Struggling to fix ABS fault

[KevinSt]

This was the scenario a couple of weeks ago:

 

2005 Octavia 2.0TDi DSG with about 70k miles.

The ABS warning light came on and the code reader showed the rear right sensor as intermittent.

 

I replaced the sensor with a new one and this did not fix the fault. The removed sensor had 'wear' marks on the tip indicating that something had been rubbing on it.

I therefore suspected the reluctor ring and peering into the sensor mounting hole with a mirror you could see a badly rusted part on the ring as the wheel was rotated. I had the hub replaced and when the garage tested it the ABS light stayed off for their test drive. However using the car after the repair the light stayed off for about 5 minutes of driving and then it came on. I have refitted the original sensor and it makes no difference, 5 minutes OK and then the light comes on.

 

The fault code is still the same so I am no further forward. The hub was definitely faulty and I cannot believe that I have 2 faulty sensors.

 

Does anyone have any suggestions what to look at next? I have trawled this forum and the web in general but cannot find any more information that helps.

 

Also does anyone know how the system works, I cannot find any information about it online. It is different to the standard inductive pickup/toothed wheel design. The sensors are semiconductors (Hall effect?) and the hub uses a 'reluctor ring'. Measuring the sensors with a multimeter reveals that there is a diode drop in one direction (0.6V) and it is open circuit in the opposite direction. The control unit supplies 12V to the sensor and given that it is only a 2 wire connection if must be configured as an open collector. Waving a magnet past the sensor whilst monitoring the voltage from the sensor with it connected to the controller did not indicate any pulses. The reluctor wheel itself doesn't appear to be particularly magnetic so I am at a loss.

[adamal]

Really odd one, and as you say the sensors are both unlikely to be duff and in any case the fault code is more likely related to the ring itself, as you found.  I think the sensors look quite hardy actually.

 

The only thought given the fault code is still the same is did they actually clear the fault code - if not, maybe the continuing presence of the fault is causing the light to come back on?

 

As to how it works, I thought it was magnetic - given the sensors are covered in a plastic coating, I can't see any other way it could work. Perhaps the magnet is just in a little part of the ring so that as it passes the sensor it causes a pulse.

 

Adam

[xman]

I had an abs sensor go on my Alhambra - first intermittently, then solid. Replaced with an ebay sensor and all ok for 2 months then intermittent then solid.

 

Both times I check the sensor resistance and it was open circuit. New sensor again and so far so good.

 

The sensor is a coil pickup with a magnet in the sensor. I guess when the ring roates it induces mV levels in the coil which goes back to the abs ecu. I'm pretty certain the abs ecu tests for continuity at start up and then at regular intervals, probably when no signal is received. It will also fault if the lack of signal is inplausible, i.e. all the other wheels have been showing 30mph for the last minute or so.

 

Things to check:

 

is the ring actually rotating properly - sometimes they come loose and jam, also check the ring/hole has no debris attached, my independent left a load of steel filings from where he cleaned the hole out and then bunged full of some kind of grey grease.

 

check the continuity of the sensor (iirc around 1.1kohm), flex the cable around the sensor while doing this as I'm pretty certain this is where my replacement went duff.

 

Check the sensor is the correct type size and length and fitted correctly all the way down in the hole, otherwise signal maybe intermittent.

 

Check the connector, does it click together? Inspect the internals etc, make sure the connectors not pushed back or damaged. Check the wiring coming out of the connector and going back to the ecu - rodent damage?

 

Lastly if VCDS says right rear, I take it that means rear offside. Disconnect the sensor and rescan to check it comes up a solid fault on that wheel.

 

ABS fault light should be self resetting, when you restart the ignition and after you've driven a short distance although the intermittent fault code may still be logged and would need to be cleared.

Edited 27 March, 2015 by xman

[lee_vrs]

Has the relucter ring been pushed slightly off the hub due to corrosion seen it a few times

[KevinSt]

Thanks for the info folks. Here's a further update:

 

These sensors are not like standard inductive pick-ups, they do not measure about 1k ohm, resistively they measure completely open circuit in one direction and about 4M ohm with the probes the other way around.

 

The reluctor ring on the back of the hub seems to be very slightly magnetic, apart from the corroded section on the original hub both seem to be equally magnetic. Just for clarity, the original hub and reluctor ring were removed and both were replaced with new. The fault code was cleared after the new hub was fitted. The warning light stayed out but only because it was a short trip.

 

What seems to happen now is that the warning light stays out for the first 5 minutes of the first trip of the day. The light then stays on for subsequent trips of the day. This suggests that there is some link between engine temperature and the fault light. I am therefore wondering is there is some issue with the control unit that is located in the engine bay.

 

I am stuck as to what to do next. The evidence of the damaged reluctor ring suggested that changing that (as others on this forum have done) would resolved the problem.

[adamal]

It sounds like it needed to be changed anyway, perhaps it hadn't quite failed but was about to, and this problem is something else.

