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Scott, Master Mechanic
Category: Australia Car
Satisfied Customers: 251
Experience:  30 yrs experience with Aussie, Japanese & British cars
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I drive a Ford Falcon (Australian) > I am having a recurring

Customer Question

I drive a Ford Falcon (Australian) > I am having a recurring problem with my ABS sensors accumulating metal filings on my front bake sensors causing the ABS to malfunction. The sensors have very strong magnets, even with the ignition turned off, and this is clearly the reason the metallic dust accumulates. Is it normal that the sensors are magnetic in the manner I have described, or is it that they have become magnetised over time from the current passing to them. Clearly if I clear the sensor of the metal filling type dust the ABS works fine. Your advice please.
Lester XXXX
Submitted: 2 years ago.
Category: Australia Car
Expert:  Scott replied 2 years ago.

Hello Lester,
Indeed the sensors are magnetic, & will accumulate ferrous metal dust or filings if they are present in the brake rotor, or if they're being generated by metal to metal contact in the hub area. That should be the focus - the source of the ferrous metal dust or filings.
Can I ask, have you had the rotors machined lately, or any brake servicing? Sometimes machining swarf can remain in the rotor & gradually collect on the permanent magnets in the sensor. Other than that, it may require a pad removal to ensure there's no metal to metal contact in the braking system..
Any further info would be appreciated..

Customer: replied 2 years ago.
The unanswered part of my question. Is the magnetic effect an electric magnet ?. If they are indeed electric magnets, with the ignition turned off the magnetic field would be lost and hopefully the metallic fragments would fall away. But as I said with the ignition turned off the sensors are still strongly magnetic.
The recurring problem started to occur after new rotors and pads were fitted, but thorough inspection of the brake area does not show any apparent reason, and the braking function is excellent, until fresh metallic dust collects after about 3000km the mechanic at my servicing Ford Dealer has not been able to offer any explanation.Lester BLACK
Expert:  Scott replied 2 years ago.

Hi Again Lester,
The sensors include a permanent magnet & a reluctor ring, so that part will attract & retain ferrous fragments etc & retain them until they're cleaned off, regardless of power signal at the module.
All I can suggest is if the rotors & calipers haven't been removed & cleaned after a frequent occurrence of the issue, well, then this should be done first. I would not expect any variance in brake performance until the ABS signal was interfered with.
There is no normal reason for the area to generate enough loose ferrous material for it to require cleaning at 3000km intervals. I wouldn't expect the brake pads to have iron content, but none the less, as a last resort, I would be changing them too..
Hoping this helps!

Customer: replied 2 years ago.
Thanks Scott,This morning I again took the front wheels off and cleaned the sensors. Only the nearside had a few very small fragments on the tip. Once removed and then, a test drive confirmed that the problem was resolved. Certainly the contamination was far less than when the new Disc’s were installed.I have one remaining question. I am wondering if the recessed inner tip of the sensors, has a different polarity to the otherwise exposed part of the sensor, and whether the metallic fragments in fact cross the polarity and cause the false message to the controller. Now if that were the case then a thin, non metallic membrane, on the exposed sensor would stop the fragments crossing the polarity, but should still allow the magnetic field to pass the membrane and do its job.Could I have your thoughts.Thanks, Lester
Expert:  Scott replied 2 years ago.

Hi again Lester,
Yes, in a sense that is how they work - the 'coil' side of the sensor is exposed the magnetic field generated by the fixed magnet, with the interruptor ring altering that field & generating a pulse in the coil, which is processed by the ECU. The problem with placing anything in there that reduces the field adequately so that iron particles etc won't stick to it is that it will also be interfering with the permanent magnet field, creating a similar issue to as what you have now ie think you would find particles stuck to the 'membrane', as it's still in the 'field' area. It would also have to be made from a heat proof material such as mylar - material such as thin plastic wrap would melt at 140C, which the area could easily get to..
As you are finding, the issue will become less apparent with vehicle use if it's indeed left-over machining swarf being emitted by the rotor..
Cheers Again,