TPMS - 8 PSI Warning

[pfreeus]

I was curious about the TPMS system and when the low tire pressure indicator warning would come on. I figured that the cheap pen tire pressure gauge wouldn't be accurate enough so decided to look into what gauge would give me decent/consistent readings without breaking the bank. I ended up getting the Accu-Gage EZ-Air 100psi gauge from getagauge.com. It arrived today.

From my limited test (below), it appears the warning comes on when the pressure drops below 8psi from what was set.

This is what I did.

FRONT DRIVER TIRE
1) Pumped the front driver side to 40 PSI
2) Set the TPMS per instuctions in the manual
3) Turned on the car then proceeded to deflate the tire by one PSI, kick the tire then checked for the warning light
4) Repeated step 3 until warning light came on
The warning came on at 32 PSI
5) Turned off the car and pumped to 36 PSI
6) Turned on the car
Warning light was not on

FRONT PASSENGER TIRE
1) Checked pressure and it was at 33 PSI
2) Deflated the tire by one PSI, kicked the tire then checked for warning light
3) Repeated step 2 until warning light came on
The warning came on at 25 PSI

The reason for inflating the front/driver tire was I wanted to determine if it was a set PSI or a percent of set pressure (such as 20 or 25%). Granted this is a very limited test but I concluded that it has to be a set PSI and not percent. If percent, say 25%, the front driver would have gone on when the PSI was at 30 but it came on at 32.
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I wanted to set my tires to 42/40 and wanted to be told when it dropped below 2 PSI (basically be warned when the fronts were at 40 or below OR the rears were at 38 or belwo) so I did the following.

1) Increased the front tires to 48 PSI
2) Increased the rear tires to 46 PSI
3) Set the TPMS per instructions in the manual
4) Deflated the front tires to 42 PSI
5) Deflated the rear tires to 40 PSI
6) Turned on the car and checked that the warning was off
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Things learned.
- Not sure if this is a factory or dealer thing but my tires were nitrogen filled. I used compressed air to fill though.
- All tire pressures before the start of the experiment were 33 PSI. I'm assuming this was done so when the tires were rotated, it would be no big thing.
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What I didn't do but wished I did.
- After the last step 6 above, I wished I deflated the tire pressure by 2 or 3 PSI to see if the warning light came on. My bike pump broke so had to borrow a small cigarette lighter pump and friend took it back already. May try to get later and deflate.
- Same for back tire.
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What I would like to know.
I'd like to know if the sensor is specific to a wheel wheel or if the set PSI is specific to the wheel well independent of what registered sensor was there. I am hoping that it is independent so that when I rotate tires, I can just increase to 42 or decrease to 40 accordingly without having to perform the past steps 1-6.
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Now reading on line they say not to mix compress air with nitrogen (whatever) so I'm not recommending that anyone try the above. I comfortably feel that I will be okay but that is just me. I am not encouraging anyone to do what I did.

[rachaelseven]

Originally Posted by pfreeus

What I would like to know.
I'd like to know if the sensor is specific to a wheel wheel or if the set PSI is specific to the wheel well independent of what registered sensor was there. I am hoping that it is independent so that when I rotate tires, I can just increase to 42 or decrease to 40 accordingly without having to perform the past steps 1-6.

I've seen the sensor output page in the Techstream system and the set pressure is tied to the specific sensor ID and not the position on the car. The TMPS system does not know where the tire is, just what four sensor IDs are associated with it. And each sensor ID has its own set point. So when you rotate tires, you'll need to go through all your steps to have the settings correct again. Sorry. I can confirm from looking at that page though, that your observation of an 8psi offset seems to be correct. My tires were at 48/46 and my trip points showed in the system as 40/38.

As for mixing nitrogen and air, don't worry about it. The air you breathe every day is 78% nitrogen to begin with, so mixing the two will have no effect at all.

[mmurphy]

Originally Posted by pfreeus

From my limited test (below), it appears the warning comes on when the pressure drops below 8psi from what was set.

Nice to know. Thank you!

Marty

[jay_man2]

You pushed me over the edge to order that gauge and the Check-a-Spare valve extender. I now won't have to pull the spare all the way out to check its pressure periodically.

Chek-a-spare, Check-a-spare, tire gauge guard

[TKY]

According to the Owners Manual, we have to activate the system. I have not done this because I check my tire pressure regularly and I plan to buy winter tires with their own wheels so I'll mount them once without transferring or installing a set of TPMS sensors in the winter tires. I could just deflate the tires to see if the warning light comes on, but thought of this question while reading this thread.

Has anyone checked their TPMS to see if the dealership activated their system as part of the new car setup and prep procedures? --TK

[a64pilot]

Originally Posted by TKY

According to the Owners Manual, we have to activate the system. I have not done this because I check my tire pressure regularly and I plan to buy winter tires with their own wheels so I'll mount them once without transferring or installing a set of TPMS sensors in the winter tires. I could just deflate the tires to see if the warning light comes on, but thought of this question while reading this thread.

Has anyone checked their TPMS to see if the dealership activated their system as part of the new car setup and prep procedures? --TK

I had a low tire from a puncture a couple of weeks after I bought the car. System worked as advertised. I hadn't done anything to activate the system.

[pfreeus]

rachealseven, thanks for letting me know that the pressure is tied directly to the tire regardless of location. The next time I rotate (and do steps again), I think I am just going to increase all to 46 PSI. That way future rotations will just have a 4 PSI range for the front and 2 PSI range for the rear. My Prius were originally at 33 PSI tfor all four tires anyway.

It would have been nice if Toyota included a sensor for the spare and monitored. It would save us from the hassle of pulling out the spare to check unless we get the extender like jay_man2 did (which I totally forgot and now have to pay $4.95 in shipping; it was my bad for forgetting so no biggy). Thanks for the reminder jay_man2.

[Philosophe]

Originally Posted by pfreeus

Things learned.
- Not sure if this is a factgory [sic] or dealer thing but my tires were nitrogen filled. I used compressed air to fill though.
[…]
Now reading on line they say not to mix compress air with nitrogen (whatever) so I'm not recommending that anyone try the above. I comfortably feel that I will be okay but that is just me. I am not encouraging anyone to do what I did.

I highly doubt that changing from 78% nitrogen (also called air) to close to 100% nitrogen would make a difference. Add this to the fact that your tires are not vacuum pumped before supposedly pure nitrogen is pumped in it. You are probably getting something around 90% nitrogen in the end. 78% or 90% should not make that much of a difference. Air humidity has a greater impact but use of nitrogen doesn’t ensure that the gas (air/nitrogen) moisture is reduced.

IMHO, the nitrogen thing is mostly a hoax, especially when you consider the price you are paying.

[stream]

Originally Posted by jay_man2

You pushed me over the edge to order that gauge and the Check-a-Spare valve extender. I now won't have to pull the spare all the way out to check its pressure periodically.

Chek-a-spare, Check-a-spare, tire gauge guard

I have one installed on my spare, and ran it so the end is in the small storage area on the left side of the cargo area, where I keep my Accu-Gage and disposable plastic gloves for pressure checking. I cut a small notch in the underside of the large storage area above the spare so the hose wouldn't get kinked.

[jay_man2]

Originally Posted by stream

I have one installed on my spare, and ran it so the end is in the small storage area on the left side of the cargo area, where I keep my Accu-Gage and disposable plastic gloves for pressure checking. I cut a small notch in the underside of the large storage area above the spare so the hose wouldn't get kinked.

Great idea. Thanks!