This has happened two times, both under the same circumstances. While driving on California's Hwy 50 on the way to Lake Tahoe, my TPS warning came on. The indicator rotated from tire to tire for 10 minutes and then turned off.

The weather was 85 degrees and I was climbing from sea level to 7K feet. I have checked the tire pressure, both times, with a digital gauge and it was spot-on.

Any thoughts...?

 

See this article: The Influence of Altitude on Tire Pressure from TireRack.com: http://www.tirerack.com/tires/tirete...jsp?techid=167

"Significant changes in altitude affects tire pressures when traveling from one elevation to another. Fortunately this influence is relatively small and can be easily accommodated. Atmospheric pressure is the force exerted on objects by the weight of the air molecules above them. While air molecules are invisible, they have mass and occupy space. However as altitude increases, atmospheric pressure decreases. For example, atmospheric pressure pushes against the earth at 14.7 pounds per square inch (1 kilogram per square centimeter) at sea level, yet drops to only 10.1 pounds per square inch at 10,000 feet as indicated in the following chart.

Altitude (ft.) Air Pressure (psi)
Sea Level ....... 14.7
1,000 .............14.2
2,000 ............ 13.7
3,000 .............13.2
4,000 .............12.7
5,000 .............12.2
6,000 .............11.7
7,000 .............11.3
8,000 .............10.9
9,000 .............10.5
10,000 ...........10.1

When it comes to measuring tire inflation pressure, it is important to realize there is a difference between atmospheric pressure and gauge pressure. Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure.

Imagine removing the core from a tire valve and allowing the air to escape. Even after the air has completely stopped rushing out of the valve, the tire is still experiencing 14.7 pounds per square inch of atmospheric pressure. However, a tire pressure gauge would read zero pounds per square inch of tire inflation pressure because the pressure outside the tire is equal to the pressure inside.

Since a tire mounted on a wheel essentially establishes a flexible airtight (at least in the short term) pressure chamber in which the tire is shaped and reinforced by internal cords, it retains the same volume of air molecules regardless of its elevation above sea level. However, if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.

However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10-degree Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations.

This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home."


Stuart

 

Stuart,

Thanks for the in-depth explanation on the altitudes influence on air pressure. The fact is, I have been driving the XK, as well as, two Land Rovers up and down the hill for 10 years. The problem, as described in my post, is a recent "phenomena" which very well may be amplified by the climb.

With that said, it's my understanding that the warning is triggered by either; (1) a difference in a tires pressure, as it compares to the others and or (2) if the tire is 4 to 6 psi lower than the required pressure.

What I am failing to understand is why the dash icon warning, that illustrates which tire is out of range, was rotating clockwise highlighting each tire?

 

Tahoe Dave,

I suspect that the rapid 7,000 foot increase in altitude caused the pressure in all 4 tires to fall below the required minimum, thereby triggering the warning for all 4 tires.

I'm not a tech (and I flunked out of engineering school, fortunately!), so that's just my WAG.

Stuart

p.s. When you're parked at home (sea level) let the air out of all 4 tires to 20 psi and see if you get the same rotating tpms display. If so, you know that the increased altitude was the culprit and the tpms did what it was supposed to do.

 

When you say "it rotated from tire to tire and then turned off"...........by turned off do you mean it stopped showing a warning and now was back to normal?

 

The article quoted confuses more than helps in this case. It is correct in pointing out that standard tires gauges indicate by comparing tire pressure to ambient pressure- but that's not how the internal sensors work on cars. They have no reference to outside pressure AFAIK.

Furthermore- a standard gauge would give the (false) impression that internal pressure increases with altitude, not a decrease which would set off the warning. The article is correct in this regard.

If a person set off at sea level (14.7 lbs ambient pressure) and 35 lbs. internal tire pressure and drove to 10K feet altutude where ambient pressure is 10.1 psi, a standard gauge would read (falsely) 39.6 psi due to the increased differential between internal and ambient pressures.

The message is don't measure and adjust tire pressure at high altitude or maybe don't drive at 10K ft altitudes

 

I have checked, again for the third time, the tire pressure post trip. The front tires, at rest, are 30PSI and the rears are 33PSI. This is the door jamb stickers recommended pressure. The warning light comes on, then "rotates" around to each tire and eventually goes out.

I am of the opinion that the under the hood sensing unit is faulty?

 

Probably the module behind the rear seats
The wiring to the INITIATORS for the rear wheels can be seen by laying on the gnd near the rear wheels and looking up infront of the rear wheel are. The front INITIATORS require removal of the front tires and the panel in front of the front tires to see the wiring. Neither of which is the cause imho.

The RECIVER is next to your right knee under the console.

Memory here: INITATORS send signal to SENSOR in the wheel .........SENSOR responds by sending data to the RECIEVER near your knee. .............Reciever is hard wired to the MODULE which sucks up that reciever data, massages it and if all is not well, sends a message on the CAN bus.

I've had this problem for a year. I checked the INITIATOR wiring. Good. I massaged the two harness plugs to the MODULE. That resulted in very long periods of no TPMS message faults. UNTIL about a week ago where nothing will fix the problem except probably installing a new MODULE. I believe once this is done a JAGUAR techy in so many words, has to mess with it to make it communicate with the CAN. I'm not real sure of that. All I know is that TPMS is and was made for politicians and is close to being a useless piece of equipment that I care not to fix. Money thrown down the drain imho. I've had flats before. I do not need this bs piece of equip to tell me a tire is low. IF I can't tell a tire i low.......then I need to sell the car and take the taxi. I'm not a cheapskate. I own a SL.

 

Hailers,

You've nailed it. I received a call from the service manager at SF British Motors, who came up with the same diagnosis, given the symptoms. I'll have the module replaced, next time I'm in the Bay Area

Thanks to everyone for helping me out...