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[Rich Vandervoort]

>>From:          "Rich Vandervoort" <richvandervoort at hotmail.com>
>
>>Here's how we used to get wheels speeds from competitors ABS systems.  Tap
>>into the wheel speed signal at the ABS controller.  Note use twisted pair
>>wiring with a grounded shield, do not route near spark plug wires or 
>>similar
>>noise sources.  The consequence of noise on wheel speeds can be no brakes.

OK, How???.  No way would a possible failed speed sensor disable the entire
braking system, maybe no abs, but that could happen anyway
Bruce

The ac noise can look like a real wheel speed to the ABS controller.  
Reference velocity is then artificially too high.  The real wheel speeds 
since they are below the high reference cause pressure dumps at  their 
wheels.  If you are real unlucky the noise gets coupled to a rear wheel.  
The rear wheels are usually hydraulically controlled together using the 
lower of the two wheels speeds as a control variable.  Therefore it is 
possible that even the wheel that has the noise on its wheels speed gets a 
full dump because its axle mate is supplying the wheel speed the entire axle 
is controlled by.  This isn't just theory I know of a vehicle that couldn't 
stop and drove through a garage door because of wheel speed noise.

Rich V.

>
>UUH...OK that cans that idea for me. in fact heres an official "don't
>try this at home" stamp.  THUMP ;-)
>
>>Just use an off the shelf F to V converter (frequency to voltage) and feed
>>the resulting analog signal into something that can use to record the 
>>signal
>>versus time.  Calibrate a voltage to speed conversion using steady speed 
>>and
>>some reliable speed measurement device(speedo?).  Note use an undriven 
>>wheel
>>and the signal will be extremely accurate as there are usually something
>>like 50 teeth on a speed ring.  Note your wheel speed tap may put noise on
>>the speed signal if you do not match the polarity of the F to V and the 
>>ABS
>>controller.  Also the newer ABS systems have quite sophisticated wheel 
>>speed
>>diagnostic circuits and may disable ABS once the F to V is hooked up.  We
>>used to buffer the signal, but I am unsure of the circuitry.


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