Abs Light (Intermittent)

[djb2]

Good morning guys.

Just took ownership of my 2004 Mondeo Zetec TDCI....huge upgrade from my 1.8 Rover 75.

I should have got a Mondeo TDCI years ago but could never afford one. So impressed by the cool and unruffled way it accelerates and deals with hills! Im used to lumbering up hills trying to coach my previuos underpowered 1.8 Rover whilst trying not to blow another head gasket...It had 3 in my 4 years ownership and I am an experienced Rover owner and know all about the K series engine and its cooling issues.

What a car the Mondeo TDCI is. I used to use them as hire cars in work and to me it was one of those dream cars which I never thought I would own.

I do have an issue though as per the title.

The ABS light comes on intermittently. Having said that Iv just driven 15 miles and no sign of it.

When it is on nothing else is affected. Braking is spot on (even under heavy load and emergency stop situations - I tried it on an open clear stretch of road). Cruise control is unaffected (I seem to remember reading someting about the cruise control not working if the ABS light illuminates - unless thats for a different car).

I suspect from reading the internet and you tube etc that it may be a wheel sensor contact / wirirng issue and will be checking this out this week.

Its one of those situations that being new to a car model hopefully someone else would have had and cured a simlar problem and so I thought I would ask.

I have the Haines manual but not really much help there on this point.

Im not scared of taking tools to the car but only if neccessary and I cant really afford to go down the diagnostic computor route unless I have to.

Thanks guys for any assisitance you may be able to give.

[BOF]

Just took ownership of my 2004 Mondeo Zetec TDCI....huge upgrade from my 1.8 Rover 75.

I should have got a Mondeo TDCI years ago but could never afford one. So impressed by the cool and unruffled way it accelerates and deals with hills! Im used to lumbering up hills trying to coach my previuos underpowered 1.8 Rover whilst trying not to blow another head gasket...It had 3 in my 4 years ownership and I am an experienced Rover owner and know all about the K series engine and its cooling issues.

As far as I know (and this is off topic), the Rover K series cooling issues can be fixed, but what happens is that people just replace the head gasket and wait for it to go again. Really, this isn't sensible, but is what seems to happen, although K series powered cars are getting a bit long in the tooth now, and people probably do want to just spend the minimum getting them back on the road. In any case, the 75 is quite a heavy old bus, and is only marginally adequately powered by a naturally-aspirated 4 cyl K, so you'd have to hope that your new mondy is going to prove a more enjoyable drive...

The ABS light comes on intermittently. Having said that Iv just driven 15 miles and no sign of it.

When it is on nothing else is affected. Braking is spot on (even under heavy load and emergency stop situations - I tried it on an open clear stretch of road). Cruise control is unaffected (I seem to remember reading someting about the cruise control not working if the ABS light illuminates - unless thats for a different car).



I suspect from reading the internet and you tube etc that it may be a wheel sensor contact / wirirng issue and will be checking this out this week.

T'internet gets something right, shock, horror! Broadly speaking, something is wrong with (at least) one of the wheel sensor signals. 'Something wrong' may be that one signal does not agree with the other three, or is intermittent, or whatever.

When the various modules that get the wheel sensor signals 'see' that there is something wrong, it is up to each to react in its own way. So, for example, in driving the dash elapsed distance display, you might not be all that bothered that one wheel signal has gone, because you can get the number that you want (distance travelled) from the other three. For ABS and stability control, it is quite a different situation (one wheel signal gone means that whatever you can do, you can't be as effective in your fundamental objective as if you had all of the signals). Cruise control is a bit different, in that it isn't a fundamentally safety-oriented thing, like ABS and Stab Control, but you really would like it working correctly if it is engaged, so you may or may not decide on keeping cruise control available if something fishy is going on, it depends.

Unfortunately, there are two likely causes:

• bad (intermittent, probably) wiring from the wheel sensor back to a module (ABS) or between modules
• problems with the sensor and the sensed teeth

and it can take a bit of poking around to find out what is going on. The wiring stuff ought to be 'the obvious', but the sensors and the teeth possibly less so. The sensors depend on magnetism and the gap between the sensor and teeth is critical (so if, eg, the ring of teeth has taken a knock, then that can easily be problematic) or if the gap is too big the sensor probably doesn't work correctly at low speeds, but does start working as the speed increases. On of the 'classics' is if a tooth or some teeth are damaged the system might work fine around one part of the circumference, but only work around the other part above a certain speed. Equally, the sensors can be subject to physical damage and a blow to the sensor can leave the signal too low to be detected at low speeds but working perfectly well at higher speeds.

This is one of those things that usually works well, but when it gives problems can be a right pain to track down.

I hope that it is something obvious, as these can be irritating, niggly, problems.

Have you got a guarantee of any kind? If you have, telling them to fix it would be a good thing! There might also be further information available from fault codes which might point you in the right direction.

[djb2]

Hi BOF.

Thanks for the reply.

The ABS light is intermittent. On two mornings drive into work it has stayed off. I can jump in start up and never see it or it just pops on for no apparant reason.

What I mean by that is nothing seems to 'cause' it to come on i.e heavy braking, particular road speed etc.

Trying to apply a little logic to it (if thats at all possible!). If I had a cracked ring (ooh that sounds painful) or a sensor was not functioning wouldn't the ABS light be on all the time (or come on as I drive almost immediatly?).

In my basic knowledge and experiece of cars over the years the pattern of complete randomness seems to point to a dodgy or loose contact or wiring.

