ABS fault & no warning light Xantia

Martintheradio · Post by **Martintheradio** » 07 Sep 2009, 11:00

Strange situation:

No ABS warning light, but ABS operation is being applied at all times. When braking above 10 to 15 mph, brakes shudder. At 30 mph extreme shudder.

Remove ABS fuse, ABS light is on, but absolutely normal smooth braking. (Except, of course, ABS would not be operating!)

Xantia HDi Estate 110Bhp

Does anyone have any ideas please.

Post by **RichardW** » 07 Sep 2009, 12:29

Hi Martin,

Welcome along to the forum...

Most likely you have got a broken (or missing!) ABS ring on one of the front CV Joints. Have a good look for a split in the ring (on the outboard end of the CV joint). I found this company http://www.reluctorrings.com/index.php the other day that supplies replacement rings.

uncle buck · Post by **uncle buck** » 07 Sep 2009, 13:09

RichardW wrote: I found this company http://www.reluctorrings.com/index.php the other day that supplies replacement rings.

Have a look on ebay for ABS rings as they are a lot cheaper on there

Cheers.

andmcit · Post by **andmcit** » 07 Sep 2009, 13:23

I have a slightly different take on the ABS ring although I'm sorry I don't
have an alternative suggestion of a fix!

No ABS ring would simply be the same as no signal from a sensor which
brings up the warning light on the dash. I made the oversight of fitting
slightly different driveshafts on a car after fitting a new autobox and had
newer healthier replacements. Problem was, mid point of IIRC 1995 the
knuckle size changed on the CV joint thus a differing ABS ring diameter.

Basically once the car passed 15mph the sensors (whilst correct resistance
etc had too much air gap on the sensor the light came on and would stay on
requiring correct fitment of driveshaft.

I'd say the ABS problem here is caused by another issue.

Andrew

VertVega · Post by **VertVega** » 07 Sep 2009, 13:50

I would also say it is caused by the gap.

Did you recently have a maintenance done on the front wheels, wheel bearing change or etc.? Most common mistake is made when fixing the heat shield and the ABS sensor. Is it [sensor + shield + screw] or [shield + sensor + screw].

Using a digital camera you can compare gaps of both front wheel ABS sensors and toothed ring. If they differ then you will have kick back when you apply brakes, in "walking speed".

PS: This is based on a true story

Martintheradio · Post by **Martintheradio** » 07 Sep 2009, 15:17

Many thanks guys for your really fast response.

When I have the opportunity I will check the toothed rings.

The only recent work was front brake pad replacement. I'm just wondering if the sensors need cleaning as the pad replacement was left until they were grinding - naughty! & perhaps the sensors have picked up metal dust from the disks?

Also recently replaced 5 spheres, but the ABS problem was already present.

Again, many thanks

MJM · Post by **MJM** » 07 Sep 2009, 17:00

As I have just found out a faulty sensor will cause the light to stay on. If it is not illuminating with the ignition then the fault lies elsewhere. The check system will not pick up a toothed ring problem until the car is driven because it has no signal to check. It seems difficult to damage a ring when changing pads.

According to Haynes, there are two fuses for the abs system, f6 in the fascia fuse box, 10 amps, and f1 in the engine compartment, 20 amps. Logic suggests that the 20 amp is for solenoid operation and the other the electronics. I'm no expert

but in this case I think the problem is looking more difficult that sensors.

Xaccers · Post by **Xaccers** » 07 Sep 2009, 17:42

VertVega wrote:I would also say it is caused by the gap.

Did you recently have a maintenance done on the front wheels, wheel bearing change or etc.? Most common mistake is made when fixing the heat shield and the ABS sensor. Is it [sensor + shield + screw] or [shield + sensor + screw].

Using a digital camera you can compare gaps of both front wheel ABS sensors and toothed ring. If they differ then you will have kick back when you apply brakes, in "walking speed".

PS: This is based on a true story

It makes sense, as if the sensor is not registering the teeth going past then it will assume that wheel is locked and not turning.
There won't be an error light just as there isn't an error light when you hit the brakes hard in the wet or on ice and the ABS cuts in.

andmcit · Post by **andmcit** » 07 Sep 2009, 18:43

I can't see why the light won't come on. I can only speak from my
inadvertent discovery of a gap being to large for the sensor to the
teeth - the self test at start up saw 4 happy sensors and the warning
light went out right up until the car's wheels turned and it didn't get a
signal it could make sense of and the light illuminates as the system
drops into failure mode.

I'd suspect the disc and backing plate iron filings playing merry hell
with the magnetic fields on the sensors and their signal to the ECU!

Andrew

Xaccers · Post by **Xaccers** » 07 Sep 2009, 20:07

When all is working, the teeth are modulating the magnetic field (and therefore the current) at the tips of the ABS sensors enabling the ABS ECU to determine the speed of each wheel.
If one locks up, it stops modulating the magnetic field, which tells the ECU that the speed is zero or below whatever the threshold is and the ABS kicks in. No lights come on.

When a sensor fails causing the light to come on, it's because the resistance of the sensor has gone out of acceptable range.

If the sensor is working but not being affected by the teeth, then it's telling the ABS ECU that wheel is not turning, so the ABS will kick in, but the light won't come on as there's no error.
Similar to if the car was being towed with the front wheels off the deck.

Martintheradio · Post by **Martintheradio** » 08 Sep 2009, 13:21

Now checked: offside toothed ring broken & it has damaged the sensor - nuisance - that's an extra £30.

Interesting: it seems that in determining a fault condition, the cpu only looks at the static state of the sensors.

Thanks again all

MikeT · Post by **MikeT** » 08 Sep 2009, 13:35

AFAIK the ABS carries out two self-tests. One on power up (ignition on) testing it's own computer and the other once the car reaches a certain speed (15mph I think) which I can only assume is to test there is a signal being produced by each sensor.

It's certainly true of the Saxo as a friend replaced a sensor and complained the light was still on but later reported it went out once driven and has been fine ever since.