VSS - Permanent Magnet style

[Ludis Langens]

One type of VSS, used with (analog style) electronic speedometers:

A small electric motor is geared to the transmission/transaxle output. 
This motor is used as a generator.  The output is a 4000 pulse-per-mile
AC sine wave.  I've heard the voltage can reach scary high values (one
hand behind the back, don't let it short to any computer chips.)

The VSS's AC output is connected to the instrument cluster.  One lead is
grounded.  The signal on the other lead is connected to a zero crossing
detector.  It outputs a 4000 ppm digital signal.  This is divided by two
to produce a 2000 ppm signal.  The instrument cluster outputs the 2000
ppm via an "open collector" driver.

The ECM provides a pullup to 12V for the OC signal.  This is then
inverted and translated to TTL levels and sent a VLSI chip.  This VLSI
chip measures the period of the 2000 ppm signal and makes it available
to software.  (That means that the duty cycle does not matter!)

...

There are at least two different instrument cluster circuit designs.  An
older design has the zero crossing detector built into a 16 pin DIP. 
This DIP also includes a power supply and speedo coil drivers.  The
divide by 2 is performed by a single chip microcontroller, also in a 16
pin DIP.  This microcontroller controls the speedometer.  The OC output
is an NPN transistor.  The power supply and driver DIP is marked 6394,
the uC DIP is 6393.

A newer design has an 8 pin DIP which includes the zero crossing
detector, divide by two, and OC output drivers.  It looks like this chip
has two 2000 ppm outputs and one 4000 ppm output (which goes to the same
microcontroller as the older design.)  The 8 pin DIP is marked 6758.

1986 and 1987 seems to be transition years where both designs were used.

...

In one group of ECMs (1226869, '6870, '7170, '7302), the
buffering/inverting function is performed by one unit of an LM258/LM358
opamp.  Resistors keep the output in a 0 to 5 volt range and provide
lots of input hysteresis.

A different group of ECMs (1226156, '6864) use a buffer that usually is
used for the MAF (frequency type) signal.

...

There are other types of VSS sensors.  This PM type was used for much of
the '80's.  The 6758 chip should be quite useful for hacks that use a PM
type sensor.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/