Newby ABS problem

[Johneboy 0]

Hi I have had my Corrado VR6 for about 3 years now. Brilliant car.... but I have the dreaded "ABS light on" problem and an MOT looming.

 

I have scanned the forum messages and can't see anything that corresponds to the precise symptoms my car is showing. These are:

90% of time ABS light comes on with the ignition and never goes out

10% of time ABS light starts off... but will typically come on 10 to 20 minutes into a trip

 

This is a situation that has got gradually worse... it used to be ok 90% of the time.

 

When the ABS light is off, the ABS system works fine... and when the light is off it definitely does not (mucho scary moments).

 

I have measured the resistance on the front 2 sensors (not got round to doing the back ones yet) and the values are as expected (12k ish).

 

Given the ABS light is typically on at startup, should I assume this cannot be a sensor problem, or does the beast have some sort of memory of previous faults?

 

The stealer "thinks" the computers knackered... but they are not sure and I forgot to ask for the VAG-COM codes.

 

The progressive degradation of the ABS makes me think its not a computer problem. I would kinda expect it to either work or not work

 

Any advice/ideas most welcome

[dr_mat 0]

It's almost certainly a wheel sensor. Ignore the dealer, they're a bit clueless...

The error codes will be stored by the ECU until read and cancelled.

It's possible the ABS controller is knacked, but it's very unusual.

 

But the point is with the ABS - you have to either trust the error code from VAG-COM (or the dealer tools), or you have to know the system inside out. And the latter is extremely rare. The guys from ATE that make it can probably answer those questions, but no-one at VW can...

[Reggit 0]

Mine's doing similar, although nowhere near as bad. VAGCOM reports brake pedal position sensor, and I'm off to try and get one tomorrow.

 

Johneboy, where abouts in the country are you. There may be some one on here with vagcom willing to help you with the codes.

[Mike Edwards 0]

Yes - pedal position sensor. WHen a wheel sensor goes the light is always on - no coming on after a few minutes.

 

That said, as they cost more than £50, get the fault codes read 1st.

[buttles 0]

New ABS relay cured mine. Light coming on after 10-20 minutes

[tonytiger 0]

WHen a wheel sensor goes the light is always on - no coming on after a few minutes.

 

I disagree - when I had a duff wheel sensor on mine, the light wasn't always on - I actually had pretty much the same symptoms described by Johneboy, but sometimes the light would go out again while travelling (for no obvious reason) - and the system could work for days at a time without problem. Once I replaced the offending sensor it the system worked perfectly (though the EDL is pretty useless).

 

In the end I bought vagcom to find out exactly what the problem was - cheaper in the long run. I would strongly recommend anybody with a vr who plans on keeping it for a while to invest in vagcom (I know that ideally means a laptop too) as I've found it invaluable - apart from using it when you know there's a problem, you can use it periodically too make sure you haven't got any problems.

[Ice White Socks 0]

It

WHen a wheel sensor goes the light is always on - no coming on after a few minutes.

Not true. Don't know how a sensor works but they can give varying resistances when on their way out- sometimes will fall within tolerance of ABS brain so no light, sometimes will fall outside and flag up the fault light. It sounds daft but it really does happen :?

 

Cheers

[dr_mat 0]

Of course. One front wheel sensor went on mine and the light would come on intermittently. The other wheel sensor went and the light was on solidly. Clearly the difference between "open circuit" and "implausible signal".

[Ice White Socks 0]

Of course. One front wheel sensor went on mine and the light would come on intermittently. The other wheel sensor went and the light was on solidly. Clearly the difference between "open circuit" and "implausible signal".

there is no difference between 'open circuit' and 'implausible signal' as far as the ABS brain is concerned- both will cause the light to come on solidly.

 

When the light comes on intermittently the sensors output is varying between a 'plausible' signal and an 'implausible' signal- thats what i found strange- you would think it would either be broken or not but that doesn't appear to be the case.

 

Wierd :?

[Henny 0]

The wheel sensors are a VERY fine coiled strand of metal wire... they have a habit of breaking along this strand which can cause intermittent ABS faults... The reason they are intermittent is very similar to how you can get a lightbulb to work again once it's blown by shaking it until the ends just touch and weld themselves back together. The self test shoves some voltage through the sensors to check for continuity and resistance, this can cause the coil to reconnect and pass the self test. The vibration as you are driving along can then cause the coil's break to re-open and therefore put the light back on again...

 

This doesn't always happen, but when it does it can be a real PITA to find the fault! :roll:

[dr_mat 0]

One front wheel sensor went on mine and the light would come on intermittently. The other wheel sensor went and the light was on solidly. Clearly the difference between "open circuit" and "implausible signal".

there is no difference between 'open circuit' and 'implausible signal' as far as the ABS brain is concerned- both will cause the light to come on solidly.

 

Of course, I never said the light didn't come on. But actually there is a difference, according the ABS ECU - it logs a different fault code.

 

When the light comes on intermittently the sensors output is varying between a 'plausible' signal and an 'implausible' signal- thats what i found strange- you would think it would either be broken or not but that doesn't appear to be the case.

 

This is exactly what we were referring to - it's not cut-and-dried. A faulty sensor CAN cause an intermittant fault.

[Ice White Socks 0]

Of course. One front wheel sensor went on mine and the light would come on intermittently. The other wheel sensor went and the light was on solidly. Clearly the difference between "open circuit" and "implausible signal".

 

Sorry Dr Mat- I must have misinterpreted your post- It looks like said you have had two sensors go, one causing an intermittent light and one causing a solid light- the difference between the two being one was an 'open circuit' and the other was an 'implausible signal'. All I was saying is that neither of those would cause an intermittent light- both would cause a solid light.

 

it's not cut-and-dried. A faulty sensor CAN cause an intermittant fault.

 

I know that- I said it myself about eight posts ago

 

Not true. Don't know how a sensor works but they can give varying resistances when on their way out

[dr_mat 0]

It looks like said you have had two sensors go, one causing an intermittent light and one causing a solid light- the difference between the two being one was an 'open circuit' and the other was an 'implausible signal'. All I was saying is that neither of those would cause an intermittent light- both would cause a solid light.

 

My meaning of "intermittent light" was just that it wouldn't come on *all* the time. Sometimes I'd go whole journeys without it coming on and sometimes it would come on shortly after startup. Once it was on, of course, it stayed on until I switch the car off, so it's a solid light in that sense..! :)

 

So we're not arguing at all, we're just not speaking clearly..! :)

[Johneboy 0]

Wow - First post to this forum and I am blown away by the volume of responses. As I see it, the next steps are:

1) Measure the resistance on the rear sensors and remeasure the front sensors

2) Get the vagcom codes (I live in Amersham, work in Milton Keynes (someone has to))... any vagcom owners nearby?

3) Check out the ABS relay - where is this and how should I tell if its duff?

4) Check out the pedal position sensor - where is this and again how should I tell if its duff?

[buttles 0]

There are threads for all of the above on this site. In a nutshell vagcom will tell you if you have any of the problems listed. ABS power relay is one of three located in front of the glovebox, you need to remove the lower shelf at least. I can't rember which one it is. 10 quid. One is abs power, one is pump relay, not sure what other one is for. Vagcom throws up code power B+. For a tenner good place to start if you can't get hold of vagcom. I have it in Redhill, bit far round maybe!