cyberdyne speed calibration box

[Shannen Durphey]

Joe Boucher wrote:

> Shannen Durphey wrote:
>
> > 4) modern speed signals, specifically 700R4, 4l60E, are AC, 40 pulses per
> > revolution of the driveline.
> >
> > 4.5) to find out how many ppm are sent to signal buffer, by 40 tooth sensor,
> > divide 5280 by tire circumference in feet, multiply by rear axle ratio, and
> > multiply by 40.  A car with 29" tires and 3.72:1 axle produces 103,483.3
> > pulses per mile.
>
> This info and calculation are correct, but the in '89 TBI system I
> installed in my Suburban, the 103,483.3 pulses are fed into the back of
> the speedometer housing where a circuit card converts it into the 4000
> pulse per mile, 12 volt signal.  I didn't have the speedo housing and
> was getting a no VSS code and no torque converter lockup. When hooked up
> to a digital VOM the tranny output would show a rising AC voltage signal
> as speed increased.  The speedo housing, VSS output to the ecm on a VOM
> would quickly jump up to 6 volts and stay there.  I was told this was an
> indication of a digital, on/off 12 volt signal.
>
> Joe Boucher
> '70 RS/SS Camaro  '81 TBI Suburban

  Yes.  The analogue signal is converted to digital.  What you are using to do the
job would be done by the DRAC in the later TBI pickups w/o speedo cable.  If you
have a DVOM capable of showing Hz, you should see 67Hz at 60 mph at the ECM when
all is correct.

Shannen