Tyre Pressure Monitoring System

[Geoffers]

A couple of days ago the TPMS warning light flashed up (in Yellow) telling me there is a problem. I am out driving at the time. Later, when I get home and after the tyres cool down, I check the pressures to find that the fronts are both 27.5 and the rears are 27 and 26!

I blow the tyres up to 32 front and 30 rear. When I go to start the car the TPMS warning light has disappeared! This is before I've started the engine and rolled the wheels!
So, just how does this system work?

[John Ratsey]

What does the handbook say?

The description of the TPMS in the MK 3's handbook explains to reset the TPMS using on option on the multi-function display after adjusting pressures / changing tyres so it can learn what is the new baseline condition. The system must be able to take account of slightly different pressure on front and back.

[Geoffers]

Thanks John but I'm not looking to see how to reset it. It appears to have reset itself without a wheel turning!
I'm wondering just how does the system work? The inwheel sensors would surely need a power supply to report a change in pressure?

[John Ratsey]

My understanding is that the TPMS works by comparing the rotational speed of the wheels, so the sensor can be on each hub. Any loss of pressure will result in a wheel spinning faster because its effective diameter will be reduced.

Given that the front wheels may have higher pressure than the rear ones it's possible that the TPMS compares the front pair and the rear pair separately rather than taking some overall average or maybe that is factored into the calibration process.

[Geoffers]

That's what I used to think John.
But, how would the system know when I hadn't turned a wheel?

Perhaps the sensors trigger a battery driven circuit when the pressure falls. 

[stiggysawdust]

Here is a link to a good explanation of how TPMS works.
http://www.atseuromaster.co.uk/tyres/tyre-pressure-monitoring-system
I read somewhere that gunge should not be used but the above makes no mention. So whether you would need a new sensor or not, I don't know.

[Geoffers]

Thanks for that stiggy - really useful! Now I understand how it works. Cars after 2014 will likely have the direct system with a monitor attached to the valve thingy (which mine has) of each wheel, which actually monitors the tyre pressure.

My handbook makes reference to not using gunge as it will likely damage the inwheel monitor. Strange that, as the car is only supplied with a gunge type jobbie.

[stiggysawdust]

Thanks for that stiggy - really useful! Now I understand how it works. Cars after 2014 will likely have the direct system with a monitor attached to the valve thingy (which mine has) of each wheel, which actually monitors the tyre pressure.

My handbook makes reference to not using gunge as it will likely damage the inwheel monitor. Strange that, as the car is only supplied with a gunge type jobbie.

I suppose that a straight forward puncture not only costs a replacement gunge bottle, but also a replacement sensor on top of the repair cost.
The cost of buying a spare wheel (with sensor) begins to make more sense than ever.

[John Ratsey]

My description above was written from memory of what was in my Mk 3 handbook.

I wasn't wrong and attach the relevant pages.

It is possible that those Mk 2 cars with the TPMS use a different system, but I doubt it. Honda inflict us with some strange design decisions (eg the uncontrollable Mk 3 auto headlights) but may have realised that direct pressure measurement and some leak repair goo might not work well together.



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[culzean]

I also understood deflation warning on MK2 to work by comparing rolling radius rather than direct pressure measurement (it also works on 6th gen Civic like that).  It is pretty sensitive - I have to reset after new tyres fitted, but not after blowing my tyres up,  mind you I check them pretty regularly so they never get more than a few PSI low (Michelins seem to hold pressure very well).

[Geoffers]

Foe completeness I have attached the relevant pages from my handbook.
This is for a 2015 Mk 2 ES plus.

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[John Ratsey]

Foe completeness I have attached the relevant pages from my handbook.
This is for a 2015 Mk 2 ES plus.

That's very interesting - your Mk 2 has a direct monitoring system while my Mk 3 has an indirect system. I wonder why Honda made the change?

[Geoffers]

Yes John, it does seem peculiar!