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Nein Kinder
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Discussion Starter · #1 · (Edited)
I just moved from Anchorage to Denver. My 2011 Lotus Elise SC stock tire pressures were set to the factory 26 front / 28 rear cold values before leaving Anchorage. When the car arrived in Denver, I checked the tire pressures cold and they were - as expected - about 3 psi high. I corrected them to spec by bleeding the pressure down. I use a high-quality AccuGage gauge that is good for +-1 psi.

I now get a TPMS low pressure warning for LF and RF, but no warning for the back tires.

Tire pressure is set to gauge pressure, not absolute pressure. Both my tires and my pressure gauge are affected by ambient pressure, so gauge pressure is always the correct setting, even as altitude changes. At 5000' MSL, there should be about a 2.5 psi drop in atmospheric pressure compared to sea level, so what I saw on my gauge was expected.

I read the comments from the final TPMS rule in the FMVSS regarding temperature and altitude correction ... NHTSA foolishly (IMO) decided to ignore any requirement for temperature or altitude correction. There are commercial systems for large vehicles that incorporate both these features, going beyond the standard. FMVSS says TPMS has to alert at 25% low (or a preset number applied to all the tires), but that would be 26 x .25 = 6.5 psi low for the fronts. It appears, however, that the system on my Lotus is alerting at a much smaller value, maybe as little as 4 psi low. And it is certainly measuring *absolute* tire pressure and reporting on that.

Setting my tire pressures too high is not an acceptable solution, despite seeing a lot of dealer posts that suggest just that. So my question is this: is there any way to modify the TPMS settings to account for altitude? Failing that, is there any way to disable TPMS without getting warnings on the dash?

Glen
 

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i just ordered a set of tpms off ebay for my exige, going to try the pvc route and not having to see that annoying light anymore.
 

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Nein Kinder
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1,568 Posts
Discussion Starter · #5 ·
Yes - I found that too. I'm a little skeptical of a reset since every description I can find of the system says no reset is necessary. If all the logical actions fail, it may be worth a try. I guess I'd tell Discount Tire it's a Saab 9-3 if they don't have something specific to the Lotus.

I also found the thread describing sealing the transmitters in a pressurized tube. That too defies the system descriptions I've found (where the transmitters don't operate unless spinning). Maybe all the descriptions are wrong...

My original adjustment was to bleed the tires down to the recommended pressure. I'm going to try dropping the pressure down to about 5 psi then bringing it back up. I can imagine a system that would have hysteresis built into it to ensure that it always met the FMVSS requirement for alerting at 25% low. That is, the TPMS pressure measurement would be designed to lag behind actual pressure (i.e., read low) during a decreasing pressure trend. Perhaps if I induce a rising pressure in the system, I can get on the other side of that hysteresis and extinguish the light. I'm only about 3 psi (absolute) low in Denver, so if the tires are filled to spec, that should still keep me about 3.5 psi above the 25% threshold. (Fronts: 26 psi x .25 = 6.5 psi low to alert.)

Glen
 

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Nein Kinder
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1,568 Posts
Discussion Starter · #6 · (Edited)
Problem Resolved

I was finally able to verify the accuracy of my bourdon tube tire gauge. It turns out the gauge I was using indicated about 3 psi higher than the actual pressure. Add the 2.5 psi absolute pressure drop between sea level and the elevation in Denver, and the result was enough to trigger the TPMS warning. With my tires pressures set correctly using a bourdon tube gauge calibrated to +-1 psi, I no longer receive a TPMS warning.

I'd have to move to an elevation of at least 10,000' and then set my tire pressures at that location to get a false TPMS warning. That probably won't happen anytime soon.

So I learned a few things here:

- the TPMS in my Lotus uses a reference to absolute pressure. It does not compensate for altitude or temperature.

- a calibrated bourdon tube type gauge should be used to set tire pressures, especially if pressures are set at elevations significantly above or below sea level. Spring-type tire gauges do not compensate for elevation and only provide a reference to absolute pressure.

- at 5000' elevation, my TPMS alerts for the front tires at 26 x 0.25 - 2.5 = 4 psi low.

The uniqueness of the Lotus plus the TPMS finally exposed the inaccurate tire gauge I've been using for years. To establish normal tire pressures for all my vehicles, I'll drive the vehicle for several miles at 55-65 mph on a fairly straight road at ambient temperatures of 60° F - 80° F. Then I use an infrared temperature gauge to measure tread temps at the inside corner, midpoint and outside corner of each tire. I then adjust the tire pressures to get the most consistent temps across the face of the tread. For example, if the midpoint of the tread was hotter than the shoulders, I'd reduce the tire pressure to put more load on the outer part of the tread.

I'm usually able to get the temps across the tread face within 2° F of each other. Elements that can affect this procedure include camber, caster, air flow around the tires and proximity to external heat sources like exhaust systems. This procedure has resulted in very even tread wear across the face of the tires that I set in this manner - for hundreds of thousands of miles.

The Elise has a lot of camber in the rear tires, and even with factory tires and pressures, the inside corner of the tread runs about 5° F hotter than the outside for straight-line driving. It's set up this way on purpose and my procedure for setting pressure using temps wouldn't be valid. My Elise is the first car I've had in 25 years that I just set to the factory pressures. With my new accurate tire gauge, I'll have to adjust the *indicated* pressures I use on all my other cars, then verify it with the temperature gauge again.

Glen
 
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