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DISCUSSION => Electrical => Topic started by: doitover on January 24, 2010, 05:36:42 AM

Title: speedo signal, what does it look like
Post by: doitover on January 24, 2010, 05:36:42 AM

Anyone know what the speedo signal from the rear axle speed sensor looks like?
Is it 0 to 12 Volts? Does it bounce on open and close?

Thanks, no access to an oscilloscope at the moment and haven't been able to find anything on it.

Title: speedo signal, what does it look like
Post by: DesktopDave on January 24, 2010, 08:49:42 AM

I found this site, seems to be a 0-8vdc signal:
http://www.unofficialbmw.com/e30/interior/e30_speedo_sender_problem.html

He noted some resistance readings as well, so I'm not sure.  You're right that there isn't a lot of info out there.

Why not jack up the rear of the car & spin the wheels slowly while having your DMM hooked up?  That'd give you a good reading...remember to post your results.

Title: speedo signal, what does it look like
Post by: doitover on January 24, 2010, 03:53:25 PM

I think I came across the answer. Knowing that the sensor is a reed switch, it is pretty unlikely that it is bounce free. I came across a circuit intended just for that application, a magnetically controlled reed switch. Just a 555 timer in one shot mode. Having spent so long in digital design I tend to forget the useful analog solutions to things.

Testing it in the driveway is a good suggestion. A multimeter wouldn't tell me what I need to know but I could have hooked a counter up to it, spun the wheel once and see if it caught 8 pulses or more.

Thanks,