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Discussion Starter · #1 ·
I was driving the car today with the dashboard info display set to the AYC/ACD screen and I noticed that when cruising at any constant speed above 50 mph on the motorway the ACD system is being activated (i.e. 5 bars stay constantly lit up in the middle of the display - the AYC displays on the left and right remain neutral). I've previously only noticed the ACD system being activated in the past under heavy acceleration so is it normal for it to activate when cruising at motorway speeds or do I have some gremlins to work out?

If it's the later then what could be causing the system to activate incorrectly, but not throw a cel - a faulty sensor?
 

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Discussion Starter · #4 ·
Can any of the other X owners confirm whether they see the same behaviour or not?

I'm pretty sure it's not normal as I seem to recall when I got the car I was doing 70mph and only the AYC indicator came on and that was because one of the tyres was seriously down on pressure. At a guess the cause is possibly due to having different brands of tyres at the front and back - the tread levels are practically identical between the front/back tyres but perhaps the different brands deform in different ways at speed (i.e. one has a softer side wall) which is messing with the ACD system.
 

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I was driving the car today with the dashboard info display set to the AYC/ACD screen and I noticed that when cruising at any constant speed above 50 mph on the motorway the ACD system is being activated (i.e. 5 bars stay constantly lit up in the middle of the display - the AYC displays on the left and right remain neutral). I've previously only noticed the ACD system being activated in the past under heavy acceleration so is it normal for it to activate when cruising at motorway speeds or do I have some gremlins to work out?

If it's the later then what could be causing the system to activate incorrectly, but not throw a cel - a faulty sensor?
Used mine on a short stretch of dual carriageway the other day so checked, no sign of ACD bars at 60 or 70

Stu
 

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Discussion Starter · #7 ·
Thanks guys.... I'm going to try switching the slightly newer front tyres with the back ones and see if that makes any difference before this problem kills my AYC/ACD pump or the diff.
 

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Discussion Starter · #8 ·
Okay so I finally got to the bottom of this and it turns out that despite all the tyres being the same size (245/40/18) with the same amount of tread/pressure different brands seem to sit differently when loaded and the result of this was that the back tyres were significantly larger than the fronts when the car was sitting on them.

Having now switched out the rears for the same brand as the fronts the problem has been resolved and I'm now able to do motorway speeds without the diff being constantly activated.
 

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I do not 'rotate' my tyres on my 'X'. Am I to assume from the above posts that this is a wise decision?
Likewise,is it best practice to renew all four tyres at once?
 

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Discussion Starter · #10 ·
When I bought my car it already had different brands of tyre for the fronts/rears and the fronts were nearly bald while the rears were brand new yet there were no ACD/AYC issues until I stuck new tyres on the front. It therefore doesn't appear that you have to strictly renew all the tyres at once, however you need to be careful about which tyres you use as my experience in this thread has shown - clearly replacing all tyres at once or at least sticking with the same make/brand as the existing tyres will make your life easier.

Under normal circumstances if the wear levels of my tyres was uneven then I'd rotate them before the difference becomes too extreme so that I can get the most out of the set before I replace them all, but the ACD/AYC system will cope with a variance between the front/back due to normal tread wear so it's not essential.
 

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When I bought my car it already had different brands of tyre for the fronts/rears and the fronts were nearly bald while the rears were brand new yet there were no ACD/AYC issues until I stuck new tyres on the front. It therefore doesn't appear that you have to strictly renew all the tyres at once, however you need to be careful about which tyres you use as my experience in this thread has shown - clearly replacing all tyres at once or at least sticking with the same make/brand as the existing tyres will make your life easier.

Under normal circumstances if the wear levels of my tyres was uneven then I'd rotate them before the difference becomes too extreme so that I can get the most out of the set before I replace them all, but the ACD/AYC system will cope with a variance between the front/back due to normal tread wear so it's not essential.
As you've found, just because the tyre size is the same, the rolling circumferance can vary between makes. I guess there is tolerance to accomodate tyre wear, but different makes can vary substancially in rolling circumferance

guess the answer is to rotate so all tyres are worn at the same time, then replace in matching 4's

Stu
 

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Phil
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Another interesting thread!

My question arising out of the above it just how sensitive is the S-AYC system, eg such that small wheel/tyre cicumference value differences can cause the system to active?

I'd pretty much presumed that running eg the same make/size of tyre all round but with varying tread depths would always be accommodated with no ACD activation? Or it is possible that the sensivity varies somewhat from car to car?
 

