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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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 Posted: Sat Jan 19, 2019 6:42 pm Post subject: Abs pulsing only at low speed 2mph-10mph
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Evening all!, basically have this issue with my 996 c4 1999 3.4 where as at low speed the abs pulses, bought it like this so no idea when it started nothing wrong with the brakes as such all tyres pretty good and matching and all tyre pressures correct, no warning lights or faults but this happends, has anyone successfully fixed this as upon a google search it seems a few people get this from time to time but nobody ever updates there threads as to how they’ve resolved it so it would be great to hear people’s solutions that have resolved them on here
Thanks ricky
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deMort
Zolder
Joined: 21 Mar 2015
Posts: 5945
Location: Brighton
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| Posted: Sat Jan 19, 2019 7:05 pm Post subject:
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It will more than likely be a wheel speed sensor thats getting old , low speed and the signal is not strong enough so the car thinks the wheel has locked up .. hence abs kicking in .
To find which one .. a fault code read out and hopefully there will be a code ..
If not then its a case of scopeing each sensor and looking for the one with the weakest signal.
You could try an ohms check on each sensor as well , youll be looking for the one thats different from the others but this is a bit of a hit an miss check.
Seperate note for any Cayenne owners .. never use an Ohm meter on these sensors .. it kills them .
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sat Jan 19, 2019 7:10 pm Post subject: .
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Great advice thank you!, I was thinking along that same path either speed sensor or the ring bit inside that the speed sensor uses, doesn’t seem to be any fault codes so I’ll try and test them see what there all reading and will update you guys either way, have you had this problem before on yours?, I guess there’s not much else it could be since there’s no warning lights etc and no other braking issues other than this at low speeds
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deMort
Zolder
Joined: 21 Mar 2015
Posts: 5945
Location: Brighton
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| Posted: Sat Jan 19, 2019 7:18 pm Post subject:
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I think the ring whilst posible might tend to give you problems at higher speeds .. have a look at each one though , your looking to see if they are heavily corroded ... and i mean VERY heavily corroded .
Off hand i would start with looking at the sensors .. some testers give you a value as the car moves so you would see x3 wheels at 5 mph and x1 at 0 if it is a sensor or a ring .
Ive not had it before on mine .. i dont actually own a Porsche im afraid .. im a Porsche Mechanic 
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sat Jan 19, 2019 7:34 pm Post subject:
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Agh I see I’m not sure if my friends diagnostics shows up individual wheel speeds x4 but I’ll look into it as that would be a much easier way to pin point the problematic corner for sure
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deMort
Zolder
Joined: 21 Mar 2015
Posts: 5945
Location: Brighton
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| Posted: Sat Jan 19, 2019 8:54 pm Post subject:
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Just for us nerds here ..
The sensor is a magnet with copper windings.
When you pass a soft iron metal over this it will generate a small voltage .. emf i belive (Electro Motive Force ).
The sensor will have a metal part then a gap then a metal part going in front of it .. this will in turn generate a voltage then no voltage then a voltage in the sensor .. an oscilateing signal basically .
The car sees this as a waveform and knows the rotation speed of the wheel due to the frequency of said wave form.
If something interupts this signal be it a poor / corroded ring the pattern will alter to a flat point at times , low speed and this is far more likely due to corroded highs and lows of the rings , corrosion will almost join two high points of the ring together .. or a poor sensor that cant generate a descent signal .
If you have a fast enough scope then you will see a square wave .. the reality is most scopes show an analog signal or a slope up / down .
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sat Jan 19, 2019 9:57 pm Post subject: .
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Thanks for that it’s quite an interesting little read I’m looking forward to getting started on it now and getting to the bottom of it now!, I’ll have it plugged in tomorrow see exactly what info we can extract from the diagnostics before I start stripping and inspecting the rings etc. Thanks for your help!
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HSC911
Long Beach
Joined: 23 Jul 2014
Posts: 6119
Location: Bedford
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| Posted: Sat Jan 19, 2019 11:18 pm Post subject:
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Don't forget to donate too 
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Griffter
Monza
Joined: 22 May 2016
Posts: 248
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| Posted: Sun Jan 20, 2019 7:47 am Post subject:
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I had exactly this on my wife’s rover years ago. It was a cracked reluctor (ABS) ring. My moneys on a senor or ring.
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sun Jan 20, 2019 3:03 pm Post subject:
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Yea I agree one or the other, don’t think will get chance to have it plugged in wile next weekend now but will certainly update you guys in the hope that it helps somone else too, can the abs ring bit be changed if it is that or is that part of the drive shaft of these?
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wasz
Sepang
Joined: 28 Dec 2012
Posts: 2791
1999 Porsche 996 Carrera 2
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deMort
Zolder
Joined: 21 Mar 2015
Posts: 5945
Location: Brighton
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| Posted: Sun Jan 20, 2019 8:47 pm Post subject:
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Im afraid the ring is part of the driveshaft .. you are looking for x2 ring high spots almost joined together .. you can scrape out the rust between the two .. that is if thats the fault .
Figure out which corner of the car is the problem first then go from there .
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Mon Jan 21, 2019 8:48 pm Post subject:
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Little bit of an update plugged it in today no faults stored, tried to get the values of all 4 speed sensors on durametric diagnostics and annoyingly I can’t seem to get it to show anything it doesn’t do anything, will check it out on my friends diagnostics at weekend
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sun Jan 27, 2019 4:11 pm Post subject:
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Bit of an update done a diagnostics and no faults, done a scan of the abs sensors and nothing flagging up as out of ordinary so guess it must be the abs ring either damaged or rusty/corroded
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JS1500
Monza
Joined: 12 Apr 2017
Posts: 196
Location: Aberdeen
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| Posted: Sun Jan 27, 2019 9:39 pm Post subject:
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I had this fault on my C4 and found a garage that could replace the reluctor rings without having to replace the driveshaft.
I actually have a couple of spare rings, as I bought four. If they're the same as the ones you need you can have them for the cost of postage...
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2000 996 C4 - Guards Red
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Sun Jan 27, 2019 9:56 pm Post subject:
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Great thanks that’s the best thing that I’ve heard as it does seem very plausible that being the fault but then I’ve not heard of anyone else having issues with the rings so worried I’m heading in the wrong direction, was your symptoms the same as in abs pulsing but only at very low speed?
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wasz
Sepang
Joined: 28 Dec 2012
Posts: 2791
1999 Porsche 996 Carrera 2
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| Posted: Mon Jan 28, 2019 11:04 am Post subject:
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| Rickyhull wrote: | | Bit of an update done a diagnostics and no faults, done a scan of the abs sensors and nothing flagging up as out of ordinary so guess it must be the abs ring either damaged or rusty/corroded |
Can this diagnostic log all 4 wheel speeds?
Then drive the car in a straight line and see which one is out compared to the others.
Then you can start to look for them.
When I fitted new front wheel bearings to my C2 Im sure the wheel bearing seal had something written on it to say it has the reluctor in. But yours being a C4 will be as DeMort says on the driveshafts.
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Rickyhull
Monza
Joined: 06 Mar 2016
Posts: 165
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| Posted: Mon Jan 28, 2019 12:30 pm Post subject:
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Unfortunately not on my friends although I have got durametric and it does show up all 4 wheel speed sensors but doesn’t give off speeds/voltages so maybe because it’s the free version I’m not sure 👎
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