Outwitting Your Vehicle's 'Brain'

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?

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?

Youtube

What a good video!

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

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.

The ABS system is not tested in an MOT. All the tester wants to see is the light coming on for a second or 2 on the turn of the ignition switch and then go out.

I am inclined to agree.

Chris M. White snipped-for-privacy@gsm.com wrote

Dunno about that particular Merc, but its unlikely to be a simple analog signal that you can do that with.

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.

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.

Chris M. White snipped-for-privacy@gsm.com wrote

Don't believe that.

Plenty of digital systems only have 2 wires.

That's not all there is at the sensor.

And even if it was, you cant just move the two wires from the faulty wheel to the wheel on the other side and connect those 2 to the same 2 on the good wheel and have it work.

Will screw up your abs and possibly traction control , might also throw a wobbly as the sensors have a resistance as does the wire putting the circuits in parallel will alter that resistance

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

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

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.

newshound presented the following explanation :

Nope - the ABS is completely independent of the canbus.

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.

What law defines how an ABS system works properly ?

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!