Using Tach w/out the CDI
 

I am trying to find out how to use the factory tachometer to monitor engine RPM without the use of the CD unit. Normally the tach connects to the CDI, and I would like to be able to use it while using another ignition source.

I know with VWs the tach connects to the coil, but this is for an aftermarket tachometer. I'm wondering if I can do the same by connecting the factory tach to an aftermarket coil with provision for a tach connection.

Please let me know what you think. Thanks, Bob (911s).

PS: I posted this thread in the Off Topic forum originally, then realized that it should have been posted here. Sorry.

The 911S tach gets its signal from the distributor.
It is just expedient to connect the tach wire to the same CD post (pin C) as the distributor input to the CD box.
If you are using an ignition system other than a CD, connect the tach to the output distributor wire. Pulses to ground will control the tach.

Thanks Doug, I'll check it out. I'll get back with you if I have more questions.

Bob (911s)

In my 75 w 3.0 engine, i hooked the tach wire (black with purple tracer) to the plus side of my coil, has been fine for years that way.

before you burn up your tach...

The CDI unit and factory tach use a sine wave, and new ignition modules use a square wave signal. they are not compatible.

When I installed a new optical pickup/electronic ignition, I had to have my tach's guts replaced with a new tach's guts. still looks like factory.

I do believe you can use a tach adapter to solve this problem, also.