Hi Guys,

I've searched the forums for an answer here and did not come up with anything specific to this fault, so hope you can help !

Purchased a 1999 XJR with 112,000 miles on the clock, running sweet as a nut.

When you drive along (slow, fast it does not matter) the ASC warning light flashes amber and the words ASC come up in the dash. You can feel the car throttleing back as if it is wheelspinning, but you can be doing 30mph or 70mph, it does not matter.

It has been on to diagnostic machine and NO FAULT CODES are coming up, it never shows NO ASC or LIMP HOME or any other message.

The car has 245x18x40 profile tyres on front and 255x18x35 on the rear, the rears are new.

The local jag specialist has checked the car over and can find no fault he has also had a look at the 'link cable' or something and they are fine. It had a dodgy top ball joint which they originally thought was the problem, but they fixed that and the ASC problem is still there !

They now think it is the different tyre sizes that are causing the problem. I dont want to shell out megabucks to replace tyres that go round just fine, but if it is 100% the problem, I will bite the bullet and change them!

Any ideas on this one ?

Obviously if you turn TRAC OFF there is no problem.

Car starts and runs fine no issue, just this fault.

When the ASC light is not on (when the problem happens) the ASC and Traction control systems work fine.

I hope you guys can help and go easy, its my first post !

Freddy

 

If you DO have different diameter tires between front and back the computer will see a difference in rotation rate between front and back and assume one set is losing traction - and turn on the traction control. In your case, the difference is small enough that it must be on the hairy edge of tripping compurer's pre-set loss of traction threshold, so things like turning will add just enough to trip it; explaining your intermittent ASC warning and actions.

The tires you listed have are about 3/4" taller in the front than in the rear, so the rears are always turning about 3% faster than the front... Don't know what the trac control threshold is, but this may certainly explain it.

 

Thanks for the answer and I really appreciate your help !

I dont have £400 to spend replacing the tyres on the back!, if its not the problem then I'd be really gutted and would not be able to go on holiday next month!!!

Anyone in the Lowestoft/Norwich/Ipswich/Yarmouth area with two wheels I could try ??? would need to be 245x18x40 !!!!!

I'd buy you a pie and a pint if I could drive a mile with em on !

Ta muchly again quad person!

 

Could this be the common dirty sensor/loose ABS module connection problem?

 

Simple test: Lower the air pressure in your front tires until they sag by 3/8" (half the difference in total height) over current height. Verify by measuring center of hub to ground and make it equal on both front and rear (you can add some pressure in the rears to increase their diameter a bit too).

Then drive for a bit and see if the trac message goes away. Depending on how much pressure you have to remove, I might not suggest running like this regularly, but it will tell you if it is the tires that are making the warning happen.

Perhaps, if it's not TOO much air pressure lost, you could increase pressure in the rear and decrease in the front a bit each, to more equalize the height and run this normally - caution, you don't want to be driving regularly on severly over or under inflated tires.