ABS fault

mro

Car has failed MOT with ABS warning light staying on & not going out.
Am taking car to garage to do ABS Diagnostic. It seems the sensors are quite expensive.

Any ideas on typical faults ?


Seems ABS system comprises:

-wheel sensors
-wheel reluctor rings
-ABS system control unit


The reluctor rings on front wheels are badly corroded.

Apparently the rear wheel sensors are in the hubs.


MOT tester said one ring was broken.

Could this be the fault causing ABS lamp to stay on ?

Would ABS system detect a broken ring, as I thought the rings were used only when wheel rotates, not when car is stationary on starting engine, during MOT test.


So am thinking problem is elsewhere like the sensors.

Any ideas/suggestions on possible/typical faults ?
.

AdrianC

If it's coming on for the self-test, then staying on, that's more likely to be a sensor (or ECU) rather than a ring.
If it self-tested OK, then came on once you started moving, that'd be quite likely to be a ring, as it sees three wheels rotating and the fourth apparently not.

Let the diagnostics do their job, and don't second-guess...

s_b

your logic all seems correct
possibly light stays on once its located the broken ring
these rings are available on ebay,i think i paid about £4 for the last one i needed for a good ole renault
unusual for the actual sensors to go faulty but not impossible
have a scan but the broken relucter ring is the fly at the moment in the soop

mro

AdrianC wrote: »

If it's coming on for the self-test, then staying on, that's more likely to be a sensor (or ECU) rather than a ring.
If it self-tested OK, then came on once you started moving, that'd be quite likely to be a ring, as it sees three wheels rotating and the fourth apparently not.

Let the diagnostics do their job, and don't second-guess...

Just trying to anticipate where this is going, as sensors seem pretty expensive.

And always possibility it could be ABS control unit, so would have nightmare trying to find one from breakers/scrapyard.

mro

s_b wrote: »

your logic all seems correct
possibly light stays on once its located the broken ring
these rings are available on ebay,i think i paid about £4 for the last one i needed for a good ole renault
unusual for the actual sensors to go faulty but not impossible
have a scan but the broken relucter ring is the fly at the moment in the soop

ABS light comes on & stays on, after switching on ignition & not starting engine, without car moving.

Think these rings are magnetic.

Think you have to take off drive shaft to put rings on, I'm not up to that at moment, haven't got time, need to get through MOT.

Johnmcl7

What car and how old is it?

IamNotAllowedToUseMyName

Just so you are clear, think of the rings as some stripes on the wheel that the sensor can see. Obviously, the stripes can be damaged which would cause some problems, but you will not sense the stripes moving when stationary, so that does point to:

1) Sensors
2) Wiring (missed that one so far)
3) Control unit.

I would have thought that any car less than 10 years old would give a proper fault on general car diagnostics to isolate the likely area of fault - it would certainly know that the control unit was throwing a fault through self-test or whether the control unit had detected a sensor that was not to spec. However, if the CU is ok, then it could not necessarily tell the difference between a wire break and a fault sensor.

colino

Unlikely to be a control unit (BMW excepted) and a lot of generic code readers are simply not good enough to distinguish between the relatively straightforward faults possible on an abs system (or even a faulty battery). Decent garage with a decent tech will let you know, quickly what the fault is and get OP on their way.

AdrianC

Indeed - generic ODB2 readers will only read the generic ODB2 codes - which are a subset of engine management codes, mainly emission-related. You'll need the manufacturer's diagnostic kit (or compatible) to read the ABS codes.

mro

Are ABS faults read via car diagnostic plug & engine ECU ?

AdrianC

mro wrote: »

Are ABS faults read via car diagnostic plug & engine ECU ?

Diagnostic plug? Yes, usually.
Engine ECU? No.