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Discussion Starter #1
My 2020 has all tires set to the same pressure (35 lbs psi) in the garage where it's been parked overnight, so no direct sun.

LF = 38
RF = 36
LR = 34
LR = 34

My digital gauge and my Snap-on dial gauge both say 35 lbs psi. My previous non-GMC car TPMS system always tracked the gauges perfectly.

The LF is reading 8% high. If I inflated the tires per the TPMS that tire would be almost halfway to being considered flat (20% below the pressure needed to sustain its load).

Is it normal for a GMC system to show this much variance?

Thanks.
 

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Not that I have noticed on my '11. There is always a difference between the TPMS and tire gage, but the difference between the tires is the same as the gage reads. I set the tire pressure by the gage and use the TPMS as a reference from the gage.
 

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Discussion Starter #3
Thanks. I do as well but when the TPMS is all over the place compared to the gauge I wonder whether it's reading accurately at all. This probably will be my first dealer visit issue.
 

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Mine are about +/- 3%. I use a good tire gauge to check and only view the tpms as a possible low tire warning device. I took one apart to see what is in the device and there is a electronic circuit that uses a pressure diaphragm through a tiny hole (transducer?) to measure the air pressure in the tire. Accuracy is subjective in my opinion. Even though my tire gauge shows all tires at 35 psi the tpms is plus or minus 1-2 psi in different tires. They also have batteries that are supposed to last about 7-10 years according to different folks but i have seen them fail as soon as 1 day to 4 years.
 

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Discussion Starter #5
I took one apart to see what is in the device and there is a electronic circuit that uses a pressure diaphragm through a tiny hole (transducer?) to measure the air pressure in the tire.
You may be able to answer my next question. Is the TPMS sensor on your 2013 removable from the valve stem? I need to replace the rubber valve stems with aluminum valve stems and if the sensor can be moved to a new valve stem it will be a whole lot cheaper.

I saw some Schrader valves at Autozone that supposedly fit my car and it looks like they have a small bolt at the bottom that secures the sensor to the valve stem. Is that how yours works?
 

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My TPM readings are usually close to my gauge. After it has sat for a day or two I need to drive a few blocks before it updates.
 

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I've found that the TPMS readings are usually within 1 or 2 PSI of measured air pressure on most vehicles I've owned for the past 20 years or so. And that is some 18 vehicles.
 

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You may be able to answer my next question. Is the TPMS sensor on your 2013 removable from the valve stem? I need to replace the rubber valve stems with aluminum valve stems and if the sensor can be moved to a new valve stem it will be a whole lot cheaper.

I saw some Schrader valves at Autozone that supposedly fit my car and it looks like they have a small bolt at the bottom that secures the sensor to the valve stem. Is that how yours works?
My original TPMS that came with the car were not removable from the valve stem. The new AC Delco ones did have that screw at the bottom of the stem and probably could be applied to a new stem. Just one of those replacement TPMS units from the Chevy dealer was $80.00 but the ones I got from Amazon (AC Delco) were $35.00 for all 4. One other issue is that the new TPMS stems did have a 1/4oz less weight so a rebalance of the tires was necessary.
 

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My 2020 has all tires set to the same pressure (35 lbs psi) in the garage where it's been parked overnight, so no direct sun.

LF = 38
RF = 36
LR = 34
LR = 34

My digital gauge and my Snap-on dial gauge both say 35 lbs psi. My previous non-GMC car TPMS system always tracked the gauges perfectly.

The LF is reading 8% high. If I inflated the tires per the TPMS that tire would be almost halfway to being considered flat (20% below the pressure needed to sustain its load).

Is it normal for a GMC system to show this much variance?

Thanks.
My 2015 is similar first thing in the morning, or after sitting for several hours. Tires all read a different pressure even though they were inflated and all measured the same on my handheld gauge. After 20 minutes of driving the TPMS readings will balance out and all show the same on the display. I agree it is kind of annoying. I have learned to use the TPMS display as a general overview instead of an exact reading. As long as none of the tires are reading below the recommended cold pressure, all is good!
 

