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theporschephile
01-09-2014, 04:57 PM
How sensitive is the TPMS supposed to be? Mine has been on for a week or so, and since it finally warmed up (33* baby!) i went to fill them up, filled to 38 in the fronts and 32 in the rears like the door jamb told me to, and i go back in the car to check and it's still complaining, specifically about the rears, saying the left rear is 31.9psi and the right rear is 30.4. I was using the gauge built into the pump, so i can understand that error (30.4 to 32), but i was surprised it was complaining about the 31.9. Is this typical? Only reason it bothers me is because every time I start the car I have to wait forever for the stupid messages to go away before I can do anything else on the EVIC.

lammie200
01-09-2014, 05:01 PM
How sensitive is the TPMS supposed to be? Mine has been on for a week or so, and since it finally warmed up (33* baby!) i went to fill them up, filled to 38 in the fronts and 32 in the rears like the door jamb told me to, and i go back in the car to check and it's still complaining, specifically about the rears, saying the left rear is 31.9psi and the right rear is 30.4. I was using the gauge built into the pump, so i can understand that error (30.4 to 32), but i was surprised it was complaining about the 31.9. Is this typical? Only reason it bothers me is because every time I start the car I have to wait forever for the stupid messages to go away before I can do anything else on the EVIC.

From my experience the warning light will come on when the air pressure is below 30 lbs. I put in air for the correct pressure and it takes 15-20 minutes of driving and the light will go out and not return. If your light comes back you may have a bad TPS. Also, I check the tire pressure with a separate gauge, not the gauge on pump. My gauge and the TPS's seem to read exactly the same. The gauge on the pump is highly inaccurate.

theporschephile
01-09-2014, 05:16 PM
d'oh, i didn't try driving afterwards, since i had just gotten home from work. I'm heading back out in a bit, so i'll report this evening if they decided to behave

Robert Nixon
01-09-2014, 06:02 PM
That sounds likely to me, you have to drive for a while for the sensors to read the new pressure.

ophidia31
01-09-2014, 07:05 PM
I usually run my tires a little low (34 front 32 rear) but with the less than 5* weather weve had on mon/tues/wed the air in the tires compressed and set the light off on the dash for me. Once it started warming back up, the air in the tires expanded and all was well. So I do think these sensors are pretty sensitive as Ive never had that happen on any other car Ive had that had tpms sensors.

theporschephile
01-09-2014, 10:21 PM
I used to have that happen on my subaru. Every single winter the light would come on without fail, in the spring it'd go back off.

also, just got home and the light remained off. Checked the pressures via the menu button and they were like 36-37 up front and 30-31 in the rear and the ligth stayed off, so not as sensitive as i thought. That would have been annoying

DuckDodgers
01-10-2014, 12:05 AM
Reading the maunal and interpreting what they are saying (their example uses a 30 psi standard tire, at 27 psi normally, a cold day taking the tire down to 23 and then driving warming it back up to 27...it then indicates the light will extingush when the tire reaches 27 again) it seems likely that the Fiat TPMS uses a hard coded ~10% below rear tire pressure (lowest normal pressure tires), or about 29 psi as the lowest value before coding. It is possible that the system also uses ~10% below the normal front pressures, or about 34 psi, but that make it much more complicated. We know the system can locate each TPMS sensor and correct its location shortly after a tire rotation, so it should be capable of this trick. However, ophidia31's experience seems to imply they use the lower bar all around, otherwise the somewhat low front would often trip a TPMS code.

The other consideration for theporschephile is that until recently a lot of cars used indirect TPMS systems or non-pressure reporting TPMS direct systems. The indirect systems mostly made use of the ABS sensors to note rotational rate differences that suggested low tire pressure. The non-pressure reporting systems simply lit a light at a set value without reporting exact pressures that the driver could view. The newest systems report values for the drivers consideration, which generally makes them appear more sensitive. In addition to this fact, I would expect the inflator pressure gauge to be somewhat inaccurate...however, it would probably tend to be slightly low, on the whole, as a safety consideration.

I assume that everyone here understands the relationship between temperature and tire pressure, and is aware of the simple formula to expect a 1 psi drop in cold tire pressure for every 10F drop in temperature.

Toad
01-10-2014, 01:10 AM
My experience is tpms doesn't know anything. It tells me that all four tires are the same pressure. Buy a tire pressure gauge.

ophidia31
01-14-2014, 11:58 AM
My experience is tpms doesn't know anything. It tells me that all four tires are the same pressure. Buy a tire pressure gauge.

This. When the light came on, I checked all my tires and there wasnt anything out of the ordinary. The cluster was saying 29psi in the front left but when I checked, both of them were ~31-32psi. Which, with the temprature of those days, is close to the formula duckdodgers said at the end of his post.