tach signal from coil with cdi

[gumba]

Can i get a tach signal from the coil to run a shift light?
I have a twin plug Bosch distributor with 2 coils and 2 cdi boxes on a 3.5 turbo motor.
I've tried a couple of shift lights T ing off the tach wire at the tach. The MSD shift light sort of works occasionally. The other box I tried didn't work at all.

[Jonny H]

No, the coil primary A terminal has a -400V pulse. You could try getting tbe RPM signal from the TD pin on back of the tacho which may be a more compatible voltage level.

P.s. Our CDI+ units have a programmable shift light output and rev limiters built in.

[gumba]

No luck coming off the back of the tach.
Your boxes would be great if I hadn't had my 2 boxes rebuilt a few years back.
Thanks for the reply.

[Big Dav]

I have one of these which I use to get a signal from a coil to a data logger. It works well.

https://www.ebay.com/itm/UNIVERSAL-TACH-ADAPTER-CONVERTER-COP-1-2-3-4-5-6-8-10-cyl-to-any-4-6-8-Tach/141931549970?hash=item210bc79512:g:YiQAAOSwwG9ZmOo p

[Trackrash]

Are they 6 pin CDIs? Have you tried the tac output on the CDI?

[gumba]

6 pin cdi's. the tach output works fine on the tach. I T'd off the tach line at the tach. The shift light is inconsistant.