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Possible ABS failure but no warning

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Old Jul 6th, 2019, 08:01   #1
id5
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Default Possible ABS failure but no warning

I had a mishap with a trench that was masquerading as a pot hole a while back that caused a strange behaviour in the steering when running on the space saver. I thought that is was AWD related, see this thread, but it seem not to be the case.

The technicians currently reckon that the reason the TPM does not work is due to a dirty ABS sensor. It begs the question of why the little goblin in the engine bay sees that the car is moving forward but does not check that the ABS thinks the wheels are turning. If it did then it could alert that one of the safety systems, the ABS, is not working correctly. When I get it back, then on a quiet, straight road I will jump on the brake pedal to check the ABS response but it begs the question is there a better way to check?
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Old Jul 11th, 2019, 08:20   #2
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I am repeating this post here as well as in the AWD section.

The ABS sensor has been cleaned and now the wait until the tyre pressures drop. I have tried hard braking at 40mph whilst steering and there seemed to be some ability to steer but it was not a good test as it was a dry road and brakes that are really good stop the car within a few lengths. I have noticed that the electronic stability control lights up on the dash over some really steep road humps but it may have always done that and I may just be noticing it. Apart from testing of the sensors does anyone know of a better test for the complete ABS function?
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Old Aug 28th, 2019, 04:13   #3
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Tyres down to 23 & 25psi again and still no warning from the TPM. I have booked it into the dealers again. Haven't seen the electronic stability control light up for a long while when going over steep speed humps.
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Old Aug 28th, 2019, 07:36   #4
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Quote:
Originally Posted by id5 View Post
I am repeating this post here as well as in the AWD section.

The ABS sensor has been cleaned and now the wait until the tyre pressures drop. I have tried hard braking at 40mph whilst steering and there seemed to be some ability to steer but it was not a good test as it was a dry road and brakes that are really good stop the car within a few lengths. I have noticed that the electronic stability control lights up on the dash over some really steep road humps but it may have always done that and I may just be noticing it. Apart from testing of the sensors does anyone know of a better test for the complete ABS function?
Slam on the anchors on a slippery surface: snow, ice, sand, gravel, wet tarmac.

Gravel may be the easiest surface for you to find

Do not do it going slow, ABS does not work at really slow speeds. Do it at about 30kmph or 20mph to be sure you are fast enough.

You'll REALLY feel the ABS kick in
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Old Aug 28th, 2019, 07:38   #5
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Tyres down to 23 & 25psi again and still no warning from the TPM. I have booked it into the dealers again. Haven't seen the electronic stability control light up for a long while when going over steep speed humps.
Depending on what type of TPMS, it may need the tyre pressure to go down by 40% before the TPMS warning is triggered.
And on the type that measures wheel rotation, it may need a straight road for some minutes to trigger the warning. So set the tyre to say 50% of normal pressure, and drive for around 15 minutes down a straight road to see if it triggers.
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Old Aug 28th, 2019, 16:35   #6
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Quote:
Originally Posted by SwissXC90 View Post
Depending on what type of TPMS, it may need the tyre pressure to go down by 40% before the TPMS warning is triggered.
And on the type that measures wheel rotation, it may need a straight road for some minutes to trigger the warning. So set the tyre to say 50% of normal pressure, and drive for around 15 minutes down a straight road to see if it triggers.
It is TPM measured by the ABS sensor rather than the valve stem version. I currently run at 36psi cold and the lowest recording so far has been just sub 20 when hot. I can run 75 miles to Derby and it does not alert. I have also put on the tempa spare which is a different rolling circumfrence and nothing happens

When they cleaned the ABS sensors the last time, I noticed that when I came off a precipitous speed hump the ESC light would flash.

I like the idea of the gravel test. I will do it down our yard the next time I come home and see what lights up on the dash.
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Old Aug 29th, 2019, 17:29   #7
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Okay, tried that. Came into the yard at 40 hit the brakes and turned it into a sledge.

Now how do I remove the word Possibly from in the title of the thread and roll on the 14th of Sept which is the earliest they can look at it.
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Old Sep 16th, 2019, 18:08   #8
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I finally managed to get the TPM warning to go off. I had to fill the tyres cold to 38psi then use the car until they finally alerted on the front right when it had dropped to 19psi. That is a 50% drop before the alert goes off.

Does anyone know what the drop should be?
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Old Sep 21st, 2019, 09:55   #9
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Dealer has had the car in for a second time. Completely ignored the TPM not going off but found a nail in one of the tyres and claimed that’s why the TPM went off. I asked them to test it by removing some of the air and driving it. They did, but they removed over 50% of the air and said, ‘Look it works.’

Now I am trying a different tack and writing to Volvo directly to find out what the percentage or PSI drop is meant to be before the TPM alert goes off and then test it myself.
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Old Sep 21st, 2019, 10:17   #10
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A 50% drop to trigger the TPMS is about right

TPMS that uses in-tyre radio transmitters are very accurate and can trigger at just a few PSI drop, as they directly measure the tyre pressure. they are not subject to distance, speed, or road geometry.

TPMS that uses wheel rotation sensors, such as yours, need a much MUCH larger drop in tyre pressure to trigger the TPMS.
Why? because it is using the average wheel rotation speed and comparing left vs right and front vs back to find out if one wheel is spinning at a higher average speed (as the circumference is lower) that the other tyres.
It relies on the tyre to have deflated enough for the circumference to drop enough on enough straight roads to get enough difference to the other tyres over enough time to trigger the TPMS.
Depending on the roads you drive on, the stiffness of the sidewalls of your tyres, and the speeds you drive, the length of the journey, it can take more, or less, air pressure and time to trigger the TPMS.

If you have ever directly monitored tyre pressures while driving,which I did recently over 2 weeks in a new Opel Corsa rental car with transmitters in the wheels, you'll see that the pressures vary as much as 10 PSI on normal driving, and can vary by 6 to 10 PSI left vs right, because the curb-side wheel is often going over more bumps, and flexing the tyre more, and thus getting hotter. And the more cornering you do, the hotter the outside tyre gets. I was amazed at the variance. Of course, none of this should trigger a low pressure alert on the TPMS, as it was completely normal pressure differences due to tyre temperature and road surfaces. Thus the TPMS must withstand easily 10 PSI difference left vs right and consider that totally normal. And it needs to average out the differences over a period of time of at least 10 minutes to remove any variations caused by cornering or bumps.

So a 50% drop in air pressure to trigger the TPMS in your case I would consider perfectly normal and a correctly operating system. Volvo won't do anything apart from tell you the same.

If you want TPMS to trigger earlier, get softer tyres... and then enjoy a softer rider, with softer cornering... but that may be not what you want!
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Last edited by SwissXC90; Sep 21st, 2019 at 10:23.
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