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I do notice when the car is sitting overnight that TPMS readings will be somewhat off.
But as @arcee says, after driving a bit the readings even out.
 

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Discussion Starter #11
My original TPMS that came with the car were not removable from the valve stem. The new AC Delco ones did have that screw at the bottom of the stem and probably could be applied to a new stem. Just one of those replacement TPMS units from the Chevy dealer was $80.00 but the ones I got from Amazon (AC Delco) were $35.00 for all 4. One other issue is that the new TPMS stems did have a 1/4oz less weight so a rebalance of the tires was necessary.
Perfect, thanks for all of the detail. Fingers crossed...
 

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Discussion Starter #12
Thanks to all who so generously offered their assistance. I decided to drive the car today and see how the TPMS responded. I pulled it out of the garage and suddenly all of the pressures changed to where I expected them to be. And in case you're wondering, no, it is not a really, really long garage. 🙂
 

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The TPMS will not operate unless there is rotation of the tires, as you observed.
 

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The TPMS will not operate unless there is rotation of the tires, as you observed.
I dunno . . . I rotated my tires the other day and the TPMS still was not correct . . . 🤣 . . . 🤣
 
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+/- 4 psi is the pressure change that is noticeable for ride, wear and performance. Any change less then that makes no measurable difference except maybe on a high precision dyno. PSI accuracy is not that important
 

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Agree, in this case, consistency may be more important than accuracy.
 

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Discussion Starter #17
The TPMS will not operate unless there is rotation of the tires, as you observed.
Great minds think alike. ;) I found this link late last night: Chevrolet Equinox Service Manual - Tire Pressure Monitor Description and Operation TPMS - Wheels/Tires

Selected excerpts:

When the vehicle is stationary, the sensor's internal accelerometer is inactive, which puts the sensors into a Stationary state. In this state the sensors sample tire pressure once every 30-seconds and do not transmit at all if the tire pressure does not change. As vehicle speed increases, centrifugal force activates the sensors internal accelerometer causing the sensors to go into Wake and then Drive mode. In Drive mode, the sensors sample tire pressure once every 30-seconds and transmit in Drive mode once every 60-seconds.

The sensors continuously compare their last pressure sample to their current pressure sample and will transmit in Learn Mode-Pressure Triggered if a 8.3-kPa-(1.2-psi) change in tire pressure has been detected in either a Stationary or Drive state.

Both the indicator icon and driver information center message can be cleared by adjusting the tire pressures to the recommended pressures and driving the vehicle above 40-km/h-(25-mph) for at least 2-minutes.

Driving the vehicle above 40-km/h-(25-mph) for at least 2-minutes will activate the sensors causing the driver information center to display the current tire pressures.


The second paragraph is the interesting one because it seems to say that a change in pressure should show up even if the car has not moved and the pressure change is more than 1.2 psi. That's the part that does not seem to happen. Whatev. At least now I understand the system better.

Thanks.
 

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TPMS is not designed to give 100% accuracy of pressures but to give warning of a impending flat.
Most will trigger at 26 -28 psi so tire is not completely flat and driver will still have adequate control of vehicle when driving and be able to pull over safely is the design strategy.
If air is low enough it will trigger immediately on a parked start up.
 
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Agree .. the TPMS sensing is for monitoring a minimum safe tire pressure with some safety margin giving enough time for an owner to address the need to remedy.
 
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My 2020 has all tires set to the same pressure (35 lbs psi) in the garage where it's been parked overnight, so no direct sun.

LF = 38
RF = 36
LR = 34
LR = 34

My digital gauge and my Snap-on dial gauge both say 35 lbs psi. My previous non-GMC car TPMS system always tracked the gauges perfectly.

The LF is reading 8% high. If I inflated the tires per the TPMS that tire would be almost halfway to being considered flat (20% below the pressure needed to sustain its load).

Is it normal for a GMC system to show this much variance?

Thanks.
 
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