Tire pressure readings

[C "T" ess]

I keep getting messages from On-Star to check my tire pressure. When I check the DIC; the numbers as you would expect, are the same reported by On*. I check my tire pressure and have my own compressor, so checking them is not an issue. I have tried three different air pressure gauges (all known to be accurate) and even when I get a dead on 35 lb. reading the DIC shows odd numbers off by a pound or two. The dealer says there is nothing wrong with the monitoring system but will not go so far as to say the information is approximate. Even when I had them check the pressures at the same time, by the time I left all of the numbers were skewed.
Has anyone else had this situation. Has anyone been able to get the DIC to match real world numbers?

[928S]

C "T" ess,

The only discrepancies that I've noticed were due to the fact that TPMS/DIC rounds and displays psi in 1.0 lb increments and my handheld digital gauge in 0.5 lb increments, so I assume that both are giving me accurate readings. Based on my experience (1 yr 8 mos), I'd think that something is amiss if you're getting more than a 1.0 lb (cold) difference between your TPMS/DIC and your handheld gauge.

[JimboVT]

The tire pressure sensors use "absolute pressure" and normally our gauges measuring the tire pressure by "relative pressure". Not sure where you are located but if you are located in mountain cities, the "relative" pressure gauge is affected by altitude while the tire sensor is transmitting "absolute" pressure. Theory is the same as it's difficult to make water boiling in mountain area. If we inflate the tire in Denver, CO (mile high) than drive to Los Angeles, CA (sea level), we will find you lost 5 psi by "relative pressure" reading. And if you compensate 5 psi while we are in Los Angels than drive back to Denver...you will find something very interesting by using the "relative pressure" gauge.

[928S]

JimboVT,

It's my understanding that all handheld tire pressure gauges are vented to ambient atmospheric pressure and, therefore, measure a tire's absolute pressure minus atmospheric. Likewise, the TPMS senses and omits variable atmospheric pressure from the DIC display. So any discrepancy between handheld gauge and TPMS/DIC must be due to calibration and/or rounding error - not altitude, elevation above sea level.

[MoFex]

I had the same problem. I hated when the DIC showed different tire pressure in every tire. So I purchased a tire pressure gauage with 0.1 PSI resolution. It solved the issue.

[gonzalezfj]

Originally Posted by 928S

JimboVT,

It's my understanding that all handheld tire pressure gauges are vented to ambient atmospheric pressure and, therefore, measure a tire's absolute pressure minus atmospheric. Likewise, the TPMS senses and omits variable atmospheric pressure from the DIC display. So any discrepancy between handheld gauge and TPMS/DIC must be due to calibration and/or rounding error - not altitude, elevation above sea level.

Sorry, but your statement is incorrect. The GM TPMS sensors (like most other manufacturer's) read pressure against a constant reference of 14.7 PSI atmospheric, while your tire pressure gauge reads true gauge pressure. For the vast majority of owners the reading discrepancy when actual atmospheric pressure changes (like when going up or down in elevation) is not significant enough to matter.

Frank Gonzalez

[st4422]

If you would like to see if the difference between gauge pressure and absolute pressure is significant at your location go to the link below. There is a table of atmospheric pressure at different elevations. The difference between atmospheric pressure at your altitude and 14.7 PSI will be the difference between a tire gauge pressure and the GM TPMS sensors.

Link

[928S]

Originally Posted by gonzalezfj

Sorry, but your statement is incorrect. The GM TPMS sensors (like most other manufacturer's) read pressure against a constant reference of 14.7 PSI atmospheric, while your tire pressure gauge reads true gauge pressure.

Oops! Thanks for the heads up.

I'm surprised that GM provides no such disclosure in the CTS Owner's Manual. After all, the USA mean elevation is 2512 feet (13.4 psi vs. 14.7 at sea level), so theoretically the TPMS understates tire inflation by about 1.3 psi on average, and in mile-high Denver CO the error rises twofold to 2.6 psi.

[ewill3rd]

The key thing to remember here is that OnStar is idiotic when it comes to things like this.
Some computer tells another computer that your pressure readings aren't right and they just automatically tell you that there is a problem.

It is possible that the readings are too high also.
The tire plackard has the recommended inflation pressure. If you set the pressures higher than that spec and then drive the car the pressures will go even higher.
I don't know at what threshold OnStar will send you a message, but honestly apart from it being annoying if you are happy with the pressures you run your tires at, I would make sure they are what you want and disregard anything you hear from OnStar. A lot of those people don't know which end of the car is the front.

[gm250061]

I had this same problem with the tire pressure then I realized that after I had the tires rotated the sensor was calling out the wrong tire. I am still trying to figure out how to reset this.

[928S]

gm250061, here ya go...

http://www.cadillacforums.com/forums...ml#post1559368 (Tire Pressure Monitor-- Reset ?)