spijun

Here at the end of the text you have an explanation

http://www.audienthusiasts.com/VagComABS.html

acadianbob

I'm still dubious since the system is not really sensing tire pressure but is, in fact, sensing the difference in rotational speed. This would be an interesting test. Set all your tires at 32 PSI and reset the system. Then, lower all 4 tires to say 24 PSI and go for another run. Since the tires would all be rotating at the same "rpm", it might not trigger the system at all. In fact, I'm getting that it wouldn't. What do you think?

MP4.2+6.0

I think the direct system has always been better. The indirect system works, but it is really more at the more basic scenario of under inflation. Think Ford Explorer roll over fiascos. Trouble is, the sensors for the direct system at OE discounted retail are in the low to mid $100 range, each. And more important for Audi, besides reducing the manufacturing cost somewhat (where are bet they get them for far less), they reduce the warranty cost a lot. And BTW if you have CPO, good chance you can get the next set during late CPO--as I did on my D3 A8. That costs Audi $ too. Pricy for us as owners too with second winter wheel sets, unless one runs w/out the sensors and maybe turns off the system via VAG COM. Thus, I think the owner of the business (Beru) sold it, a clue they saw the negative writing on the wall.

Meanwhile, technically take the 8PSI drop a bit further. Now lets say it is happening pretty fast--a big tire failure or cut--instead of over months or w/ a sudden big temperature drop. The direct sensors also respond if they see a sudden pressure drop, and a lot quicker (they look for both absolute pressure, and rapid pressure change and at a lot less than 8 PSI for sudden change AFAIK). Unlike the indirect systems, I don't think they are assuming anything about the other three tires too, theoretically relevant both for unlikely scenarios like a nail/spike strip field, but also for all for tires going low gradually. And frankly at 70 + MPH if it is going down fast I want to know that RIGHT NOW, not in some seconds as the tire starts to flatten visibly or as I start to feel it in the drivetrain more obviously and wonder what is going on. By the time I feel it on a freeway, at a minimum the tire is probably already toast, let alone the safety issues. Had that happen on two Audis over the years, and seconds matter, even more so in the back actually. Or at Autobahn speeds frankly it could be literally a life or death delta.

Net, I'm not a fan of manufacturers cheapening out systems when lives matter. That's my view of the bottom line here, in more ways than one! And yes, it probably still covers 90% + of the scenarios good enough, and certainly better than no system. On a middling performance Toyota, where I didn't expect much more and ran into the indirect systems the first time in 2004, so be it and it didn't have any great driving potential anyway. On better Euro vehicles, I expect better when the technology is there. And as NHTSA, I would hope for better longer run too. I don't drive vehicles that fit the Ford Explorer/Firestone rollover fiasco with barely adequate tires, body on frame construction married to manufacturer desire to plush out the ride, and owners who seemingly never checked the pressure, but I expect them to be thinking at a higher standard than those too. Instead, I now get an indirect system that "meets code" and a U.S. mandated uniform print/type door jamb sticker in three colors with the maximum load PSI only, assuming I can't read a manual or understand tire pressure settings or maintenance as the dumbed down lowest common denominator approach.

Bob Petruska

That is correct, they won't trigger the warning. One tire needs to have a less pressure from the other tires by 8 psi or more.

81Coupe

The direct system has one more advantage: it gives you each tire's psi reading at the click of a button (at least that's how my wife's RDX works). That way, you can check your tires much more easily and much more frequently. Hence it is far less likely that any tire's pressure can gradually decrease to the mid 20's without being noticed.

MP4.2+6.0

both tire pressure AND temperature at all four road wheels, AND that often forgotten spare that can disappoint when/if finally needed. On my W12 I even know the right tire typically runs 5-8 degrees warmer than the drivers side, apparently relating to a buried aux cooler just in front of the (vented) right front fender liner.

All gone now with the U.S. D4, let alone the Q5. Of course on the spare that was back in the day when they were full sized, often on the OE road wheel and didn't just come with either a compressor or a can of goo (Hybrids) and some cracker jax instructions.

snagitseven

With Audi, AFAIK, only the A8/S8/R8 have provided actual PSI readouts. My '09 A4 with direct sensors didn't even identify the individual problem tire much less PSI readouts, just a general warning that one of the tires was low.

acadianbob

Yes or one that is over-inflated as in my service case

This really disturbs me. It is -17 actual here in MN at this very minute. Really cold weather affects tire temps and you can actually lose the seal to the bead if your pressure gets too low; resulting in sudden deflation. This system would never let you know that you might be in trouble.

NABS4

I like Audi's system. It is simple and it works. No tire sensors to replace and no additional senors required with winter wheels. Stop being lazy and check your tire pressures periodically.