I finally got around to checking my tire pressures on my 2007 MCS for the first time since March. All tires were down to about 20 pounds. The puzzling thing is not that they were low but why did the low pressure warning system not pick this up?

 

If you've had the car since March (or earlier), than you have the old "indirect" Tire Pressure Monitoring System.

The indirect TPMS detects a leaking tire by comparing the rotational speed of a tire to the speed of the tire on the other side of the car (i.e., it compares front-left to front-right, and rear-left to rear-right.) A significant difference in the speeds of the two tires tells the TPMS that a tire's low.

The drawback to the indirect TPMS is that if both tires at one end of the car (either front or rear) lose air at the same rate, their rotational speed will be about the same, and the TPMS will never know anything's wrong.

I had the exact same thing happen today - I went too long between checking my pressures, and all four tires had dropped from 35psig to 28psig. Since they all lost air equally, I never heard a peep from the TPMS.

 

BINGO!

X2

 

Learn something new everyday! I did not know this..Thanks for the info!

 

So, the FTM only compares front wheel with wheel tire, and rear wheel with rear wheel? That seems odd. Why not just take a ratio of each wheel and compare them all. It might take an extra nanosecond or two, but gee...

FWIW, one can't rely on the new TPMS to maintain good tire pressure either. It only alerts when pressure drops 30% or more.

We still have to keep an eye on our tire pressures, no matter what system we have.

 

The only reason I can think of is that comparing the ratio of each wheel's speed to the average of all four wheels wouldn't give you any extra warning, except in the case where the wheels at one end of the car are leaking equally, while the wheels at the other end aren't leaking at all (or are leaking together, but at a much slower rate).

Plus, averaging all four wheels and checking each wheel against the average might give you false positives if you run significantly different pressures in the front and rear tires.

Regardless, like you said, the TPMS (indirect or direct) is no substitute for staying on top of your pressures in the first place.

 

I almost hate to admit this...I'm usually on my tire's pressure.

Two weeks ago, while pulling out of my driveway, the pressure warning came on. (newer system, original run flats) Work is 9 miles away and an air compressor is available, so I proceeded to work. I waited until morning break to check the pressure so I could get a cold reading. Both fronts were 31 psi, as was the left rear. The right rear was 29 psi. Two psi difference and the system alerted me! That's tighter than I thought it would be.

I topped them all off and re-initialized the system. Last check (yesterday)showed all of them to be good.

 

Ok I read the manual and the TPMS is a little confusing to me. You only reinitialize the tire pressure when they are changed... do you check the pressure manually or does the system actually tell you what pressure each tire is?

This might be a dumb question.

 

With the newer system I was alerted, at startup, that my tires had all dropped to 30psi while the car sat idle for a week in the colder weather. I kinda like it.

 

What were your tire pressures initially? They should have been 36 to 38 psi. So, they were down somewhere between 5 to 9 lbs. That isn't really good enough for getting the best gas mileage and tire life.

 

So, is there a software upgrade to go to the new system or is it a hardware change?

 

You buy a new MINI.

Not only would you need new software, you would need the receiving unit attached to the computer, and a sending unit in each wheel. The sending units are no problem. I'm not sure what it would take to add a receiving unit and software. It may or may not be possible.

 

30% is only the maximum threshold mandated by the government. These electronic systems are capable of being much more sensitive, at no additional cost. It would be up to each manufacturer to choose their sensitivity.

The system helps to prevent accidents caused by tire blowouts. It wasn't expected to maintain tires at their optimal pressure, and we still have to monitor that ourselves.

 

Depends on tire size also I believe... My 17's say 33lbs

 

Check page 142 of your manual. For 17" it recommends 36 psi., unless you are travelling over 100 mph with four passengers & luggage. Then use 41 psi.

Now check the sticker in the driver-side door sill. My MCS (came with 17" wheels) says 38 psi.

Only 16" get a 33 psi recommendation, and that is for 195 tires. Wider 16" need more air.

 

The active system really does seem to be pretty accurate. I was running 34.5 in the fronts, and 33 in the rears. I rotated my tires, and on the way to the station to adjust the pressures, it did not trip the TPS lights, but the obc had a triangle with an exclamation point in it. When I checked the computer, the error was the TPS. After a reset, it is fine.

 

They were 33 when I got the car from the dealer. (16" stock run flats)

 

The indicator came on 2 days after I picked up my 2007 from the dealer. Seems to be OK now. what happened to the quality check?

 

from what i understand, even the new system has no way of telling you the pressure in each tire... correct?

i would think that this would be possible since my dads 04 caddy has direct pressure monitoring and can tell you the pressure in each tire on the OBC.

 

Ya.. I just checked, the 17 is 38.. I was recalling the 16" pressure..

 

If the valve stems are silver then a software update should update you to the new system. If your valve stems are black then it cannot be updated. The silver stemmed wheels use the individual tire pressures instead of rotational change.

 

i have these but i still cant check the individual tire pressures... i guess i will only know when a tire is low. too bad, it would be nice to be able to see each tire pressure at the OBC screen.

 

Those poor, poor passengers.

 

I don't believe it shows individual pressures it just alerts you to which tire is low instead of an indication that "a" tire is low.

 

I don't have my car yet but according to the MA if you have a NAV system it will tell you which tire is flat but if you don't it will just tell you that a tire is low (but not which one)

I also read somewhere that the tire pressure system averages pressure over time so if you have a very slow leak (or more likely if the pressure goes down from a slow change in temperature) it may not pick it up.