yea, right

 

Yeah it could be a couple of things, so correcting the pressures and taking her for a spin will at least provide you help in troubleshooting. If you have a dead battery in a sensor or a malfunctioning TPMS SYSTEM it will help isolate a little. Since it stores (the TPMS head) a number of codes, you have to do at least one trip to correct/isolate if one tire on a previous trip was low/beyond settings. Once you make sure the pressures are correct, a drive of over 20 for a minimum of 2 or so minutes should eliminate a actual low or critical reading and reset. If it is a bad battery or problems with communication, your not going to see a resolution. Oh, don't forget a minimal battery/batteries in the sensors. They are the kinds that when they warm up to a higher temperature (in cold weather places) they will provide minimal sufficient voltage/current to operate correctly. But then when they get colder again, POOF you get erratic indications (if system is nominal). Its a good read there in the service manual, I suggest you review it and understand its operation (unique) to the XF platform. Not all TPMS systems/hardware are the same, and unless the same system/programming hardware/software in installed in both cars, they results may be different.

 

Easy answer to "Now, I askya, how in heck did the computer NOT turn the light on ?", the transmitters couldn't sent the signal to the TPMS receiver so as far as the puters were concerned, nothing is wrong. Its the same as removing the power pin (that mod) to the TPMS system. You disabled it by non-sending wheels/tires/sensors!

Logical, me friend, Logical!

 

Logical ?????
so, you are saying that if both batteries in my rear wheels happen to die at the same time, that no error would be indicated ? ( transmitters couldn't send the signal )

 

George, thats exactly correct but the condition of both batteries probably would never happen... The two wheels that have no sensors would never be sending a low or critical condition in regards to the pressure (psi), so while driving no signal would reach the TPMS receiver so there is nothing for the receiver to process. No signal, no triggering of the lamp. Kapishe(sp)?

Going out on a limb here, I think the TPMS sensors in our cars wheels only transmit a low or critical PSI signal. The registering/programming is there ONLY to help jack up the price (diagnostic) that the tech charges you to locate the errant sensor(s). They have to generate $ for those damn machines, just another way to bend you over in my opinion. Thats why when my sensor battery(s) start triggering due to some crazy reason, I'll be pulling that receivers positive power pin and tucking it up with tape on it. Also, there is a diagnostic feature on this setup, as when the indicator lamp on the dash flashes repeatedly (about a minute I think), and then stays lit, it means something is wrong with the TPMS system. It does not necessarily mean a tire is low. In this condition, it would be prudent to do the pin removal mod as that would be very expensive to fix. Oh, a footnote, if you do the power pin removal thing, and the TPMS light does not reset after a while, you may have to either get the light reset (dunno if a standard OBDII tool will work or not) or suspect something other than the TPMS is causing the flashing lamp on the TPMS circuit. There are a few (if not many) things connected to the trigger output on the TPMS bus.

 

Cool .. that explains why you don't need to put the sensors in new wheel purchases.. Just remove them all and you'll be fine..

 

I would think so... Nothing beats the checking weekly (or more often if you like) the manual checking of tire pressures. You just cannot legislate safety in tire maintenance. TPMS just makes it a reminded issue providing the system is working properly and the manufacturer doesn't make it harder and more expensive to keep it that way. We all know the manufacturer will make it easier for you to give them your monies, right? If removing those TPMS sensors at the next tire purchase doesn't work, just do the pin removal mod and if you still have the TPMS indication, you'll either need to have it reset and/or you have another problem which is triggering the lamp in the dash.

 

It is the signal that keeps the lamp off, no signal and the lamp goes on.
Maybe when the pressure is low there is no signal sent.
Also no signal from a dead battery and the light goes on.
See Beaners post here.

 

I don't have a reason why... or a driving desire to find out why

.. but THEY WILL SEND A SIGNAL OF A SORT WITHOUT HAVING TO DRIVE IT.
Been there, seen it for myself, and

"I think" from the Pope himself isn't going to change my mind on it.

 

Then your centrifugal switch on one of your sensors is stuck, or the second part of my post below is causing it.



I see nothing in the service manual that proves this but thats not to say the TPMS receiver doesn't have a code in it that requires a signal from any one of 4 sensors, or one from each, over a period of time or miles. The systems tie all in together so if there is a time/mileage limit somewhere in the systems interweave, then it is beyond me. If you get the light and don't want to **** with it, just do the pin mod. Seems to me if the pin mod works though, then the timer/mileage signal to the indicator lamp MUST come from the TPMS receiver. Remember, the TPMS system is mostly comprised of 5 components. The TPMS RECEIVER (just receiver) and the 4 TPMS sensors (IE TRANSMITTERS). There is no data flowing from the receiver in any way to the sensors. Just an illumination of a dash light when it (the receiver) gets a signal from any one or more of the sensors. Just no other explanation that I can come up with.

 

I think TPMS is a work of the devil not intended to be understood by mortals.

 

**Update**

Started my roadster today for the first time since I noticed the low tire pressure light last December. It did not come back on this time, so it obviously reset itself during the time the car was sitting in the garage. I figured it would not reset until I drove the car but that is not the case.

I still can't figure out how these things work, but they do. Bugs the hell out of me trying to figure out the operation of these but I need to move on.

Again, thanx for everyone's input. Happy Motoring all.