Stock tach wiring w/ rotary and dual msd?

[heywier427]

So a lot of info points to needing a msd tach adapter 8920 and wiring goes like this out of the msd adapter:

 

black = ground

 

violet = not used

 

white to tach output plug on msd box.

 

red to black/green at back of tach.

 

Above is how it is hooked up with elec dizzy on a L series.

 

 

My dilemma is I have 2 MSD boxes.  Will I be getting the correct signal to the tach only using 1 white wire off 1 msd box?

 

I see msd makes a duel msd jumper wire (which is just the 2 wires coming off each msd box and joining them into one).

 

Anyone know if I will need to do that?

 

Any reason why I can just run the wires off the stock tach to the coil and back like done in factory wiring?

 

When hooking up a regular tach in a rotary, you only take the signal off 1 coil (stock has 2 coils) and it doesn't matter which one.

 

Car does not yet run, so I cant do any testing on my own.

 

Just seeing if anyone has run into this question.

 

Thanks.

[70 goon]

All you need to do is tap into the lead coil and your good.

[shacks510]

MSD work different than factory ignition systems, so you can't hook a factory tach straight up to the coil and get accurate readings. I have a 6AL with a factory two wire tach (white loop) that I hooked up to the negative side of the coil just to see what would happen. The needle bounced around and wouldn't give an accurate reading. You'll need to get some kind of tach adapter that basically acts as a resistor for the tach to read true. I'd say get one from MSD so you know it'll work. The higher current can also kill the tach (so I've been told). Mines just for looks at the moment.

[heywier427]

Ok cool.

 

thanks guys.

 

Will get the tach adapter and only use the tach output on the leading spark msd.

 

I will post the results.