 

I wonder if the fault is similar to that experienced by softscoop (see his recent thread), which was essentially a loom failure.  It could be only apparent with a bit of expansion as wheels get hotter?  Might be worth asking him for more info as I believe it was easily fixed by an auto spark...

[softscoop]

Yeah mine was ns front loom. It had a break somewhere in the wheel arch. It had previously been repaired apparently.

Mine lit up as soon as the ignition was turned on and would instantly reappear when clearing the fault code.

Have you got Vcds? You can measure the speed readings from the sensors and see what they're showing if anything.

[adamal]

Being an intermittent fault this might be a loose connection, perhaps exacerbated by the heat, as opposed to an actual break. VCDS speed readings before and after the light comes on would potentially be informative as softscoop suggests.

[xman]

These sensors are not like standard inductive pick-ups, they do not measure about 1k ohm, resistively they measure completely open circuit in one direction and about 4M ohm with the probes the other way around.

Are you sure? They may be coils with clamping diodes in parallel to stop big volts being generated. If it was an open collector transistor, reverse breakdown is usually around 5 volts not 0.6 volts for silicon transistor.

I would check another sensor to compare.

Edited 31 March, 2015 by xman

[Gti Jazz Blue]

Also was it a genuine hub - there have been instances with pattern replacements causing issues.

[KevinSt]

Thanks for the additional info.

I have measured all the other sensors and they all measure the same. Perhaps the sensor output is more open drain rather than open collector. The 0.6V that I measured is probably the protection diode in parallel with the output MOSFET. If there is a coil then it is before the output semiconductor and you cannot measure it.

I am beginning to think that the cheap pattern hub that I had fitted is the problem. I think my cheapest course of action is to buy myself the correct torque wrench, spline but and a genuine hub.

I suspect that if I connected up a VCDS it would only show what we all suspect that is the speed pulses disappear when the light comes on.

[hatchy]

Got on and changed my offside rear hub today to hopefully sort out my ABS,ESP and TPMS lights coming on (and it did). Got the more expensive one (FAG) of the two that ECP sell and when I took the old OE one off that was a FAG unit too.

[KevinSt]

I've just found this excellent web site where you can buy just the reluctor ring:

 

http://www.ukpartsdirect.com/

 

This is how they describe the operation of the later type ABS sensors:

 

Q. How does an Active ABS Reluctor Ring work?

A. Active ABS Reluctor Rings work with Active ABS Sensors. Active ABS Sensors can be likened to the Hall Type Sensor (used in tachometers). 

The sensor is mounted in close proximity to the Acttive (Magnetic) ABS Reluctor Ring. The Active (Magnetic) type ABS Ring consists of a multi-pole (north / south / north / south …) magnetic ring. More and more vehicles are now being produced with integral ABS within the wheel bearing seal and use magnetic powder instead of fixed magnets.

The rotating, alternating, magnetic poles produce a magnetic flux within the sensor element which then amplifies and controls the signal for the ECU (vehicle's computer) to use as wheel speed information. The output of an Active sensor is digital with a square wave signal, being capable of sending wheel speed information down to 0mph. In contrast, the Passive sensor’s accuracy is usually dubious below 20mph.

[Tony1234]

I'm in exactly same boat with a mk1 fabia. Abs light and other one on, got scanner to tell me which wheel changed sensor and still no joy. Stripped wheel down again and cleaned hole of any crud and spun wheel to see the ring moving under the hole - cleared codes still no joy. Am poised now ready to buy a gen2 fitting tool and new bearing and change the bearing - which I really don't want to do but I can see any other way to test the circuits.

What can I measure to the sensor - voltage across the two connections with the sensor out?

:-)

[softscoop]

Are you sure it's not the cable?

 

Mine said it was electrical fault, changed the sensor and made no difference.

 

The cable had broken in the wheel arch.

[j0hn]

The fault code is still the same so I am no further forward. The hub was definitely faulty and I cannot believe that I have 2 faulty sensors.

 

 

What is the fault code? As far as I can tell there are fault codes that distinguish between electrical/open circuit and mechanical failures. I currently have a mechanical failure which is preventing me from doing the steering stop limit reset and having properly working steering. As it's mechanical, I am 99% sure that it's the reluctor ring on the hub and it was finished off by heavy rain in recent bad weather.

[Tony1234]

I'll check but not sure where it goes from the clip- front near side

:-)

[KevinSt]

Tony1234,

 

I've just come across this excellent video:

 

http://www.youtube.com/watch?v=UBNo_yg2gFI

 

The most interesting bit for me is how he makes electrical connection to the sensor with what he describes as a 'T pin'. It looks like a formed piece of wire that is thin enough to poke down the back of the weather seal on the wire in the back of the sensor. This would allow you to make voltage measurements with the ignition switched on whilst rotating the hub. One pin will be permanently at battery voltage the other with change between 2 states as the hub is rotated.

