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Doh!!! Tire Pressure


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25 replies to this topic

#21 OFFLINE   SnowStorm

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Posted 15 November 2018 - 09:07 AM

Just another reminder that the TPMS is no good for day-to-day monitoring of low pressure!

 

Today I lowered tire pressure to 25 psi because of a critical trip we had to make in potentially nasty "winter mix" conditions.  I was surprised that there was no warning from the TPMS system until after driving about 10 miles!  Trip was completed without incident (although there had been an accident on top of the mountain) and tires are back to "normal".  I'm left wondering if there's always such a long delay after starting the car or is the trip point down around 24 psi and I had to leave the garage and drive in 32F weather for a while before the pressure dropped that low.  If there's always a delay, a tire could go almost flat overnight and you'd drive off down the road for miles before knowing it!

 

I really need to get a full-time Forscan display installed!


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#22 OFFLINE   Plus 3 Golfer

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Posted 15 November 2018 - 01:44 PM

Usually the TPMS trigger point is around 25% below placard.(don't know what Ford uses).   With placard at 38 PSI, the trigger point would then be around 26.5 psi.   A few years ago, my wife got the low pressure message about 1.5 miles from home and quickly returned home.  Pressure was at 22 psi via ForScan when she got home.  No air leaking past screw at that time.  

 

Perhaps, it took 10 miles of driving in colder conditions for psi to fall below threshold.  TPMS is continuous with about one RF transmission per minute when speed is over 20 mph.  So, there should be no delay of 10 miles if psi is below threshold. Other possibility is batteries are failing (colder weather) / some RF interference.  BCM will only alarm for failed RF transmitter if no signal received in 18 minutes and speed greater than 20 mph (could this be equivalent to 10 miles?).  It appears the dash alarm is the same for low pressure or no RF signal.  

 


Edited by Plus 3 Golfer, 15 November 2018 - 01:44 PM.


#23 OFFLINE   kyledamron

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Posted 15 November 2018 - 05:57 PM

I haven't had any issues with that previously. When I took a trip after it had been hot for several weeks and then got cold. My tp light came on fairly quickly when the tire was low. Around 28 PSI.

 

You can "train" your tire pressure sensors to recognize what levels you want by following these steps. Note, this is for vehicles equipped with push-button start.

 

1. Push your start/stop button, without holding the brake

2. Press the hazard switch, on and off, three times.

3. The vehicle will honk and you will be in "training" mode.

4. Go to left front wheel and release air in the tire until the vehicle honks twice.

5. Go to each wheel (Right front, right rear, left rear) and repeat the procedure until all wheels have been "trained"



#24 OFFLINE   fbov

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Posted 15 November 2018 - 07:15 PM

...lowered tire pressure to 25 psi because of a critical trip we had to make in potentially nasty "winter mix" conditions....

You did this because?

 

There is a myth that you get better snow traction at lower tire pressure. It's like the ABS myth that locked wheels stop you faster - true in some circumstances. ABS is disadvantaged in deep snow or gravel, where a locked wheel plows a wall in front of it, rather than sliding over it. Similarly, in a few cases you can get more traction by reducing tire pressure. Driving in snow on paved roads is not one of them. Modern tire designs want a small contact patch to maximize the tread bite into the snow, and hopefully through it to the road. A long contact patch has advantages, but you only get that from tall, narrow tires. 

 

Have fun,

Frank


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#25 OFFLINE   Plus 3 Golfer

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Posted 15 November 2018 - 07:20 PM

I haven't had any issues with that previously. When I took a trip after it had been hot for several weeks and then got cold. My tp light came on fairly quickly when the tire was low. Around 28 PSI.

 

You can "train" your tire pressure sensors to recognize what levels you want by following these steps. Note, this is for vehicles equipped with push-button start.

 

1. Push your start/stop button, without holding the brake

2. Press the hazard switch, on and off, three times.

3. The vehicle will honk and you will be in "training" mode.

4. Go to left front wheel and release air in the tire until the vehicle honks twice.

5. Go to each wheel (Right front, right rear, left rear) and repeat the procedure until all wheels have been "trained"

NO, one cannot change the trigger level (safety). The procedure simply allows the Body Control Module to relearn the positions of the sensors so the PIDs identify the correct wheel position when a DTC is thrown.  I use this procedure every time I rotate tires.


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#26 OFFLINE   Plus 3 Golfer

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Posted 15 November 2018 - 07:54 PM

SS,  I believe the snip below is why TPMS didn't trigger for 10 miles.  Yep, ForScan is it.  I certainly don't want to drive 20 minutes before being notified my tire pressure is dropping.

 

S4.2 TPMS detection requirements. The tire pressure monitoring system must:

(a) Illuminate a low tire pressure warning telltale not more than 20 minutes after the inflation pressure in one or more of the vehicle's tires, up to a total of four tires, is equal to or less than either the pressure 25 percent below the vehicle manufacturer's recommended cold inflation pressure, or the pressure specified in the 3rd column of Table 1 of this standard for the corresponding type of tire, whichever is higher;

...

S4.4 TPMS malfunction.

(a) The vehicle shall be equipped with a tire pressure monitoring system that includes a telltale that provides a warning to the driver not more than 20 minutes after the occurrence of a malfunction that affects the generation or transmission of control or response signals in the vehicle's tire pressure monitoring system. The vehicle's TPMS malfunction indicator shall meet the requirements of either S4.4(b) or S4.4©.








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