Not sure I get what you mean by the three ohm coil will not drive the tach.

I am using a 3 ohm coil and my tach works

If you are still using the stock tach with a 3 ohm coil and ballast resistor, my experience was that the tach would work up to about 3000 rpm and then cut out. If I bypassed the ballast resistor it would make it to about 4000 rpms and cut out. I thought you had indicated the same symptoms in your post. After putting in the 1.5 ohm coil and leaving the original ballast resistor in the tach worked at least up to red line.

YOu know I will check that. My tach usually works, and yes, at times, it does act wonky, but it is not consistent. It is always in the upper RPM band though, I will give you that. But it is not predictable at all. I can sometimes rev to 5000 rpm with no issues, other times, I can rev to 6500 rpm with no issues. Every once in a while it acts crazy at a high rpm.

But just for kicks, I will order a 1.5 Ohm coil and see what happens. Cannot possibly hurt.

YOu know I will check that. My tach usually works, and yes, at times, it does act wonky, but it is not consistent. It is always in the upper RPM band though, I will give you that. But it is not predictable at all. I can sometimes rev to 5000 rpm with no issues, other times, I can rev to 6500 rpm with no issues. Every once in a while it acts crazy at a high rpm.

But just for kicks, I will order a 1.5 Ohm coil and see what happens. Cannot possibly hurt.

I have it wired this way with a 3.0 Pertronix Coil and Ignitor in my 240z. Still waiting for someone in the fórum to tell me if it is wired wrong or not. I am having low spark and rough idle problems at traffic jams.

Bump as I am facing the same tach issues above.  Hooked it up exactly as shown here: http://www.zhome.com/ZCMnL/tech/ZXPertronix.htm The ballast resistor is in place still and the green/white wire still connected to the resisted side, but the unresisted side has both the black/white factory wire plus the Pertronix Red Wire.

I suppose I can place all 3 wires on the unresisted side effectively removing the ballast resistor altogether... hopefully that doesn't damage the tach?  Who knows if the tach will work then, but based on the discussion here it may not.

My tach doesn't make it above 3-4k before going nuts.  I have the 3 ohm resistor.  Looks like i'll be trying the 1.5 ohm and wiring back in the Ballast Resistor?   Any reason why the 1.5 wouldn't be as good as the 3 ohm?  

If I recall the 240Z circuit right they have the a resistor before the tach and the coil is after the tach.  Seems a weird way to lock the tach and coil together in to one circuit. If I had a 240Z and had this problem I think that I'd run a separate power wire to the coil, in parallel with the tach, and then add a potentiometer (rheostat, whatever) to the tach circuit.  Use a resistor or the coil itself to determine overall coil circuit resistance, and the potentiometer to control the current through the tach.  Seems like the main problem is getting the right amount of current through the tach.  I don't know if the problems described are too much or too little.  If it's too little then this wouldn't work.

SteveJ, or CO, or Mark M or Dave WM or one of the other guys that thinks about electrons alot might have some thoughts on it.