ABS light comes on when hitting cats eyes - Alfa Romeo Forum
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(Post Link) post #1 of 6 Old 23-06-09 Thread Starter
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ABS light comes on when hitting cats eyes


I've had an intermittent fault with my ABS light coming on in my 156 sportwagon for the last two years which has now become very frequent. After a lot of theories and a changed front N/S sensor I have been able to prove that the light comes on when I have built up some speed and I hit a cats eye with my O/S wheel.

The garage had connected their diagnostics to the car when the light was on and it stated an sensor air gap problem and a fault with the teeth wheel (can't remember the name) but didn't state which wheel the problem was with (hence me guessing, incorrectly, the N/S wheel).

The garage are now saying that I should change the O/S sensor and if this doesn't fix it then start with other components. Before I start down this expensive route I wondered if any of you guys could apply a bit of your technical knowledge to give me a better chance of doing the right thing.

Hoping you can help.

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The O/S Wheel is where I would have started, with it being where the cats eyes are.

All the garage had to do is unplug the sensors 1 by 1 till they got the same code, basic diagnostics really.
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(Post Link) post #3 of 6 Old 23-07-09 Thread Starter
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I've now had both front speed sensors changed and still no better. The problem is now also happening when the car is travelling at speed (70 mph+) or under sharp breaking. I have also noticed that the abs kicks in much earlier than one would expect.

As I was fed up with paying 200 for sensors I didn't need I went to an alfa dealer. They gave me the same explanation as the indie, then 'scoped' out the front wheels and changed me 70 but still no explanation. They did however give me the print out from the diagnostics:

"Incorrect number of teeth on phonic wheel - Intermittent". With a little searching I have found that errors in the speed sensors will always be reported with the wheel identified whereas the phonic wheel error does not indicate the wheel. So that's 200 wasted on new speed sensors!

Anyway, I'm now in the position that I don't know what to do next. I can get the ABS ECU checked out by a local specialist firm or the alfa dealer has quoted me 500 for replacing both O/S phonic wheels (apparently the rear phonic wheel also contains the speed sensor!). They will do this individually so if the O/S rear wheel is the problem then it's "only" 350!

I'm really confused about what to do next, I could be looking at 1000 for all four phonic wheels and still not fixing the problem.

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are you tyres the same width & profile?

same manufacturer?

different sized tyres will effect how the abs works

also rem you have 2 sensors on the rear wheels too, again they need to be the same width & profile as the fronts.

on the rear hubs there are 2 magnatised discs that rear sensors pick up a signal from.

afaik, on the front the sensor picks up the ball bearings within the front bearings [ I've yet to find a magnatised disc at the front ]

really you need your own diagnsotics to get ove rthis final hurdle

* laptop

* interface [ which I can sell to you ]

* KWP71 Software [ FREE ]
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They are magnetic Sears as the balls in a bearing are not fixed to the wheel or hub therefore a velocity calculation would be impossible.... but to help this man I would check for play on the wheelbearings seeing as the problem is happening with cornering forces...
(Post Link) post #6 of 6 Old 01-10-09 Thread Starter
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It's fixed!!!!! But the total cost has now come to 1060

It turned out to be the N/S rear phonic wheel giving intermittent reading. The alfa garage tech specialist connected his laptop so that he could scope out the ECU signal from inside the car whilst travelling at 80 mph. He had done this for the front two wheels in the garage however he had to do this from the car for the rears.

This was the last chance for the car, I had incurred the following costs up to this point:

1. Both front speed sensors - 200 - Don't change these unless the ECU error code says change them
2. Scoping of front wheels - 60
3. Both O/S wheel bearings (include the phonic wheels and the rears have the speed senor integrated) - 500 - done on a 'hunch' by the garage
4. ECU test - 50

Once they scoped out the rear wheels (see attached images) and found the problem (see the dip in the middle of the N/SRear image) I had to pay 250 for the N/S wheel bearing. Fortunately changing this wheel bearing also cured the ABS kicking in too early when breaking hard.

Along the way I have gone through disconnecting the front speed sensors in turn and seeing if the handbrake warning light came on in addition to the ABS warning light (this happens when the second error is registered by the ECU). Note that disconnecting the front O/S speed sensor meant that a second error was not registered - it must be linked to the N/S rear some way. As a way of working around the car excluding each wheel in turn to find the problem wheel I had also tried connecting one speed sensor to both ECU inputs however the ECU spotted this.

I hope this post helps anyone else with this error to determine the problem without having to spend a grand like me!!!
Attached Images
File Type: jpg NSRear.jpg (36.9 KB, 10 views)
File Type: jpg OSRear.jpg (37.7 KB, 5 views)
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abs , cats , eyes , hitting , light

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