[lucasspencer]

Hi fellow Karma devotees,

My tire pressure readings are set to over 50 via the factory and have been since the beginning. My service guy in Palm Beach (yes, still one there) has no clue about it. A manual read with a gauge gives me 40 and 36 as it should be. Does anyone have any idea how to match the default readings on the system with the real-life numbers? Is this a metric vs. imperial thing?
Thanks,
Josh

2012 Eco-Sport, Shadow

 

[ct-fiskerbuzz]

... A manual read with a gauge gives me 40 and 36 as it should be. Does anyone have any idea how to match the default readings on the system with the real-life numbers? Is this a metric vs. imperial thing?

The readings in the TPMS screen (which are only visible when one is out of range, so that you get an orange System button etc) are in Arbitrary Units rather than PSI. Not actually metric-vs-Imperial, but might as well be.

 

[lucasspencer]

The readings in the TPMS screen (which are only visible when one is out of range, so that you get an orange System button etc) are in Arbitrary Units rather than PSI. Not actually metric-vs-Imperial, but might as well be.

Thanks for the response. Even though it says 50 psi on the TPMS I never understood the correlation between what that and the actual reading which was 10 points lower, plus it never set off the orange...until now. Perhaps a faulty sensor is at work.

 

[Fisker_Fan]

YES TPMS are known to be faulty to many cars, but the ones in the Karma do actually work.

The reason for the wrong displayed PSI value is relative pressure vs. absolute pressure.

The TPMS reads the absolute pressure within the tires. What the display SHOULD read is absolute pressure - atmospheric value (~11.4psi at sea level) = relative pressure (your typical tire gauge reading)

This was a simple bug that never got fixed.....

 

[lucasspencer]

YES TPMS are known to be faulty to many cars, but the ones in the Karma do actually work.

The reason for the wrong displayed PSI value is relative pressure vs. absolute pressure.

The TPMS reads the absolute pressure within the tires. What the display SHOULD read is absolute pressure - atmospheric value (~11.4psi at sea level) = relative pressure (your typical tire gauge reading)

This was a simple bug that never got fixed.....

Thanks for clarifying. 11.4 seems like the right difference then to what I'm seeing.

 

[SoCalGuy]

Wow that's the first time someone has actually explained the difference - most of the techs I've spoken to just say the sensors are just off/don't work. Thanks for the info!

 

[ct-fiskerbuzz]

... What the display SHOULD read is absolute pressure - atmospheric value (~11.4psi at sea level) = relative pressure (your typical tire gauge reading)

Hm, atmospheric pressure at sea level is 760 torr or ~ 14.7 psi. It falls off with altitude, down to about 80% at a bit over a mile high (which is high enough for most practical purposes), to ~11.7 psi at that point.

(It also varies with weather, which is why barometers are useful )