Rear of wrong Bosch alternator to run a diesel tach
This is the Bosch alternator with "W" Terminal at
the lower corner of the regulatorthis alternator is used on the 1984 Jetta diesel and outputs a square wave pulse of 10V and varying dwell duplicating the pulse of an Ignition coil
VW uses this signal to run the upshift indicator on Rabbit and Jetta models according to the wiring schematic in my Haynes manual this wire also drives a tach
on the 83 on Jetta models.
I Literally scoped out the "W" signal on My Rabbit and a good friends Suburban Tach output from the HEI to compare them...
The VW has an excellent square wave pulse with varying width and fixed amplitude (10 volts of clean signal).
The HEI signal rings from the 50kv spark energy coupling with the tach wire but is otherwise much the same type of signal
Ill probably have to build a small circuit to back down the frequency as the vw has a large
# of pulses per rev (more than the 350 Chevy!!).. Ill not be hacking apart a perfectly good tach unless I have to....