Classic Audi Technical Mech/Tech Suspension & Brakes » ABS operation

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Old 18-05-19, 08:44 PM   #11
tractor Dave
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Could it be the coolant after run pump you can hear?
I don't think any pump should be running on the abs when not activated. Try disconnecting the abs completely by removing the fuse or control box and see if that stops the noise.
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Old 18-05-19, 08:58 PM   #12
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Quote:
Originally Posted by tractor Dave View Post
Could it be the coolant after run pump you can hear?
I don't think any pump should be running on the abs when not activated. Try disconnecting the abs completely by removing the fuse or control box and see if that stops the noise.
Too noisy for the after run pump.

If I switch ABS off, I get no noise so it does seem to be the ABS. I suppose I could get it tested then figure this out after.
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Old 18-05-19, 09:14 PM   #13
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I've been searching the S2 forum for you and similar issues have been faulty bomb or the servo. A conversion to Vac brakes is recommended and ditch the bomb! Not sure if you have sufficient space though. Alex on the S2 forum (error404) does a very good kit and will advise if you decide to go that route.
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Old 18-05-19, 09:24 PM   #14
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Or oil pressure switch at 23
https://audi.7zap.com/en/rdw/audi+qu...6/6/612-53005/

Also check you have good earth on the ABS pump (underneath on mine)
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Old 18-05-19, 10:31 PM   #15
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if it's only happening at really slow speeds as you are coming to a stop, it's the ABS modulator making the noise.
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Old 19-05-19, 08:31 AM   #16
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I've been thinking more about this, in conjunction with comments and Thai site:
https://www.knowyourparts.com/techni...ator-problems/

I'm tending now towards sensor issues at low speeds making system think that a wheel is locked. The noise could be the valves opening and closing.
EDIT(AS per Mike above-ABS modulator)

At faster speeds the sensor outputs are more stable, so less likely to cause an issue.

Of course it's difficult to drive around in the private roads here at 40mph to confirm this, so I'll have to take a look at the rear sender teeth and the connections before trying to remove and clean the sensors themselves (last option!)

Off to Duxford now, so more in the week.

Edit:I'll test the bomb first because the first check is simple.
The interesting thing is the noise happens whether I'm braking or not, which (I think) only makes sense if the system is getting a bad signal from the sensors.
Generally, the noise happens when braking or not, but certainly more often when braking.
The noise doesn't seem to happen at higher (30mph) speeds, but does happen more often at lower speeds (less than 10), and also more often when braking-in particular braking to a stop.

Last edited by BackintheFold; 20-05-19 at 09:30 AM.
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Old 19-05-19, 10:38 AM   #17
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I get a similar effect at low speed with the V8, when coming to a halt, and especially when turning.
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Old 10-09-19, 11:51 AM   #18
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I've bought a 20 oscilloscope from eBay, so i can check the signals coming from each sensor-the re is NO WAY i'm prepared to remove any of them (and risk breaking them) until I know that that specific sensor is a problem!!
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Old 14-09-20, 09:00 AM   #19
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Re-starting this thread.
My rear left sensor is giving a bad signal, so will check it out before too long-hopefully by end of September.
The waveform showing on my oscilloscope is a square (ish) wave from three sensors, and a perfect sine wave from one.

I'm aiming my sights at the sine wave, on the basis that I have read that the output should be square on this system. I may remove the driveshaft to clean the ring, or i may try to remove the sensor and clean the ring through the hole, then re-set the sensor. I don't want to disturb the tracking so will have to see if i can remove the driveshaft without disturbing it, then decide what to do.
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Last edited by BackintheFold; 14-09-20 at 10:22 AM.
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