Circuits for magnetic pickups

[Donald Whisnant]

> 
> From: "Oliver Scholz" <SOZ at iis.fhg.de>
> Date: Thu, 1 Aug 1996 09:24:35 +0200
> Subject: Re: Circuits for magnetic pickups
> 
> Hi all,
> 
> regarding the recent discussion of magnetic pickups I have a question:
> 
> On older transmissions GM used a PM generator as the Vehicle Speed Sensor.
> It outputs a sine wave and about 4000 pulses per mile.
> 
> The later model transmissions (it is Getrags I am particularly interested
> in) they use a hall sensor of some kind that is triggered by a slotted
> disc on the axle. That signal is run to the ECM/PCM which drives the
> speedo at the usual 4000ppm.
> 
> Now, I would like to use the new transmission on an older car, so I
> would have to run the Hall sensor signal into a microprocessor that
> generates the proper speed signal from it.
> 
> Now the question: Does anybody know the waveform of the signal?
> Is it sine, square, ??? Which circuit would be recommended to buffer it?
> Also, how many pulses does it generate per revolution of the drive wheel?
> 
> Any help would be appreciated!
> 
> Best regards,
> 
> - -Oliver
> 


:)  I've recently had a tremendous amount of experience with this
application, as I'm presently building a digital speedometer to go on
my '85 Grand Prix for the '94 4L60E transmission I just put in (which
has an electronic pickup -- versus the old mechanical gear/cable)...  This
is the same sensor used on 93-95 applications (and maybe even before that).
It is basically a coil and magnet that generates an "AC" waveform that
corresponds to the teeth on the output gear...  These ECM/PCM systems
typically used 2000 pulses per mile...  The newer ones, as you mention,
have switched to a hall-effect in place of the coil assembly (which 
eliminates not only an expensive [by comparison] coil but a buffer like
the LM1815 or HIP9020, etc)...  For higher resolution, these ECM/PCM
systems have gone to using 4000 pulses per mile...  HOWEVER, I don't 
think <-(key word here) they have changed the number of teeth on the output
gear (if someone knows otherwise, please tell us) -- but on all the ones
I've encountered, they have 40 teeth ...  So for my circuit, with my
tire size of 235/60R15 and diff ratio of 2.41 and given that you get
40 pulses (teeth) per revolution, I had to make a divider of 37.25 to
get the 2000 ppm that my PCM needed (and it works very well) ....

The signal the PCM, speedo, etc, wants to see is a 0 to 5v square wave.
These are driven by OPEN COLLECTOR outputs in the divider circuit with
pull-up resistors (the PCM has an internal pull-up to 5 volts) ...  With
the Hall-Effect, if it is like most hall effects, it already has an
open-collector output -- SO, some of these PCMs don't even use an external
divider (my 95 Formula Firebird is this way) -- it is all done inside
the PCM ...  However, on other vehicles, they still have the external
divider...

The typical hall-effect output looks like this:


               _____ Output (open collector)
             |/
    H-E  ----|
             |\>____GND

Most hall-effects require a +5v power source, so typically 3 or 4 leads
are used (+5V, Gnd, open-collector signal out, and optionally an extra
"signal ground") ...  This can go straight to the PCM (if the PCM has
the internal dividers to handle the ratio)...

On the older ones with the "generators", a buffer like the LM1815 and
or HIP9020 is needed...  There are only 2 wires from the sensor on
these (signal and ground).  Typically with this setup, they just make
the "buffer circuit" into a "divider circuit" as well to handle the
ratio conversions...  (so the pcm doesn't have to know the tire
sizes and gear ratios) ...

And to complete the list, the older systems with the mechanical cable
typically has an output gear that turns the "intermediate cable" -- The
intermediate cable goes through a small gear box that via gear ratios
does the division as needed for tire size, etc -- the output cable
(that goes to the speedometer) turns at 1000 rotations per mile.  On
the back of the speedometer is a metal surface that has two "holes"
evenly spaced...  An LED interruptor is used for the ECM/PCM to pick
up the speedometer cable turn... Since there are 2 holes and the cable
turns at 1000 rot per mi, the result is 2000 pulses per mile -- with
the output of the interruptor being the open-collector NPN driver
transistor that all of the other setups use....

Hope this helps...  If you need to make a "digital divider" circuit
and don't know how to handle the fractions (assuming you want to -- I
don't think GM even bothers with the fractions), let me know .. I've
got several different circuits (both stand alone and cpu controlled)
that can do divides down as far as 1/256 ...

Donald Whisnant
dewhisna at ix.netcom.com