TPMS Summary Thread

I went to the dollar store and bought a sheet of happy face stickers. So, during the winter when I have my winter wheels on and the summer ones are in my storage locker, I put the sticker over the little light and all is good. The sheet has lots of stickers left for the coming years. Unfortunately, the clear plastic is actually quite far from the light itself, so the paralax means that while I can place the sticker so I'm not annoyed by the light, my passengers can still see part of it.... oh well... they can buy their own stickers if it annoys them.

 

I just had Michelin tires installed at Costco and they suggested that they would replace the valve cores with ones of stainless steel to prevent oxidation, put on new metal valve caps, and replace the rubber mounting washers which deteriate with age and heat - all for $2 per wheel.
The rubber parts should always be replaced on metal valve stems regardless of type when putting on new tires. The same reason that they replace the rubber valve stems on ordinary rims.
Walt

 

If you can use a code to set the ECU to read 5 sensors, you might be able to use it to set ZERO sensors, and thus shut system off.....

 

It's springtime in the northern hemisphere and I thought a thread revival was appropriate. For those who have 2 sets of rims, you might want to consider purchasing a BARTEC scanner which will let your Prius "relearn" TPMS sensor codes.

Toyota Prius TPMS Bartec Wheelrite TECH400 | Hybrid Diagnostics

 

It's a bit pricey at $1400...

 

Sorry, I should have pointed out that the BARTEC Tech300 scanner is $350. At $50 per swap, the unit pays for itself in 3.5 years. If you have a buddy who swaps wheels too, and change out their TPMS for like $20, it's even faster.

Bartec Trackside TPMS tool discontinued. Should we carry the Bartec Tech300? - Corvette Forum

This is the DIY alternative if you're like me and you don't actually care if the sensors are on the wheels or not (because I'm not another dumb consumer who can't do tire pressure checks): http://www.tundrasolutions.com/forums/tundra/179120-easy-tpms-issues-solved-pictures-included/

 

But the 300 can't connect to the OBD2 port. The Prius (all Toyotas?) requires the data to be input via OBD2...

The DIY alternative at the TundraSolutions site is cute. Put the sensors inside a piece of PVC pipe, seal it, and pump it up to whatever you think TPMS wants to see.

 

Here is a new problem to wonder about.

Since the original tires were rated maximum 44 psi, is the TPMS on the Prius rated for a tire whose maximum rated psi is 51?

Is it possible to just rebuild the original ones with a higher rated valve stem?

 

The standard sensor works fine on my spare tire at 60 psi. The protocol has a limit of 77.9 psi, which I've never tested.

 

My TPMS works fine with tires rated to 51psi, FWIW.

 

That's such a weird number that I have to assume it's a translation of a different unit. Perhaps it's transmitted as a byte (0-255) where each step is a change of 2 kilopascal (kPa)? That would make the range 510 kPa which is 73.97 psi. No other unit seems close to a power of two. Well, 77.9psi is 4024 millimetres of mercury (mm Hg) which is near to 4096, or 4080 if it's 16 x 255 which would be 78.88 psi. However, I think using a proper SI unit would be more likely - pressures are at least measured in kPa or bar in metric countries.

Toyota have form for using scaled integers like this: the HV battery voltage is given in the service manual as being reported (to the diagnostic tester) as a range of 0 to 510V.

 

My notes from several months ago show that the data is a single byte. Sample values are: 0x00 = -14.5 lb, 0x28 = 0 lb, 0xff = 77.9 lb. These are values forced into the data stream and displayed by Techstream. Several other values collected from the warning level message (probably same units) are 0x7d = 30.8 lb, 0x80 = 31.9 lb, 0x83 = 33.0 lb, 0x85 = 33.7 lb. So the values appear to be absolute pressure, but not quite kPa units.

TPMS also reports tire temperature. The single byte appears to be degrees Celcius, biased so that 0x00 = -40 C.

The full set of TPMS messages that I decoded are in post #15 of http://priuschat.com/forums/gen-iii-2010-prius-technical-discussion/73944-obdii-codes.html

 

That decoding was very impressive. I was amazed too that temperature was also monitored. I am wondering if it is possible to set something up to monitor the temperature in addition to the tire pressure.

However, just because the TPMS can monitor a pressure of 77.9 lbs doesn't necessarily mean that the valve stem was designed for that pressure. It could have just been a result of the way the TPMS was programmed, which could also be somewhat generic for other Toyota vehicles too.

 

Spare tire TPMS is standard on at least some Lexus. It's one of the things that Toyota uses to differentiate Lexus from Toyota. But they apparently use the same ECU and sensors, and the sensors are designed to work with the 60 psi used in donut spares. It would be interesting to know how the 77.9 lbs was speced out, since the units don't match anything in the known universe...

 

The TPMS ECU is on the KWP bus, not the CAN bus. The ScanGauge can't cope with the Toyota KWP bus. Neither can my $25 Hong Kong ELM clone. So even though KWP is just 5 volt RS232 on a common send/receive wire, it takes an expensive interface to talk to it. The $500 Mongoose works, as does the Denso device used in the Toyota dealer version of Techstream. $10 worth of parts should be adequate to connect to a standard serial port on a laptop (if laptops had serial ports any more...)

 

Thread thanks, and possible leaky cap story

Or spend yet another $12.50 to get a USB/serial convertor. But better check that there are favorable reports of the specific model working OK on your OS, talking to your software, and talking to your hardware. The emulation is never perfect, and drivers for old ones may not be available on newer OSs.

By the way, thanks for all the info on this thread. Our TPMS lit up last night. My wife was on a priority mission and ignored it. This morning the tire read 6 psi. I hope it was continuing to leak overnight, as otherwise the tire must have overheated at least some (trip was about ten miles one-way, with a four hour parked time in the middle).

I have hopes that the fault was in the cap. I lost one of the OEM caps while checking pressure a year or two ago, and just grabbed the first cap out of my spares bin. It happened to be one that was designed to mechanically detect and display low pressure. This means it has to depress the valve stem, then contain the air in itself. While it has never failed me before in some years of on-tire mounting, and thousands of hours of driving, I speculate that a bit of dust in the wrong place could put a leakage path clean through. I replaced the cap with a different type this morning, and could not detect a drop from the 38 psi I had put in after an hour or two, so I think the current leakage rate is low. The outer sidewall does not look either thermally or mechanically stressed to my inexpert eye.

 

Re: Thread thanks, and possible leaky cap story

The TPMS warning light should have come on at about 75% of normal pressure. That would be about 24 psi if the system was initialized with the pressure at 32 psi.

 

sequence details

My wife used the car that afternoon. She is pretty confident that she would have seen the TPMS icon had it stayed on on initial startup in our dark garage--less confident that she would have noticed it on startup at church, and less confident that she would have noticed it if it came on mid-run in daylight.

But I think she would have, so most likely the tire went down from over 24 in late afternoon to under 24 in evening and was 6 in the morning.

As I run a bit higher than standard (high 30's), I want to take the recalibration measure I learned from this thread to get the warning threshold up to the low 30's. I plan to cold inflate moderately above my normal, recalibrate, drive around a little, then bleed back to my normal.

 

RobH's data fit very well to a straight line:

Pressure (psi) = -14.5037738 + 0.362594344 * (byte value) , or

Pressure (kPa) = -100 + 2.5 * (byte value)

I will test on my car soon.