Ignition troubleshooting
 

On my 78SC, ignition stopped working all of a sudden.
I took the cap off and noticed the rotor tip was broken, which explains the loss of ignition. It looks like rotor got slightly dislodged and hit the distributor inside, for some reason. I replaced the rotor and distributor, but still got no spark.
Now, I suspect that my coil burnt out because of the rotor failure. Having no rotor working would be like cranking the engine with the coil disconnected, which is known to burn them. Are my assumptions correct, has anyone else experienced something similar?

http://forums.pelicanparts.com/uploa...1580052627.jpg

I tested the resistances in the coil, 0.9 ohms for the primary and 5k ohms for the secondary, so that looks ok. It would have been an easy fix...

Do you have the Bentley manual? Check for a pulse from the distributor as described on page 280-9

This thread kinda covered it. http://forums.pelicanparts.com/porsche-911-technical-forum/334432-green-wire.html

Yeah, it could be the green wire again, but I replaced her a while ago...although that was like 15 years ago, how time flies...Thanks for the reminder on the green wire.

Type of CDI?

MSD streetfire with MSD coil blaster. Been working great for 5 years.

I found a neat test to check the MSD and the coil:

http://forums.pelicanparts.com/porsche-911-technical-forum/996192-quick-tip-msd-streetfire-users.html

Is it possible the MSD is cooking the resistor in the rotors?
Check the resistance from rotor tip to center to make sure.

It appears that the rotor tip has been over heated and may have failed similarly as the resistor due when using MSDs. I would also check the distributor play to rule out a mechanical failure from worn bearings.

The MSD WILL cook the resistor. Either use a VW rotor witn no resistor, or carefully remove the resistor with a dremel and solder in a solid copper wire and then pot it.

I have a Daytona-Sensors CDI and this is what I had to do. My rotor has been "rotoring" for 10 years with no problems.

I use the built in soft rev limitor instead of the centrifugal weight rotor rev limiter.

If only the resistor in the rotor got cooked, then the new rotor should have fixed the issue, at least temporarily. Something else is preventing the spark now. But I have good leads to investigate, thanks to this thread.

Update: I tested the spark at the coil and got an orange spark. Can a coil be weak? I also switched to resistor-less distributor, that did not help.
Next move: I could spend $45 on a new coil...

Final update: I put in a new coil, the spark was brighter, and the car started right away. In hindsight, the failure was the rotor resistor, so I went resistor less. And somehow the coil got weak too, but now all is fixed.