distributor "green wire" output
 

In the course of diagnosing an ignition issue, I checked the distributor signal to the CDI (the "green wire") using an inexpensive tablet oscilloscope. I've never seen a post here with this information, so I thought this might be of general interest.

Here's the trace:
http://forums.pelicanparts.com/uploa...1670948018.png

This was obtained by connecting the probe and its ground to the two pins on the green wire connector and cranking the engine. I have a connector because I replaced the sensor and added a connector to the factory wire. The stock configuration has a coaxial wire direct from the sensor mounted in the distributor to the CDI plug.

Vehicle is 1983 911SC track car with stock CDI ignition.

I wish my heart looked that good and didn't show that leaky valve!

Not info I've ever seen before either - thanks for contributing to the public knowledge!

(Which tablet oscilloscope? I need to grab one myself and would rather not drag out a bench top unit to the garage.)

Yep and you have the signal the correct way up too! The CDI fires when the steeper negative going part of the wave passes through 0V. This point in the trace occurs when the rotor is aligned with the posts in the cap (give or take advance). The signal varies in amplitude from about 2v at idle to 50v depending on RPM.

Side note: You will find lots of threads about swapping the two wires round to solve 'bad' running. The reason people get so confused about the polarity of the signal is all MSD's fault! Their boxes fire on the rising 'edge' which is why you have to swap the wires round.

Side note 2: There is a myth that direction that you spin the distributor produces a different wave form. This is not true - the waveform is the same is either direction. This one is Porsche's fault. For some reason, Porsche wired the CW and CCW distributors differently - swapping the two signal pins around so the wave form appears upside down on one. I read somewhere that there used to be a green wire AND a blue wire to be used with each type so that the swap is resolved by using the correct wire. Two problems with that - one was the blue wire faded to almost the same shade as the green wire. Secondly, the blue wire is now NLA.

I feel the same way about my EKG. ;)

Tablet oscilloscope. Nothing high spec, but adequate for my purposes. They're sold under various brands - mine is a FNIRSI 1013D.

I got lucky on the connection, 50-50 chance. Thanks for the details and your previous comments on ignition systems.

As far as I’ve seen I think the blue wire was used on early Turbos (CW) and some early SC (ccw). If you replaced it with a green wire it’s possible the wires would be switched and the waveform would appear in reverse of what’s shown…I recently installed a new harness on a 77 turbo and expected to have issues with the green wire polarity but all was good on the scope