I've got a Pertronix Ignitor III w/Flamethrower III coil on my 289. The engine is back in the car post-rebuild. From reading, it looks like I will not require the ballast resistor with the "III" system. And the wiring is simple--from coil to distro is just plus-to-plus and neg-to-neg. Assuming that is good, any recommendations on how the stock tach is wired into this? Thanks.
Tachometer wiring with Pertronix Ignitor III
- Thread starter KLtiger
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Austin Healer
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my experience is that you will have to send the tach off to be modified to work... Or you can buy one of the new tachs from sunbeamcarparts in the UK. These have a set of switches that allow the tach to be configured for several different ignition set-ups. They are identical in appearance to the original units and I was pleased with one I ordered.
While not an automotive electrician, looking at the wiring diagram for Mark1, 1a, It appears power to tach goes through the ballast resister, then to coil and gets signal through distributor points.. Would it work to run tach power through now unused ballast resister then to coil as before? Just a thought, and don't know if anyone would be willing to take a chance on messing up a tach. Happy to be corrected on this thought.
Austin Healer
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it'll work with, or without, the ballast. the reason for the ballast is to extend points life originally. Most electronic ignition systems are design to run a ballasted coil, eliminating the need for a separate ballast resistor. The aftermarket wiring (Ron Francis and the others) is not designed to support an induction type tach. Induction tach do not have a feed that actually connects to the tach. they have a hot lead (fused Green in color) that supplies voltage with the key on. The white wire from the coil routed through a metal "loop" that detects the square wave when the points open and close. In order to use a stop, unmodified, tachometer you would have to run a separate wire from the coil, through the loop and terminating at the ignition switch.
The other option is having the tach internally modified to be a "pulse" type tach and then it'll work with aftermarket wiring systems.
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The Pertronix and other points-replacement type ignitions have a longer dwell time than points, and that can mess up the current pulse triggering on stock tachs. The electronics module that I make (usually available through Tom Hall as well as from me) will properly operate using the current pulse transformer with stock or electronic ignitions, and it also has a voltage trigger input that allows it to be connected direct to the coil or to voltage tach triggers such as the MSD units have.