will relearn after a wheel rotation without any coding due to ABS relearn.
I think ( but not 100% sure ) the ZS EV Gen2 had an upgraded TPMS system with a relearn facility, so you would expect that the S5 has a similar system .

 

[Data]

Yes, the MK2 ZS has actual pressure sensors in the rim rather than relying on abs deflation detection. I've just updated my tyre pressures & started the car. After a few minutes the dash readings updated without driving the car. Cool!

 

[hmadsen]

As others said...To preserve power (TPMS sensors are battery operated) the sensor will only send a signal if the tyre rotates.

Last pressure you have in the dash, before driving off, will be from the last drive, where the tyres were probably nice and warm (Friction heats them up, even in cold weather)

So you have 2.8, the car was then sitting in the cold, the pressure dropped to 2.6 which you measured.

Even when you are driving, the sensor will only transmit the pressure "once in a while", as long as the pressure is stable.

If the pressure drops, it will offcourse transmit the pressure at once and keep doing it with greater frequency.

This is offcourse to conserve the battery life, they generally last 5-6 years with normal use. (Battery is not replacable, the whole sensor is replaced.)

 

[Data]

Ah yes. Although I didn't drive it after updating the tyre pressures I did move the car a few feet on my driveway. So just starting the car doesn't cause it to update the dash readings. It definitely must be driven!