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Another interesting thread!

My question arising out of the above it just how sensitive is the S-AYC system, eg such that small wheel/tyre cicumference value differences can cause the system to active?

I'd pretty much presumed that running eg the same make/size of tyre all round but with varying tread depths would always be accommodated with no ACD activation? Or it is possible that the sensivity varies somewhat from car to car?
my guess (complete guess) is it will accomodate tyre wear of the same make, but not the different rolling circumferances of varying makes

would be interesting if the OP posts the brand type and size of tyres he had a problem with, see if their rolling circumferance are listed online

Stu
 

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Phil
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my guess (complete guess) is it will accomodate tyre wear of the same make, but not the different rolling circumferances of varying makes

would be interesting if the OP posts the brand type and size of tyres he had a problem with, see if their rolling circumferance are listed online

Stu
Maybe so, but is there any significance in that the OP says all five bars were lit? Isn't it a progressive kind of thing, and one bar only illuminated might be most plausible?
 

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Maybe so, but is there any significance in that the OP says all five bars were lit? Isn't it a progressive kind of thing, and one bar only illuminated my be most plausible?
No idea, to be honest, on the road i've never seen 5 bars lit, 1 or occasionally 3 on hard acceleration. it actually surprised me on a daily drive on the road, how often a small amount of acd and ayc is activated

Stu
 

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Discussion Starter · #16 ·
With the mismatched tyres the faster I went the more ACD lights came on so 50mph was 3 and 70mph was five, which is what you'd expect as the difference in wheel speed increases with road speed when the diameter of the tyres is different.

I've dug through the service manuals but I couldn't find anything that states the tolerance of the AWC system in terms of tyre wear, however someone over on the evo x forum asked the dealer who then contacted Mitsu and was told anything up to 5/32" (4mm) is fine. A new tyre normally starts with around 8mm and 1.6mm is the legal minimum so there is a small chance to fall outside the recommended operating limit if you have the same brand of tyre on all four corners, but I suspect there will be some additional tolerance above the recommended value given by Mitsu which means that you will probably be fine unless you've got different brands involved.

I didn't even bother to accurately measure the diameter of the old tyres (Kumho vs Firestone) because when I eventually checked the clearance between the bottom of the rim and the ground with my fingers it was obvious that the difference was around 1cm so the overall difference of the diameter of the tyre would have been around 2cm. Up until that point I didn't even consider that tyres with the same size could vary so dramatically in diameter, but lesson learned as they say.
 

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With the mismatched tyres the faster I went the more ACD lights came on so 50mph was 3 and 70mph was five, which is what you'd expect as the difference in wheel speed increases with road speed when the diameter of the tyres is different.

I've dug through the service manuals but I couldn't find anything that states the tolerance of the AWC system in terms of tyre wear, however someone over on the evo x forum asked the dealer who then contacted Mitsu and was told anything up to 5/32" (4mm) is fine. A new tyre normally starts with around 8mm and 1.6mm is the legal minimum so there is a small chance to fall outside the recommended operating limit if you have the same brand of tyre on all four corners, but I suspect there will be some additional tolerance above the recommended value given by Mitsu which means that you will probably be fine unless you've got different brands involved.

I didn't even bother to accurately measure the diameter of the old tyres (Kumho vs Firestone) because when I eventually checked the clearance between the bottom of the rim and the ground with my fingers it was obvious that the difference was around 1cm so the overall difference of the diameter of the tyre would have been around 2cm. Up until that point I didn't even consider that tyres with the same size could vary so dramatically in diameter, but lesson learned as they say.
a difference of 20mm diameter on the tyres will give a difference of rolling circumferance approx 63mm, so if 1 tyre is travelling 63mm further than the other for 1 full rotation, it would easily explain the different wheel speed sensed

Stu
 

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Discussion Starter · #18 ·
a difference of 20mm diameter on the tyres will give a difference of rolling circumference approx 63mm, so if 1 tyre is travelling 63mm further than the other for 1 full rotation, it would easily explain the different wheel speed sensed
Yeah as soon as I noticed the difference in diameter I knew that's what the problem was - it's just that I had made an incorrect assumption that with both sets being 245/40/18 tyres that they would be fairly close in diameter similar to the previous mix of tyre brands that were on the car.

Amusingly when I mentioned the issue to my local Mitsu dealer they immediately blamed the AYC pump and advised a replacement which I knew was blatantly wrong at the time. Anyway hopefully this thread will save someone else a similar problem.
 
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