UKC

The ABS system has started to play up on my Peugeot 406. It randomly and intermitently kicks in whilst braking in situations where there is absolutely no need for it to do so. The pulsing felt through the pedal can be remarkably strong, disconcertingly so at times and it certainly feels that the braking is less efficient when it does.

I feel an expensive repair coming on but wondered whether I could just detach the electrical supply to the ABS unit, very easy to do - just pull the connector off, so as to de-activate the ABS altogether.
I'm perfectly happy to drive a car without ABS, I've never had one with it before and as I understand it, if the ABS system "fails" you still retain normal braking anyway.

Anyone else done this or any mechanics advice welcomed. Thanks.

In reply to pec:

Accompanied by a noise like someone twanging a slinky spring?

It is usually a dirty/rusty encoder ring on the wheel hub (check the front wheels first.) The rings are a shrink fit onto the outer driveshaft joint, with a pickup on the wheel hub.
If you can get to them make sure that they are still attached to the driveshaft, sometimes they break and come loose (replace joint about £45 plus 1hr labour)
Visual check of the wire to the pickup. If you can get the pickup out of the hub, (usually retained with one bolt) give it a clean, if you cant, dont force it it will break.
Back wheel sensors very rarely give trouble.
Any ABS fault is an MOT failure, will check the new manual as things changed after christmas, I think that as of 1st march any ABS system fitted has to work.

Cheers Adam

In reply to butteredfrog:


I'd struggle to see which part of the MOT would identify a faulty ABS system?

In reply to timjones:

From someone who sounds like they know what they are talking about.

"The ABS system will have a special sequence of warning lights when the ignition is switched on to indicate that the system is functioning correctly. That is tested for the MOT. The ABS as such is not tested. So if, as a result of your ABS failure the warning lights show an incorrect indication, then that will result in an MOT failure."

In reply to timjones:
If fitted, ABS has to work for the MOT, upon ignition, the ABS light has to self extinguish. If it stays on it means the car senses a fault somewhere and the test will declare a failure.
It is most likely a faulty sensor on one of the wheels. Undo the wiring connector one sensor at a time until you have isolated the faulty sensor. As you'll have one sensor disconneted the light will stay on but the juddering <ABS activation> should stop.

In reply to EeeByGum:
Ditto!

In reply to pec:

Check the ABS sensors with a multimeter, they are attached by push fit plugs a little way along the wiring, from memory the resistance across its contacts should be 2.3 MOhms. Also check the ABS target rings (reluctor rings) and make sure all the teeth are there. The missus' clio had exactly the same issues and it was a corroded ring that had lost a tooth that was causeing the issues. Renault wont supply reluctor rings but they're available aftermarket for about a tenner, we bought the part and paid a garage to fit it cost half an hours labour.

In reply to pec:
You can just take the fuse out for the time being and the ABS will be disconnected.

The MOT failure thing is the real problem though. I had the same problem and fixing the ABS would cost more that the value of the car so I took the ABS warning bulb out and the car got through another 2 MOTs and is still going strong now.

In reply to pec: Of course the easy fix for the MOT is to wire the warning light to the alternator/battery light. Works for airbag warning lights too.

In reply to pec:

Is it actually displaying a warning light?

You may simply have been skidding on dust, diesel, poor tyres etc. ABS (especially old ABS) can be pretty poor on loose surfaces. You could also have a tired damper or other suspension wear causing reduced traction under braking. Just a thought.

jk

In reply to pec:

One of the worst engineering aspects of road safety at the moment (and the underlying trend is a good safe one) is that so long as the ABS light goes off at the right time and the car passes the dyno test everyone thinks it's safe. Think again. The car could be all over the place in a heavy braking event. Most cars over six or seven years old probably need to have the fuse pulled on the ABS and just get the brakes to work properly. Oh, and aftermarket brake pads that work properly over 30mph would be a plus as well.

In reply to pec:
If you were to disconnect the ABS yourself and then have a crash I'd be pretty sure that your insurance company would try and wangle their way out of paying any resulting claim.

I'm not not saying that the car will be dangerous without ABS (my car hasn't got it and I'm happy enough to drive it) but, most insurance companies include a clause stating that the car must be properly maintained and disconnecting the ABS to avoid paying for the repair isn't what most people would consider "properly maintained".

In reply to pec: Check the reluctor rings and sensors.

I don't know the set up on 406 but the front nearside is usually the first to fail (potholes/drains). I'd bet that on that age of car they'll be passive sensors so can be tested with a multimeter for consistent impedance. Reluctor rung is most likely since an expensive fault like abs pump would probably throw a warning light. Reluctor ring failure doesn't give an error - it just leads to overactive abs.

The front reluctor rings (the toothed ring bit) may be built into the CV joints rather than the brake disk. which can make them a more expensive issue.

If you disconnect the system it should fail an MOT - similarly wiring it up to the alternator bulb should also fail - if it doesn't I'd be worried about what else the tester is too incompetent to notice.

In reply to pec:


ABS works by effectively cancelling the force you're applying to the brakes. If it's kicking in when it shouldn't it is almost certainly making your braking less efficient, possibly dangerously so. Best get it checked out sooner rather than when someone steps into the road in front of you!

In reply to pec: Thanks for the advice. I'll start by checking the sensors on the front wheels first when I get chance but may disconnect the electric supply in the meantime.

In reply to pec:

But don't forget the comment made about dampers.

In reply to Ashley:


I've had a look today at the front wheels, nothing obviously wrong, haven't checked the back yet. The reluctor ring on one of the driveshafts was a bit more rusty than the other, a bit pitted on a couple of teeth but none missing or even half missing. Is that bad enough to cause a problem?

Re measuring the resistance. Do I trace the wiring back from the sensor to where there's a connector plug, disconnect it and find 2 contacts to stick the probes of my multimeter on?
How do I know if there's a problem, will the defective one be higher or lower or just different from the other 3?

Pulling each of the front sensors off (one at a time) and tying it out of the way solves the problem but the dash warning light comes on. Solves the problem in the short term but won't get through an MOT!

In reply to pec: http://www.jmg-stock-photo-video.com/index.php?module=media&pId=102&...

In reply to pec: You'd need to check the signal generated by the reluctor rings on an oscilloscope to see if the rust and pitting are really the cause but I'd say that is probably what is causing your problem.
The algorithm in the ABS ECU counts teeth and throws and ABS triggering event when it sees a speed differential from any of the rings. Rusted or pitted teeth can show a weak signal that would be enough to put the system into warning mode but you'd need to check on the scope.

In reply to pec: If there is a heavy build up of rust it can cause a problem, clean them up to be on the safe side. Sure there are no cracks? Can sometimes be difficult to see on a rusty ring (fnarr)

The other thing worth mentioning is that the positioning of the sensor is quite important. Did they come out easily? How are they spaced on the 406? Is there a plastic sleeve or something?

You'll generally find a connector just on the other side of the grommet where the cable passes into the body of the car.

You should be able to look up the resistance of the sensor on the web - There could be differnt ones fitted depending on the system. Is there a readable part number on the sensors?

In reply to another_mark:

Had a look at the back ones today, they're much cleaner than the front ones so that points to the rust as a possible cause

>
They were very firmly held in a slot, no chance they could have moved.


I disconnected the front ones after that grommet and stuck a multimeter on the terminals but it gave no result, ie. infinite resistance like there's no connection at all between the terminals. Am I not measuring it right or are they both completely broken? I suspect the former!

Thanks anyway.