Johnmor
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| posted on 20/4/06 at 12:48 PM |
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Speedo sensor
Things are becoming complicated in my attemps to use a speedo from an Alfa 156 in my Viento.
The speedo accepts a signale from the ABS system and then displays the average speed of all four wheels, I know that the speedo calibration is close
to my rquirements ( one rotation =1.93m).
And there is only one connection to the speedo from the AbS system.
How do ABS sensors work , are they similar in operation to magnetic sensors for electronic speedos,
Do they open and close in rapid response to the wheel movement .
With the speedo activated there is 12v from the speedo to signale input and if that is earthed raipidly the needle responds.
Is this a hopless case or can I make this work with a normale speedo pick up?
I know there are some electrical wizards out there, can you help?
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02GF74
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| posted on 20/4/06 at 01:19 PM |
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someone will correct me if I a mwrong but ABS sensors use a notched wheel, like crank position sensors and some electronic distributors; they ave a
sensor that detects the humps going past and this genereates a pulse.
So in theory you may need some simple electonics to amplify/reshape the pulse and can use with a speedo.
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britishtrident
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| posted on 20/4/06 at 01:20 PM |
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ABS transducers are always variable reluctance, Speedo transducers are usually either hall effect devices or at the low tech end reed switches.
The raw signal from a VR sensor is a low voltage ac wavefgorm see :
http://www.picotech.com/auto/waveforms/abs_generic.html
A hall effect sensor opereates like a switch and and goes high when the magenet passes the sensor see;
http://www.picotech.com/auto/waveforms/rss_sensor.html
With both the VR sensor and the Hall Effect you would need additinal circuitry to pull the speed signal line low.
A reed switch and magnet(s) (as used in bycycle speedo and police traffic car speed computers ) should work but before doing anything you would need
to know how many pulses per mile the speedo is calibrated for.
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Johnmor
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| posted on 20/4/06 at 04:35 PM |
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Thanks for the response.
The speedo is calibrated at one rotation of the drive shaft =1.93m, that is about correct for my wheels. How many pulses are produced per rotation is
another question, but I suppose I could use trail and error.
I would imagine the ABS ECU produces a signal similar to that produced by a Hall effect sensor, and passes this to the speedo, There is only one
wire connected from the ABS ECU to the speedo. If i can reproduce this by buying and fitting a sensor, I may have a chance.
I have sussed the rest of the wiring and can use the Rev counter and other clocks, they are pretty smart and more importantly "cheap" so I
hope i can sort this out.
Thanks
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