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 Post subject: Re: ABS Problem!
PostPosted: Mon Jun 07, 2010 8:51 pm 
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Joined: Thu Nov 15, 2007 12:55 pm
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Location: Taunton, Somerset
TIPEC membership: 3980
zola1 wrote:
try that test to eliminate the sensors and let us know how you get on. A BOB box to place in line to the ECU is great tool. Time eh :)
cheers

Yes, and a spare socket and plug to make up a BOB would be really useful!!

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'88 944 Turbo S, a few mods...


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 Post subject: Re: ABS Problem!
PostPosted: Tue Jun 08, 2010 9:31 am 
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Joined: Mon Nov 05, 2007 11:26 pm
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Location: Pickmere
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zola1 wrote:
i did some ABS work with FoMoCo and Aston's, we had some issues with high mileage cars on the front magnetic sensors, they have an o-ring at the top section of the metal casing of the sensor and water ingress used to form corrosion around the chamfer where the o-ring sits, this used to "lift" the sensor slightly and would cause a irregular output... the ECU then picks this up as wheel beginning to lock (even thought is not actually taking place) and you would get symptons like the 1).ABS flashing on or 2)a slight brake pedal vibration 3)ABS valve body chatter on application of the brake pedal.


Very interesting. My ABS light has been on since August 2008. I found that by jacking up the near-side front wheel and spinning it by hand I could make the ABS light come on - which I believe to be correct because the wheel is spinning +/- 6% of the other wheels speed (they are not moving). I then turn the ignition off and on to reset the light, jack up the off-side front wheel and do the same again with the same results. However I couldn't make this method work with the back wheels and found that the rear near-side sensor was u/s. I replaced it but still had the same fault with the light and couldn't make it come on by spinning the wheel. I then found that I also had a bad earth from the rear off-side sensor to the brake back plate. As the plate is physically broken I cannot repair it.

Any ideas please?

My other threads on this problem -

viewtopic.php?f=25&t=1628&hilit=+abs

viewtopic.php?f=25&t=4273&hilit=+abs

I'm off to Cyprus in the morning so don't think me rude if I don't reply for a week or so.

Thank you.

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1991 944S2, 222000 miles.
2007 Audi A4 Avant Tdi 140 S-Line
2014 Mercedes SLK 250 AMG
1995 Triumph TR6


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 Post subject: Re: ABS Problem!
PostPosted: Tue Jun 08, 2010 11:24 am 
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Location: Taunton, Somerset
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Tri-Man wrote:
I then found that I also had a bad earth from the rear off-side sensor to the brake back plate. As the plate is physically broken I cannot repair it.

Interesting... I noticed those earth leads on the back of the backplates - however the circuit diagram for the ABS sensors does not indicate any earths...!? So, anyone know whether the earths on the back plates (and those above the gearbox) are associated with the ABS??

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'88 944 Turbo S, a few mods...


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 Post subject: Re: ABS Problem!
PostPosted: Tue Jun 08, 2010 5:46 pm 
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Joined: Wed Nov 07, 2007 12:59 pm
Posts: 224
Location: Bedford
starboard147 wrote:
zola1 wrote:
try that test to eliminate the sensors and let us know how you get on. A BOB box to place in line to the ECU is great tool. Time eh :)
cheers

Yes, and a spare socket and plug to make up a BOB would be really useful!!


I had this mind for a while now, we need to source a header from a u/s ABS ecu and use the housing to mount 4mm banana type socket and use this to interrogate the wiring and i/o's.
This would be a great addition to the 944 diagnostic tool kit :D

To make the holy grai...l a dynamic BOB that has switches in-line to the circuits and power ok bi-colour LEDS etc oh for the time

i'll take a look at the circuits and get back to you, i think its the 0volts for the Low pad sensors

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 Post subject: Re: ABS Problem - updated
PostPosted: Wed Oct 06, 2010 8:29 pm 
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Location: Taunton, Somerset
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Recap: start engine, no warnings, start rolling then ABS warning comes on. Disconnect ABS control unit above driver's right knee and check resistance of sensors and all are 1k or thereabout.

Now have obtained a scope... jack up rear and run in 1st gear - photos of scope reading for RHS attached, and LHS looks identical. Obviously, a bit more difficult to do with the front however a vigorous spin gets a sine wave that also looks very similar (the peak to peak reading is definitely the same) for both front sensors. Sine wave is 2.5V peak to peak, 12.5 mS per cycle, which I make to be 80 Hz.

Slow shutter speed led to double trace.

Where next? Help...


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'88 944 Turbo S, a few mods...
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 Post subject: Re: ABS Problem!
PostPosted: Sun Oct 17, 2010 7:02 pm 
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Location: Taunton, Somerset
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bump

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