I just experienced a minor but interesting problem.
Before departing for my regular Pittsburgh-NY weekly trips, I always check fluids and tire pressure (not that I don't trust the electronics, but I'd rather avoid dealing with a fault message in the middle of a 400m trip..). So, on Thursday I checked all four tires, using the BluePoint gage recommended by Michael elsewhere in this forum.
Three tires were fine, but I just could not get a reading of the left rear: every time I tried, air would escape from the valve before the gage could register a reading. I tried a different gage (vdo): same problem. So, i looked closely and touched the valve stem, and.....the top 1/5 of an inch of the thread ring (where the small grey plastic cap is threaded) came right off! In other words, it was broken and barely holding, and came loose completely when i connected the tire gage. Note that there is absolutely no sign of road damage to the wheel, not even kerb rash. I took a lot of pictures, will try posting when I get back home next weekend.
In any case, I called my dealer, Imports by Day in Pittsburgh, first thing Friday morning: Larry the Phaeton tech was off for the day, but they had a replacement valve in stock, so I drove over and they replaced it in less than a half hour (again, I have pictures). I'm very grateful to them for taking care of this so rapidly and efficiently, as I had to drive back to NY Friday afternoon.
Now, to reach them I have to drive approximately 15 miles, of which three or so on a winding back road, and the rest on interstate 76. This is what I observed from the TPMS system:
1) Driving out: after about 5 minutes, "defective wheel on board" message in the small screen (yellow alert). The large screen shows no pressure readings available on any tire, system "temporarily not available".
2) Maybe 2 minutes later, red "flat tire" alert, large screen shows the problem is the left rear tire (the one with the broken valve stem), the other three and the spare now read ok.
3) At the dealer I specifically asked them not to re-set the tpms after repairing the tire valve, since I wanted to observe its behavior.
4) Driving back from the dealer: an immediate "defective wheel on board" yellow alert, same as #1 above.
5) after about 5 minutes, a red "flat tire" alert, located at left rear tire, all others now read ok. In other words, the exact same beahvior as #2 above, despite the fact that the tire is now inflated at the right pressure.
6) After a further several minutes of driving at approximately 65 on the interstate, warning goes away, tpms now reports all tires ok. And so it remained all the way back to NY.
Now, here are my conclusions, keeping in mind that the day was unusually mild for the season: about 55F and sunny, in fact the dealer's service department was operating with the doors wide open:
- As for #1 and 2 above, I believe they are related to the interaction of the temperature and the air loss during my repeated attempts to get a pressure reading (foiled by the broken valve stem). Since I started the trip to the dealer with the car cool (but not cold: in my garage in unusually mild weather), the tpms first matched the air loss to its "minor loss over time" curve, but after a few minutes of observing the same loss (or perhaps a slightly higher loss due to some minor leak from the damaged valve) in the presence of rsising temperature, it identified it as a "flat tire"condition, hence red alarm.
- The tpms readings after the repair are a little harder to justify. The only explanation I can think of is that the system kept recording pressures throughout the repair, because the key remained in the ignition and the repair itself took less than an hour. Despite the pressure being normal after the repair, there was a loss of signal while the wheel was off the vehicle, hence faulty wheel. Later, on the interstate, the wheel temperature rises fast, making the system reevalute the readings of the prior hour or so (low pressure on rising temperature-->loss of radio contact-->normal pressure on colder tire-->rapidly rising tire temperature): somehow, this sequence must fit a predetermined "flat tire" profile in its program. Finally, more time goes by and, in the absence of any further abnormal readings, the cpu concludes that everything is now fine.
One final observation: the valve and stem are of different material and much lighter than anything I have on my other cars. The first impression is of "cheaper" material, but I suspect there's another explanation: lighter (as in weight reduction), perhaps to compensate for the added weight of the Beru pressure transmitter mounted at the back end inside the rim. A small weight saving, to be sure, but perhaps enough to make a difference when balancing.
P.S.: Michael, I will need your help to post the pictures, as I have never done it.