[Healeys] Fw: Possible explanation on the Petronix positive ground and Smith loop style Tach

[Bob Spidell]

Unless Pertronix has changed the Ignitor design in the 8-10 years since I installed mine, it will work with a Smiths electronic tach on a pos-ground car. Mine works fine and, in fact, since I calibrated it is absolutely spot on accurate and stable. Hard to believe, I know, but it is as verified by my Sears dwell-tach and by inference with the GPS/speedometer/gearing/etc. 

All I recall doing was reversing the loop formed by the white wire on the back of the tach, but I might have rerouted the white wire. Sorry, I don't have a schematic, but the thing to think about is the Ignitor switches battery current to the coil--you have to provide an alternative ground to the coil (I used the notorious battery cutoff switch and white/black coil ground wire moved to a different terminal on the switch)--whereas points make and break a circuit to ground from the coil. The tach is probably (has to be) wired in series with the coil and the points, so you may (probably) have to change the routing of the white wire. I did not remove any wires from my harness; just rerouted them so that I can easily go back to points in an emergency. 

The loop on the back of the tach is an inductive pickup. If switched current flows through the wire--with the proper polarity--the tach will work. The needle on the tach moves in proportion to the frequency of the current pulses (in effect, it is a simple frequency counter). Not that I doubt a reputable outfit like Nisonger, but the Ignitor I ('one') is hardly more than an electronic points set (with some inductive-capacitive circuitry to even out 'dwell'). 

Bob 



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Bob Spidell - San Jose, CA 

----- Original Message -----


A club member recently installed a Petronix ignition in his positive ground 
BJ8, 
Car fine s fine but the tachometer does not operate. This is of course 
because 
it is not connected. 


Several attempts to wire to operate the tach 
ahve proven unsuccessful. He has 
talked with Petronix tech support but they 
could not provide a definitive 
answer. I'm sure there are many positive 
ground BJ8's in the owrld that have 
properly functioning tachometers. What 
is the wiring??