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?

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.

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.

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