As I said before I am going to check the wiring to all sensors for security and cleanliness as a starting point. I think I can access the rear ones simply by parking half on the pavemnent and crawling underneath the rear of the car (but hey, not in this weather!). I believe I may have to remove the front wheels to access and check each front sensor. I will also trace back the wiring for any obvious signs of damage and or chaffing etc.

If that has not cured the appearance of the ABS light coming on, I will purchase a replacement sensor (fully tested from the breakers yard) and replace each wheel in turn to see if it makes a difference. If that still has not cured it I will replace the YAW sensor.

As far as the Rover 75 is concerned It is a beautiful car but unfortunalty too underpowered. I was always reluctant to ask too much power from it for fear of busting the headgasket again. I will never buy a car again with a K series lump.

[BOF]

Yes, but just bear in mind that it might be 'multi-factorial'. That is, there may be a problem that only occurs when the car is going at a certain speed and you brake (or steer) at the same time and maybe only that when round a certain part of the wheel circumference. This can be quite confusing.

Obviously, intermittent wiring and/or connectors is a strong possibility, just not the only one.

And, as to the k series, note that it is widely used in Motorsport (under quite different conditions) where it is really quite reliable. This is quite against popular prejudice (that Motorsport is bound to be a harder test than 'ordinary' use), provided that the engine isn't tuned to quite extraordinary outputs with inadequate care for the details.

[djb2]

Thanks once again BOF.

I haven't had a chance to look yet but I notice it stay off longer than its on. I am wondering how the system works in terms of the warning light.

i.e. If you start up and drive and the system then detects a fault (say a momentary loose of contact with a sensor due to a dirty contact/wiring issue) it illuminates the dash light which stays on.

Once on, the light does not ever go out if driving..however if you stop and restart the engine often the light stays off.

Is the system set up so that the warning light stays on in a 'driving cycle' to warn the driver that a fault has been detected until ignition has been turned off even if the fault has rectified itself?

If so that points even stronger towards a loose contact etc.

If I had a broken sensor or sensor ring surely the light would remain illuminated.

I take on board everything you say about the fault being multi-factorial and that may be the case. I also understand that the ABS system is so very complicated and intelligent in terms of the factors the car takes into account in monitoring and activating the ABS system.

Any comments as ever greatly appreciated.

thanks

[BOF]

haven't had a chance to look yet but I notice it stay off longer than its on. I am wondering how the system works in terms of the warning light....i.e. If you start up and drive and the system then detects a fault (say a momentary loose of contact with a sensor due to a dirty contact/wiring issue) it illuminates the dash light which stays on.

Once on, the light does not ever go out if driving..however if you stop and restart the engine often the light stays off.

Yes, I believe that is just down to how the software works. That is, if a fault is detected, it keeps displaying a fault until the system is reset.

The actual fault detection may only be present for a time of some milliseconds, so you'd probably never see or ignore the fault if it only displayed for the duration of the detection being positive.

I haven't had a chance to look yet but I notice it stay off longer than its on. I am wondering how the system works in terms of the warning light.

i.e. If you start up and drive and the system then detects a fault (say a momentary loose of contact with a sensor due to a dirty contact/wiring issue) it illuminates the dash light which stays on. If so that points even stronger towards a loose contact etc. If I had a broken sensor or sensor ring surely the light would remain illuminated.

Well, most systems don't have a direct 'check whether the sensor is broken' function. It would be possible to do that (detect, for example, open circuits) but it would involve additional complication, and something else to fail...

What most systems do is to look at all of the wheel sensor results, and there are some conditions which just cannot make sense if everything is working. So, if a wheel sensor indicates that the wheel has gone from 1000 rpm to 0 when the others have carried on at 1000 rpm, you'd question that. Equally, if three wheels are accelerating and one is doing nothing at all, you'd question that. And, any of these conditions might have to be present for a period of time or possibly even repeatedly before you lit the dashboard lamp.

Now, what makes this more complicated is that the output of the sensors themselves is zero at zero speed, so there is some speed threshold below which they don't really work. Usually this is of the order of 5 mph, so they work well for normal driving, but are a bit useless at really low speeds. that itself isn't the biggest problem, but the threshold is dependant on the sensor gap, sensor condition, condition of the teeth. So, it could happen that one sensor - a good one, set up correctly, with the right gap - works at 5 mph, and another one -with a larger gap, a damaged pole piece, mounted less than firmly, so it vibrates, or with the teeth in slightly damaged condition or the sensor wheel slightly out of concentricity - only works correctly from, say, 10 mph upwards, or doesn't work over part of a revolution or doesn't work correctly over a limited range of speeds.

When combined with software that might not tell you when something first happens, but some time later, this can result in a pretty confused picture and it is easy to go off saying 'well, logically, this implies that, and that implies the other...' and be wrong. It could be an intermittent in the wiring - that's not all that improbable - but it could be something else, too.

Just as an example, you could, for example, have a situation in which one tooth on one wheel is failing to be detected. Now, that may not be enough to trigger the warning on its own, but it could be that if you go around a corner and that wheel goes slower it is enough to go over the threshold and it may be that you don't actually get informed until a hundred yards later. Would you look at that and say, for example, 'Well it is obviously a left side wheel sensor because it always goes off a hundred yards after I've made a left turn between 5 and 10 mph'. I'm not sure that you would, so I'm just cautioning that these can be confusing. (And, if it is just one tooth, it may also depend where in the rotation of the wheel that tooth is positioned when you start the turn, so it might not happen every time; I just think you'd take a while to work out the connections.)