ABS Light caused by size of wheels?

[was8v]

Very nearly bought a 944 S2 earlier in the year, but changed job and had to commute 50 miles each way, so I bought a bargain Audi TT 225 coupe to rack up the miles. The TT is a very comfortable commuter and has a fair lick of speed but doesn't have true "sports car" handling or presence.

Changed jobs again and the commute is no more so I'm back on the lookout.

Now I've spotted a nice S2 for sale, however the ABS light is intermittent - the owner suspects its because of 18" wheels fitted and the workshop manual apparently backs this up.

Has anyone heard of this? I can't see how the wheel size can effect the ABS?

Is there a way of reading fault codes from the ABS ECU (like VAG-COM for VWs and Audis)?

Thanks.

 

[Mike_Pollock]

Dont know about fault codes but abs compares relative rotation speeds of the wheels so sometimes even old tyre/ new tyre combinations can upset them.

Also muck on sensors or rotors can.

Mike

 

[Eldavo]

Don't think you could do anything similar to VAG-Com as there isn't an OBD port as the car doesn't have a sophisticated engine management computer.

 

[Diver944]

As long as the tyres are pretty matched in rolling radius it doesn't cause a problem.

I've had vastly different sized tyres on front and rear and still not triggered a fault.

More likely is one of the sensors is dirty, or the wire leading to it has a short

 

[sawood12]

The manual states that if you fit wheels where the rolling radius is too far away from the standard then that can confuse the ABS and in that case it bombs out and the red light will come on. ABS actually tries to predict when the wheels are about to lock up and applies the intermittent braking before wheels lock up. In order for it to do this it needs to know reasonably accurately the rolling radius of the wheel. However the diameter of the wheel is irrelevant in this - it is the diameter of the tyre that is important so i'd take what the owner says with a pinch of salt. If the tyre diameter is outside of tolerance (which I think is + or - 17mm) then why does the light come on intermittently? Surely the tyres are of constant radius so the light should be on all the time. Either way it shouldn't be a show stopper, just assume the ABS system needs some attention and factor that into your negotiations.

Cant remember the stock tyre sizes but the way to calculate the diameter of a tyre is:-

Dw + (2 x (Wt x AR/100) where Dw is wheel diameter, Wt is tyre width and AR is aspect ratio of the tyre. Obvioulsy all units must be the same.

e.g.

205/50/17 = (17*25.4) + (2x(205 x 50/100)) = 636.8mm (or 25.07 inches)

 

[pegasus]

I would agree that the problem is a bad connection or dirt on one of the sensors,I would favour a bad connection as dirt build up on a sensor would not tend to be intermittant,get a good contact cleaner unplug the sensors give them a good spray and plug and unplug each connection about a dozen times,this will clean the pins. the abs relies on all wheels turnin at the same speed this means that irrespective of what size of wheels/tyres you have as long as the diameter/circumference of the tread is the same the system will work.hope this helps, John.

 

[greenfingers964]

just picked up this thread and i am running a 1989 944 S2 with failed abs.
The abs light functions ie it comes on with ignition and goes off once engine started.
But abs doesn't work!!!
I know that there is a VAG system that diagnoses older S"s but haven't fllowed up a fix yet.

My guess is that its likely failed sensors are most likely to bring the light on.

 

[944Turbo]

Over some years on the forums dirty sensors/broken wires are most common followed by roughly equally by ecu and abs pump. I think there are about 3 of the former for each case of the latter, but that is just a guestimate.