 

I have not tried this myself on my Octavia but I may have a go to confirm that it works.

 

When I looked at the reluctor ring through my sensor hole on my Octavia I could see the damaged section which is why I have changed the hub. If you want to check the loom then you could disconnect the sensor, connect a voltmeter to the loom contacts and switch on the ignition. You should see 12V across the terminals, if you flex the cable and the 12V comes and goes then you have a break in the cable. Ideally you need to load the contacts by placing a 1000 ohm resistor across the terminals whilst measuring the voltage.

 

You can make a very primative test of the sensor by switching your multimeter to diode test and you should get approximately 0.5-0.8V in one direction and open circuit in the other. This does not test the detector part of the sensor but tells you that the output circuit is connected to the terminals.

 

 

I am still waiting for my genuine hub to arrive so that I can see whether that fully fixes my problem.

 

Kevin

 

[Tony1234]

I'm loving that detAil. I'll have a go tomorrow - might even get a pack of resistors off fleabay

:-)

[KevinSt]

Good news!!! Fitting a genuine FAG hub appears to have finally fixed my original problem. Just for reference, the hub that I had fitted that did not work had the part number PLB23644 and had a 32mm centre to the bearing.

 

The following may be of interest to others:

 

1. I peeled back the rubber boot on the back of the sensor.
2. Using a pin, I pushed it down the back of the blue wire so that it made contact on the terminal inside the housing.
3. I connected a voltmeter between chassis and the pin. I used the rear door catch striker as a convenient chassis 0V connection.
4. Turning on the ignition the voltage read 11.25V.
5. Rotating the hub slowly the reading alternated between 11.55V and 11.25V.

 

I experimented with a sensor off the car connected to a 12V supply using a 1000 ohm pull-up resistor (it draws about 6mA). Placing either a magnet or a reluctor ring near the tip of the sensor I measured 5.0V and 3.3V as the polarity of the magnetism changed.

 

I haven't yet felt the urge to get out my iron filings or my plotting compass to count the number of poles on the original hub and compare it to the pattern part. I suspect that they are different and that the ABS controller eventually decided that one of the back wheels was rotating at a different speed to the other and so it dropped out bringing on the ABS warning light.

 

Kevin

[j0hn]

Kevin was it a hub or a hub/bearing combo?

Cheers

[KevinSt]

Yes it was a hub+bearing+reluctor ring+bolt+dust cap combo ;-)

 

This is the part:

 

http://www.eurocarparts.com/ecp/c/Skoda_Octavia_2.0_2005/p/car-parts/car-transmission/transmission-parts/wheel-bearing-and-wheel-hub/?628440300&1&1790f483f3f516df80886480bcd14f0e3eba1af6&000370

 

Kevin

[Tony1234]

Hmmm no 12v across the terminals when pulling the sensor connector off and the ignition on. Can't trace for continuity as don't know where the wires go once inside the wheel arch ( front left). Any clues (skoda fabia mk1)

:-)

[KevinSt]

Tony,

 

Are you sure that you have active type sensors? What if you measure one of the other wheels? If it is not an active type then the sensors should produce a voltage when the hub is rotated, unfortunately I suspect that you need to rotate the hub at a reasonable speed in order to measure the AC voltage across the terminals.

 

How to tell if you have active sensors:

If you measure the resistance between the sensor pins on a working wheel and it measures something like 1kohms then it is inductive. The tone ring on the hub is usually toothed.

If you measure the resistance between the sensor pins on a working wheel and it measures something like 1M ohms then it is active. The tone ring on the hub is usually smooth.

 

If you think it is active and you don't have 12V then check one of the other wheels. If that has 12V then more than likely you have a wiring fault. If you let me have the following details of your car I'll look at the workshop manual:

 

Year

Engine code

Gearbox code

 

I've had a quick look and the rear sensors appear to go to a yellow 9 way connector mounted at the 'front wall, left' which I guess is the engine bulkhead. The front sensors have no connector other than the main one going into the controller. The system appears to use active type sensors. There is a write-up of how to replace faulty ABS sensor cables but it is not easy to follow. Here is an extract:

 

–  Release plug connection on wheel speed sensor and separate plug connector. –  Unclip wheel speed sensor cable on rear axle and body. –  Remove the seat bench → Body work → Rep. gr.72. –  Remove trim panel at bottom of pillar C → Body Work → Rep. gr.70. –  Remove the entrance plate → Body Work → Rep. gr.70. –  Fold back the floor covering around the seat bench.

 

Kevin

[Tony1234]

It's front left.

Also it's not a toothed sensor.

I'm going to whip the other front wheel off this morning and see if I can get a voltage.

I'm pretty sure it's a break in the wire as when the fault first started the abs light and the esr light only came on after I braked over 10mph. Might even be time to treat myself to a Haynes manual.

It's a fabia by the way 6y2

1.9 tdi on a 52 plate that I've had from new

:-)