Abs Problem Aaaggghhhh

[blippy]

Hi,

 

I have an ABS issue that's doing my head in!

 

It all started when I replaced the front O/S wheel bearing on my 53 plate. About a week later the ABS light lit and VCDS reports an O/S front ABS mechanical failure. Now I'm pretty sure this is the DTC you get when the sensor is electrically OK, but there is no output from it - like when the ring is broken/fallen off.

 

I stripped it this morning and found that the little pole piece on the end of the sensor was missing - I think I must have bashed it with the end of the driveshaft when I did the bearing and it subsequently fell off causing the fault.

 

So I got a new sensor, cleaned everything up and installed it.

 

I cleared the fault and went for a test drive and within 20 yards it was faulting again - same fault - ABS mechanical failure.

 

I've taken everything apart again, double-checked the ring is intact and aligned with the sensor (yes and yes) made sure there's no magnetic rubbish on the end of the sensor, but there's still no output.

 

Looking at the measurement blocks from VCDS shows there is no signal from that sensor - the other 3 are fine. Unplugging the sensor causes a new code from the ABS controller identifying a wiring fault, so I don't think there is anything wrong with the wiring or connectors - if so that fault would be there all the time??

 

Only thing I can think of is that the new sensor is duff???

 

Does anyone have any ideas?

 

Thanks,

Blippy

[dlt1617]

If the ring is ok and the sensor has been replaced I would double check the sensor is fully in on the stub axle - I did mine and thought all was well but had similar problems. only other thought would be excessive play in the wheel bearing but I'd try the sensor first

 

Dave

[blippy]

If the ring is ok and the sensor has been replaced I would double check the sensor is fully in on the stub axle - I did mine and thought all was well but had similar problems. only other thought would be excessive play in the wheel bearing but I'd try the sensor first

 

Dave

 

Dave,

 

I'm pretty sure the sensor is fully seated - I ground the face of the hub to clean metal on my second attempt to get the thing working. I'm also sure the bearing is fine - it's only done about 1000 miles + I put the car through an MOT this morning (yes - I knew it would fail on the ABS light), and the ABS was the only problem reported. Surely that would have picked up a bearing problem.

 

Not sure if this is significant or not, but when I did the bearing I couldn't get the hub off the car to push it out, so I cut the outermost bearing shell out with a dremel tool. Could the resulting fine metallic dust have caused the problem?

 

I think I'm going to try to extend the wire from the N/S sensor, which is proven to work, into the O/S sensor socket and see if the ABS unit will see that - that should eliminate the O/S harness and the ABS controller shouldn't it?? I dare not pull the N/S sensor to test it directly for fear of messing that up too!

 

Blippy

[blippy]

This morning's update:

 

I rewired the N/S sensor into the O/S sensor socket with a extension cable across the bonnet :angry:

 

While monitoring VCDS, when the N/S wheel was spun the ABS ECU showed the wheel speed on the O/S front input. Therefore I can eliminate everything except the new sensor, the O/S ring and hub/bearing assembly.

 

When I was beeping-out my makeshift extension cable I discovered that the new sensor is several hundred ohms higher resistance than the N/S one. Anybody know what they should read? It's an ATE MK25 system.

 

I'll post an update when I have more.

 

Blippy

[blippy]

OK, got to the bottom of this one - the new sensor was either the wrong part or broken out-of-the-box.

 

In the end I bit the bullet and pulled the N/S sensor, managed to get it out without smashing it. I was surprised to discover there is no pole-piece on the end of this one either, so it was physically unlike the new one, which came from a local motor factor.

 

The original has VAG part number 7M3 927 807M - going to try and get one from TPS in Newcastle tomorrow.

 

Blippy

[dlt1617]

Sorry I didn't check the forum until today so no chance of giving you any other ideas but hey sounds like you've sussed it any way :)

 

Dave