LTPWS question. "Scottfab"...where are you?

[Hib Halverson]

As I recall, "Scottfab" is the resident expert on the LTPWS and repair of the sensors. Hopefully, he'll see this.

I can't remember and couldn't find my old info on the C4 version of that system.

What is the pressure threshold of that system. 26-psi sticks in my mind but I'm not sure about that.

[Kevin]

i think when the ones on my 90 went off once it was around 27. but this was years ago and I can't remember exactly

[DaveK]

A couple of write-ups from Scott in the Technet section of the website that might help.

Article 1

Article 2

Thanks

Dave

[scottfab]

Originally Posted by Hib Halverson

As I recall, "Scottfab" is the resident expert on the LTPWS and repair of the sensors. Hopefully, he'll see this.

I can't remember and couldn't find my old info on the C4 version of that system.

What is the pressure threshold of that system. 26-psi sticks in my mind but I'm not sure about that.

Hib
The write-up on this has become a bit distorted over the years from being moved from one system to another. It's to be expected since each time (three times now) stuff gets slightly altered.
In the current incarnation the images are a bit small and some text is missing. All will be fixed in good time but.... in the interim here is a good sized image of the transducer. While labeled 25psi and are +/- 1psi by my measurement, by the time they trigger an alert and you pull over to check the tire temp goes up and the actual reading will be 3-4psi higher.
This system is another one that I refuse rip out when it fails. I just fix it.

[Hib Halverson]

Do those sensors fail gradually or instantly?

I have one that's acting like it's threshold has changed but, I know that's impossible.

[scottfab]

Originally Posted by Hib Halverson

Do those sensors fail gradually or instantly?

I have one that's acting like it's threshold has changed but, I know that's impossible.

One failure scenario is "slow response". The the piezo cell pendulum starts to stick (see Article 2) it takes more wheel vibration to build up a charge such that the device can "check in". Either that or the capacitor begins to leak current which requires more peizo cell energy to build up enough energy to send the "check in" signal. The rubbing issue is more common.

The real challenge is to isolate which wheel is slow to respond.
I use the road for this. That is find an area with "ribs" on the road so
you can run one side of the car over them. (left or right) From there you get the DIC light to go out and you know it's either left or right side.

What are the symptoms? When testing purposely over inflate to 40psi.
You get quicker response to vibration. The tires that I've had have 44psi as max pressure.

[Hib Halverson]

Symptoms...

For many years, for "normal" street driving I've set my tires at 25-26 psi cold. Then, a while back, in cold weather, I began to get the low/flat tire warning after a cold start and short drive. If I'd drive a ways then key-up again, the light would stay off, presumably because of the small pressure rise after driving several miles

Recently, this has started to be a regular thing, so several days ago, I set each tire at exactly 26 psi. The light stayed off until two nights ago, when after sitting outside at a friends house on a cold night, I came out about 10-pm to go home and the light came on after start-up and a short drive. About three miles later, I pulled over and recycled the ign and the light stayed off.

Last night, I raised the tires to 27 psi cold, went for a road test and the light stayed off.

This is what got me wondering about a sensor that is "slowly" failing.

[scottfab]

Originally Posted by Hib Halverson

Symptoms...

... snip...

Last night, I raised the tires to 27 psi cold, went for a road test and the light stayed off.

This is what got me wondering about a sensor that is "slowly" failing.

It's important to know which DIC light came on, the SERVICE LTPWS or the "LOW TIRE PRESSURE".

If it's LOW TIRE PRESSURE it's likely to be related to low wheel temp and proximity of the 27psi to the 25psi built in limit. BUT it could also be that one transducer in one of the sensors has separated from it's mounting bracket. When it separates it's still attached by one solder point but the vibration upsets the trip point of the transducer. This was the very first failure I saw and was a POS to isolate.

The first thing to try is to run at 35psi on all tires for a time. If the light still does it's thing intermittently you're in for some work. Now you get to the fun part, isolating which tire it is. Unfortunately this failure requires some guess work. Pick two, any two tires and dismount and inspect the transducer in the sensors. Look to see if the transducer puck has separated from the bracket. Do this by grabbing the puck and try to
wiggle it back and forth. You have a 50/50 chance of finding it on the first try. Then you get to go the the next two tires. The method for isolation in the FSM is useless for this failure. (BTW if you get in this deep you might as well service each sensor for the failure below as preventative maintenance)

FWIW I have never tried this next method for isolating which wheel but theoretically it should work.
Set pressure to 35psi on all tires except one. Keep one tire at a time at 23 psi for just long enough for the LOW TIRE PRESSURE light to come on. Measure the time it takes for each tire to trigger the light. Look for an outlier. Either longer or shorter than all the rest.

If the light you see is "SERVICE LTPWS" then you may have one of those piezo cell pendulum issues where it starts to rub on the inside plastic.
This rubbing retards the build of of charge necessary to transmit the pressure health signal. FWIW since it's cool out I would think the other failure mechanism for this less likely.(leaky cap)
The good news is this is an easy fix and is written up in the article mentioned in a previous post. Well it's easy after you isolate which wheel. The best way I've
managed to do this is with those "wake up" grooves on the side of the road. The ones intended to wake drivers up that wonder off the road.
If you edge over just enough to hit one front tire at a time you'll see the
light go out after about 10 - 30 secs (that is after vibrating the tire for 5 seconds and wait 10 - 30seconds)
Alternatively that new untested procedure above may be instructive for finding the rogue sensor.

Good Luck.

[SteelBlueZR1]

Timely post, as my Service LPTWS light has come on for a few minutes on two occassions. Both times I had spun the tires on full throttle first / second gear runs, and then came into the same part of my town, where the light came on. Is it possible there is some radio interference causing this, or a different in average wheel rotations from the tire spinning runs?

[Hib Halverson]

It's not the service LPTWS.

On a 95, when the system detects a low tire the light that comes on says "Low/Flat Tire"