I've just had my second coil failure on my SC, each with very few miles on the coil. The first was a new silver Bosch unit, which I assumed at the time was the culprit. Now the second failure, yesterday, was a Parts Klassic unit. I got an interesting response from Parts Klassic about this:
Bosch has discontinued the correct ignition rotor for the 911SC and 930 models as February 2016. Bosch is now suppling the 930.602.902.01 rotor as a replacement rotor for these cars. The issue is the replacement rotor is the one used in the 1984-89 Porsche 3.2L Carrera that no longer uses a CDI ignition system. The rotor used in that system only has a 1000 Ohm resistor installed in it.
Another problem with the 911SC and 930 models is that the current replacement ignition wires supplied by Beru have a fitment issue. The 1K resistor on the connector that is supposed to plug into the cap has a nominal size of 8 mm. This is now too big to fit into the distributor cap. This results in the wire not contacting the brass connector in the cap, and the spark energy must jump from the cap to the plug wire. This in turn results in energy ringing back through the transformer and into the CDI box. Both issues if not corrected can and will result in transformer and / or CDI box failure.
To avoid ignition system failures:
1. Maintain factory correct resistance values, including correct location of resistance
2. Check for correct contact of secondary wiring to prevent spark jumps within the system
3. Ensure replacement components comply with factory specifications and fit for secondary resistance
4. When building/servicing twin plug vehicles check for correct resistance in the secondary system
5. When using non-resistance rotors, make sure to add resistance to the coil wire
I use Clewett wires so I wasn't worried about the second part, but the rotor resistance comment did ring a bell, as I'm pretty sure I replaced my rotor around 2016. I inquired about #5, and here was their response:
If you or your mechanic check the resistance on your rotor, you may find that it’s 0 to 1K ohms even though the factory part was about 5K ohms. Because of this, we sell a 5K resistor that attaches to the cap-coil wire. We also offer a cap-coil wire with the 5K resistor already installed. Here are the links:
https://www.************.com/p-3363-ignition-wire-connector-5k-resistance.aspx
https://www.************.com/p-3591-coil-wire-with-5k-connector.aspx
I measured my resistance and it is indeed 1k. I've ordered the 5K wire of course.