ABS light is on

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Without regard to what Chris is saying, if the ABS light is on, then you should consider the ABS feature to be inactive and your car may skid in a situation that would otherwise trigger an ABS event.
The only way to isolate the cause is through the diagnostic port. You could buy some speed sensors and swap them around until you find a combination of new andn old that work, but this is a more costly and time-consuming approach than paying somebody to plug into the diagnostic port.
The diagnostic port will identify the controller module as the fault, if it is the source of the problem.
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wrote:

Actually you can diagnose them with an oscilloscope or most decent digital multimeters. There's enough voltage output to drive the frequency meter on anything 1/2 decent. http://www.picoauto.com/waveforms/popular/wave86.html
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On Tue, 04 Jan 2011 19:13:32 +0000, Duncan Wood wrote:
[...]

Yep, even with a fairly basic DMM you can spot the most common fault of no output.
Chris
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You can. I could probably figure it out, but the OP thought the CHECK button on his OBC held the key to his troubles. Clearly he's gonna need help with this or he wouldn't be here.
If he could figure it out AND had an o-scope or a DVM, then his question might have been, how to I use my scope or meter to check a speed sensor?
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Chris Whelan wrote:

Oh? I've seen ABS-sensor "output" on a scope. I don't think most meters would register, at low wheel-speeds (think, jacking-up the wheel and spinning it by hand, or by belt-sander). Most DMM's are optimized for 60Hz AC.
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Dunno about with a DMM but I can see the E28 ABS sensor pulses with a Simpson 260. The needle bounces visibly.
I don't know if the newer systems have any different sensor outputs although I know they now have much more sophisticated actuators. --scott
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Not over here they aren't.
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Duncan Wood wrote:

Oh, you must live out in the boondocks. 8)
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No, but nobody optimises them for 60HZ, that would be plain wierd.
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Duncan Wood wrote:

You got me. Most are optimized for the 50-60Hz range, since that's all most people ever measure.
They will still "work" outside of that range, with accuracy dropping-off.
In the case of the ABS sensor, the accuracy is not important, of course.
But don't measure, say, your HiFi output with a 1KHz test tone and expect anything approaching accuracy.
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I've just tried this, unsurprisingly my flukes are just as accurate at 1kHz as 50Hz, but the crap one someones' left here rolls of from 500HZ quite slowly.
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Yup, and the front end electronics bandwidth isn't so hot.

My cheap B&K is about 20% off at 1KHz. It actually reads peak voltage and uses a fudge factor to get RMS, which is fine for sine waves but not so fine otherwise.
The Fluke 77 is right on the money, but then again the calibration specs certify it to be.
Surprisingly, my Simpson 260 seems to be accurate up to above 10 Khz, which I never would have believed if I hadn't tested it. --scott
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On Tue, 04 Jan 2011 21:30:10 -0600, dizzy wrote:
[...]

Your thoughts are inaccurate.

Really?
The aim here is to determine if a single sensor has a failure mode of zero output; there is no requirement to examine wave form, or to measure current levels.
Chris
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On Wed, 05 Jan 2011 09:33:24 -0000, Chris Whelan

Most of the time you can just use the resistance option and spot the odd one out.
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On Wed, 05 Jan 2011 09:41:21 +0000, Duncan Wood wrote:
[...]

Yep. A half-decent DIYer, or a clued-up local garage would be able to carry out this type of diagnosis easily.
It's certainly a better starting point than taking a 15-year-old BMW to a main dealer for diagnosis, as has been suggested...
(As an aside, a local VW dealer has recently put his prices up to 95UKP +VAT/hr!)
Chris
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much like they were saying:

£30/hr less than our local Audi dealer is reputed to be...
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On Wed, 05 Jan 2011 09:52:06 +0000, Adrian wrote:

Yep, and a BMW dealer is likely to be more expensive still.
The car industry seems to be trying to cover its losses in the recession by increasing repair and servicing costs; parts prices are going through the roof.
Any reliable non-franchised garage ought to be in a very strong position.
Chris
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Three years ago, my local BMW dealer wanted £130 + VAT per hour. Don't know what it is now.
No Thanks.
David
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Technically, the government wanted the VAT, not the dealer. Technically.
I'm just saying that you gotta put the taxes due into the right heap. The dealer is only the messenger.
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On 05/01/2011 20:33, Jeff Strickland wrote:

But, I still have to pay it.
Whereas businesses get to claim it back. Plus all the 'write downs' companies get against tax on company cars, whom the rest of us have to subsidise including pensioners unable to afford to properly heat their homes.
Rant over.
David
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