Just did a Pertronix ignition swap on a 72 automatic trans. It had the dual points dizzy but I swapped out with a single point before doing ignition swap. Engine runs fantastic, but I've lost the tach somewhere in this . The auto has additional wires that might be part of my issue. I did away with the ballast and tied those wires together . They seem to have no bearing on whether the engine runs or the tach works- which kind of confused me. The autos also had this relay set up for the secondary points, not sure if they are tied in somehow to the tach . Any insight would be appreciated

You can't wire the ballast back in, to check?  Should be simple to do and not hurt anything.  Maybe you have too much current through the tach circuit.  If so, a parallel circuit might get it back working but still get high current through the ignition circuit.

Train driver CO or Steve J might know for sure, among others.

My understanding is that the 3ohm coil eliminates the need for the resistor. I could wire it back in but have doubts that's the issue

Didn't know you were using the 3 ohm coil.  There's a newer version that doesn't need it, the Ignitor II, that is more of a high energy system and would pass more current through the tach circuit.  Sounds like there might be other wires involved?  Auto, dual point, relay?  Maybe it's more than just a ballast bypass.?  Does the tach do anything at all that indicates there's power running through it?  Any twitching?  

 

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Certain combos caused some twitching of the tach but usually the engine wouldn't start . I can wire a complete MS 3 unit in my car but this is messing with me. 

 

 

   

When I put my Pertronix in last spring I got the 2 black/white stripe wires mixed up, switched them around and the tach worked... up until 4000 rpm

The diagram shows a G/W and a B/W.  But, either way, that kind of shows that current flow has an effect.  If you had them switched from correct then you would have had more current than design, since the resistor is not used for Starting.  You probably had Start switched with Run.  Too much current kept your tach from working.  Maybe.

My 72 tach works good following this diagram.  If this helps.

Of course I am using the 1.5 ohm coil with ballast resistor.

This is my install result according to the diagram.

I wanted to do it this way so the tach got the same signal as origional so no issues would develop.

Lumens ,
I like this diagram since it puts the tach back in series with the coil. I will try it


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Well that didn't work. Lumens-!was this an automatic trans car?


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If I was stuck where you're at, I'd put a potentiometer in the circuit and throttle the current until the tach started working or a misfire started.  A cheap test.  If it works then you'll know it's a current problem and you can devise a way to get the tach what it needs.  If it doesn't you haven't lost much.