rich951

my problem is my abs light stays on, and the ABS is not working/being deactivated.
Has anyone had problems with the ABS being deactivated after fitting aftermarket wheels?
or know how to get it working again?

When I start the car it goes out, then when I start going forward it lights back up.
Im sure its because Ive fitted aftermarket wheels from a boxster. The Porsche manual states :

'the ABS control unit is matched to the approved tire dimensions.
Unapproved tires can lead to different wheel speeds which the control unit interprets as different road speeds at the cars axles.
If the difference in rolling radius exceeds a certain amount.the control unit deactivates the ABS and the ABS pilot lamp lights up.'

I assume that the control unit has to be reprogrammed- a: is this possible? b: how??
any advice help out there? anyone with similar experiences

IceShark

What size tires are you running?

I believe the ABS compares the wheel rotations and decides if they are within spec but doesn't compare to gear and RPM to make this decision.

So, in typical situations you would have front and rear tires with a fair difference in diameter. I don't know how much differential it takes to cause failure but I do know it makes the check - just try spinning your rear tires on jackstands.

You may have buggered up a sensor when changing out the wheels.

rich951

thanks for reply-I'll try checking the sensors first.
tyres are front: 235/40/17 rear:255/45/17

IceShark

Those tires may be enough to do it as they likely have a 7% difference in rolling radius. Don't trust me on this, but I heard 5% was the trigger. I forget exactly who told me this, I believe it was an old Porsche factory team driver's nephew from Austria when he was helping me pick new tires.

Check your particular manufacturer rolling radius as well as the sensors, but the tire sizes may need to be changed. See if you can test out my theory with some used rubber.

GrantG

Rich - those sizes are a bit outside the norm. Get 255/40/17 in the rear, at least. This will lower your gearing, giving better acceleration and less turbo lag also. Not to mention, will probably cure your abs problem,,,

Pat Kennedy

Can't recall where I originally learned it, but I have always used 3% as the threshold for tire size variance. Go to <a href="http://www.powerdog.com" target="_blank">www.powerdog.com</a> to check prospective tire sizes against the stock sizes.

RPK

IceShark

I don't have any facts, but 3% seems a bit tight to me for failure mode way back when this system was introduced to "cheap" cars. Maybe in aircraft.

rich, let us know how this turns out.

rich951

Thanks guys for advice,
my rear tyres are 255/40/17 front 235/45/17- typed in wromg sizes!
a bit of an expense changing tyres, so maybe I can find a way of borrowing a set of original wheels/tyres to try.
But I want to keep the wheels/tyres- there must be a way surely???
cant the threshold be changed by reprogramming the control box- I thought it was coomonplace that people put different wheels/tyres on Porsches, so has anyone put bigger wheels/tyres and had no problem with ABS. as well as problems?
thanks.......

IceShark

I thought those tire dimensions were a bit funny. You are well within ranges so go back to the buggered sensor idea. At the moment I'm running 235/45/17 front and 275/40/17 rear. No problem.

Your rears at ~ 827 RPM are even closer to the fronts at 823 than my rears at 811 (revolutions per mile for a Bridgestone RE 730). I'll bet you good money that it is not the tires/wheels. We can have John D. hold the pot.