Outwitting Your Vehicle's 'Brain'

[Chris M. White]

Guys,

I have an aging (18 year old) Mercedes W211 which has developed a fault which is causing the dash to light up like a Christmas tree. This one fault is causing 3 different warning lights and associated text warnings to come up: ABS, ESP and SB. I took it to an independent garage that specialises in Mercs and has all the software etc. Turns out the problem is a magnetic ring buried deep inside the n/s rear stub axle. The part costs peanuts, but is *murder* to get at. Even for a Merc main dealer it's half a day's work and requires several special service tools. I was quoted over 500 quid to fix it! +VAT!!

So I've come up with an ingenious work-around I'd like to run past you all; see if there's any adverse consequences I haven't thought of. The car's main CPU thinks the rear n/s wheel is losing grip, because it's sensing (wrongly) that the speed of that wheel is diffrerent to the speed of the one on the opposite side. How about I snip the feed from the n/s sensor and connect it in parallel with the feed from the o/s one? That way, the CPU will see the signals as perfectly in synch. The sensor on the 'good side' stub axle will then be supplying speed info for *both* sides and the duff side will be snipped out of circuit altogether. Your thoughts on anything I haven't considered (I mean *technical* issues, not daft observations about legality etc.) I would remind you all there are plenty of classic cars on the road that don't have ABS at all so it's not inherently dangerous.

[Tim+]

Probably not as issue on a vehicle that age but I imagine that might upset any stability control software (or at least stop it working properly).

Whilst I take your point that many older cars didn?t have ABS, I think as far as MOT is concerned, if it was fitted, it has to work.

Might be worth getting a second opinion on the work involved. Hard to believe that any work on a stub axle can be that complicated. Have you searched YouTube?

This any help?

Tim

[alan_m]

Is the wire to it on a connector located near the stub axle? If so is it a fault with the sensor or just a bad connection or corrosion in the connector?

[alan_m]

Youtube

the £500 repair

[newshound]

What a good video!

[Theo]

I had that problem (not on a Merc) - in the beginning driving above 80 mph would cause the ABS light to trip, then it was 70mph, then 60mph. The car was also juddery under sustained acceleration as the ABS kicked in.

I thought the reluctor disc was rusty and it would need a new wheel bearing (including the reluctor), but turned out the sensor was faulty. Unfortunately since I'd taken it into the garage with the expectation of them doing the wheel bearing, I didn't have the opportunity of DIYing it.

ebay says the W211 sensor can be had for about a tenner, so I'd try changing that first if you can DIY it.

Theo

[newshound]

Might this be CAN bus? In which case it might give Marvin a terrible pain in all the diodes down its left side.

Too risky for me. £600 doesn't sound so bad all of a sudden.

[Nick Finnigan]

That would not be enough to determine that the n/s sensor is faulty.

How about I snip the feed from

If the CPU is comparing with the n/s rear with the n/s front sensor, to allow for cornering, you still have a christmas tree.

[Chris M. White]

Seems it is, though. Only 2 wires to each sensor feed. The sensors appear to be simple electromagnetic generators which generate a pulse whenever the magnetic strip embedded in the stub axle ring goes past them.

[Brian Gaff (Sofa]

That depends whether the handling of the car depends on the abs being active, If it can be disabled, then you are probably safe, but might need to indicate somewhere that the re enable switch is inoperative. You might get insurance issues unless you declare that its not working if its there by default.

Brian

[Brian Gaff (Sofa]

Also all sorts of strange things happen in modern cars, even things like the car has been in an accident which has bent something just enough to affect things like this or suspension issues. I'm actually glad these days that I was never able to drive, since the complications seem endless when stuff begins to fail. Brian

[Harry Bloomfield, Esq.]

Chris M. White wrote on 29/07/2021 :

I think it should probably work, if it passes the ignition on tests, but it is highly illegal. If an ABS system is fitted then it it legally required to work properly.

The way they work, is a Hall sensor is fixed to the static part of the hub, the rotating part has a series of tiny magnets around its periphery, perhaps 40, which produces pulses to the ABS ECU.

What can happen is that the magnets can attract rust, which can then upset the pulse count from a wheel.

You might be able to remove the sensor and poke an even stronger magnet through the hole, then rotate the hub, to try to 'suck' loose rust off the hub.If the warning lights come on and stay on before the car is moved, the likely fault is the sensor, or its wiring.

If they appear when you first get to a low speed, then it points to the magnetic hub issue or rust.

[Harry Bloomfield, Esq.]

newshound presented the following explanation :

Nope - the ABS is completely independent of the canbus.

[Harry Bloomfield, Esq.]

on 30/07/2021, Chris M. White supposed :

They are the older type, just a coil of wire like a guitar pick-up. Modern version is a Hall sensor.

[Nick Finnigan]

What law defines how an ABS system works properly ?

[Chris M. White]

The most accurate summation so far. You obviously know your stuff! This car is too old to have serious money spent on it. The next owner will be the scrappie so I'm only going to run it into the ground anyway. I'll try your magnet idea in the first instance and fingers crossed that may work, as the lights don't come on until the car's moved some distance. Thanks for your input!

[Spike]

If you can't find a fix that keeps the car legal to use on the road at a price you're willing to pay, then it *has* run into the ground...

[Adrian Caspersz]

I've recently bought (new to me) a Skoda with much of this tech, cool to play with, but the tech has the assumed IT longevity and disposability of an iPhone. The car's mechanics will easily outlast that, which was a similar story with the OP's car.

Open source engine management is a thing, hopeful dream that someone comes up with the whole car replacement operating system (OSS, of course) so that we don't needlessly throw cars away because the "radio console" doesn't work.

Or maybe a partitioning of Convenience systems and manufacturer's safety systems, so ye could replace the convenience system, just like ye changed ya car radio of old by removing it from the dash.

In the long term for me, I'm game for some silicon hacking about to keep my new car on the road, but would still keep the mechanical maintenance a priority above that. A sensor failing, I'd change it.

I had my previous Octavia for 13 years, a solid thing for me without any serious maintenance bills, going by reports this Superb shouldn't be a complete surprise to the pocket.

[Scott]

Also, as a modification I assume it would require to be declared to the insurers. I would certainly speak to the underwriter first.

[Theo]

I'm not sure that's a 'modification' on insurers' usual lists, but 'tampered with the braking system' is not a good look, either to insurers or if it came up in court. And so if there was an accident I could well imagine the insurer declining to pay out, which would leave you liable for third party losses (don't drive behind a supercar, you can't afford it).

Theo