# Tire pressure monitor sensitivity?



## alotomeat (Jul 22, 2004)

What is the tire pressure monitor sensitivity on 3 series, or BMW cars in general? I'm asking how many psi must drop for the tire pressure light to come on?

Thanks!


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## Ajax (Feb 1, 2002)

alotomeat said:


> What is the tire pressure monitor sensitivity on 3 series, or BMW cars in general? I'm asking how many psi must drop for the tire pressure light to come on?
> 
> Thanks!


IIR, I don't think the system works on an ambient pressure basis, but from a geometric one. The system somehow measures the diameter of each wheel/tire (an underinflated tire's diameter is less than a properly inflated one.) I reckon it then measures how many times wheel/tire x rotates in some distance. If one is rotating at some percentage less than the others, the pressure alarm gets trippped.

If all your tires are leaking at about the same rate though (normal for some "seepage) there is no delta to set it off.


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## mtbscott (Jul 16, 2003)

Yeah, kind of like that. My understanding is that when you reset the button, the system then uses the current state as the "norm." If a tire starts losing pressure in relation to the others, it will shrink, thus setting off the alarm. I had two small nails in my OEM tires, and a drop of 3 PSI or so was enough to set it off. After repairing the tire, I reset the button/system as per owner's manual instructions. I also did this after getting a new set of tires and setting the tire pressures myself.


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## bren (Jul 1, 2002)

The system uses DSC and measures rotation across the car (left front compared with right rear, right front compared with left rear.) Not sure at what % difference it actually triggers.


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## safetywork (Dec 10, 2003)

mtbscott said:


> Yeah, kind of like that. My understanding is that when you reset the button, the system then uses the current state as the "norm." If a tire starts losing pressure in relation to the others, it will shrink, thus setting off the alarm. I had two small nails in my OEM tires, and a drop of 3 PSI or so was enough to set it off. After repairing the tire, I reset the button/system as per owner's manual instructions. I also did this after getting a new set of tires and setting the tire pressures myself.


3 PSI drop and it set off your warning?!!! That's awesome! This is also the first time I've heard of a 3 PSI variation that set off the warning. I usually hear a minimum of 10 PSI drop before the thing goes off from friends and from fellow forum members. :dunno:


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## TGray5 (Jan 22, 2002)

alotomeat said:


> What is the tire pressure monitor sensitivity on 3 series, or BMW cars in general? I'm asking how many psi must drop for the tire pressure light to come on?
> 
> Thanks!


Its really a flat tire monitor and as such it will only go off if you are about 20% or more below the pressure at which you calibrated it initially. I don't think it compares pressure with the other tires to determine when to indicate a problem, otherwise there would be no need to calibrate the flat tire monitor.


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## Technic (Jun 24, 2002)

alotomeat said:


> What is the tire pressure monitor sensitivity on 3 series, or BMW cars in general? I'm asking how many psi must drop for the tire pressure light to come on?
> 
> Thanks!


I just been having this tire pressure alert telling me that one of the tires is low in my M3... once I checked, the low tire was around 7 psi less than required.

I have a screw punching my tire...


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## andy_thomas (Oct 7, 2002)

AJAX said:


> IIR, I don't think the system works on an ambient pressure basis, but from a geometric one. The system somehow measures the diameter of each wheel/tire (an underinflated tire's diameter is less than a properly inflated one.) I reckon it then measures how many times wheel/tire x rotates in some distance. If one is rotating at some percentage less than the others, the pressure alarm gets trippped.
> 
> If all your tires are leaking at about the same rate though (normal for some "seepage) there is no delta to set it off.


The previous system (RDC) used pressure sensors in each wheel. The newer system, from September '03 onwards, uses wheel rotation calculations as you say. It is also a lot cheaper!


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## flashinthepan (Jul 25, 2003)

mtbscott said:


> Yeah, kind of like that. My understanding is that when you reset the button, the system then uses the current state as the "norm." If a tire starts losing pressure in relation to the others, it will shrink, thus setting off the alarm. I had two small nails in my OEM tires, and a drop of 3 PSI or so was enough to set it off. After repairing the tire, I reset the button/system as per owner's manual instructions. I also did this after getting a new set of tires and setting the tire pressures myself.


It has been my experience as well that it only takes 3 lbs diff.

When I put my new tires on - the alarm went off, I told myself -crap !!, this may become annoying, but, sure enough after checking carefully...one tire was slightly low.

Its a great safety device..imo.


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## HUMMM 3 (May 10, 2003)

During a recent trip on a very hot day at high sustained speeds my alarm went off. Turned out the tire pressures in all 4 tires had gone up significantly (about 8 psi/tire). Upon restarting the car after shutting it down to fuel and check the tires, the warning light went out. The tires cooled overnight and the next day the warning did not go off, so I assume the system didn't reintialize itself to the higher pressures. 

I think the best thing you can do is buy an accurate tire pressure gauge and use it often. Each time I check tire pressure I reinitialize the system. :thumbup:


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## obmd1 (Jan 17, 2005)

Is it possible to get a false positive with the system. Had a flat tire warning last night.... no tire looks flat, which I know means nothing. Measured pressures today, and all within spec. Reset the monitor, and I cannot get the thing to go off again.

330CI '05 with ZHP (not that that should matter)

and I found this thread searching, yes I did....:angel:


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## mitzipearl (Dec 28, 2005)

*False Alarms*

Every time I drove over 100mph with theOEM Continentals on my 2004 M3 conv the tire indicator light would go off. I replaced the tires with michelins and it has not goneoff once in the last 12,00 miles. BMW customer service line claims they areunaware of any tire pressure indicator problems. They just don't want to admit that their OEM tires suck

Ps I drive over 100mph every day at least once to justify owning this awesome car.
Soon to be replaced with a triple black M Z4 roadster performance center delivery 
03-15-05.


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