TPMS technical question

Do the TPMS sensors in the wheels actually allow the measurement of the actual tire pressure in the tires, or do they measure the difference (if any) from the tire pressure at the time of the last reset? You are instructed to set the tire pressure to spec before performing the reset procedure, so is that required to set the "baseline" tire pressure, from which the sensor then measures the "delta" (difference), or are the sensors providing a signal which allows the computer to determine the actual pressure in PSI? I am trying to understand whether I should consider the TPMS given tire pressure to be a very accurate PSI value, as compared to my quality tires gauges, or not?

 

The TPMS sensors do read the actual tire pressure. When hooked up to tech stream it shows the pressures of each tire down to a tenth.

 

The sensors measure the absolute pressure, and set it over to a digital signal , probably only in steps of 1 or 0.5 psi.
This is sent to the little computer in the dashboard, wich substracts the outside pressure of fixed 14.7 psi, and this is the pressure you read,

So on high altitude , where outside pressure is lower, it still gives pressure as if it where on sealevel.

Also temperature is measured by the sensors, and internal sensors give exact the temp of air in tire, but external sensors give temperature something in between of in - and out- side temperature, so not reliable.

There are tmps systems that compensate the pressure for temperature. Then the little computer calculates the pressure back to 18 or 20 degr C ( 65 or 68 degr F) . This using the temp the sensor gives.
So this only workes with internal sensors.

 

So the sensors send the actual pressure to your vehicle computer which then calculates the percentage off the baseline and throws the light if it's a certain percent off. The reset button was toyotas way of allowing you to have different baseline pressures front and rear which many tacomas actually ask for on the door placard. There is still a limit however. I remember I used to air down to 15 psi and hit the reset button and the light would still stay on constantly because it was just too low of a pressure.

Also, most sensors live in a very low-power state since they are battery operated and are expected to last 5-10 years. They do not constantly transmit the pressures to the computer as that would severely cut battery life. Depending on if it's Hi-Line or Lo-Line it may only transmit occasionally or when the pressure changes.

All tpms sensors actually measure the pressure down to an exact number although many do not display it for you