If you have ever turned the ignition key on your SC only to find your engine not coming to life, then read on as this could be one the cause of the issue you are facing.



The ignition system in 911 SC models is called Capacitor Discharge Ignition (CDI).
The distributor generates a voltage pulse which travels to the CDI module via a green wire that comes out of the back of the distributor. The CDI amplifies the pulse to around 450 volts before sending it to the coil where it is transformed to around 20,000 volts. One reason why I keep my hands well away from the distributor when the engine is running !!!

The green wire (number 9 on the drawing) is shielded and this can break down over time shorting the voltage pulse to earth. This is usually due to the heat cycling of the engine causing the plastic insulation of the green wire to go brittle. The plastic cracks causing the wire to have no strain relief and eventually failing either by shorting out or open circuit. The result is no spark.




Our Assistant Register Roy Saberton had this happen a few years ago when the engine would not start and he traced his problem to this green wire.

Obviously, no spark could also mean faulty CDI or coil.

To check out the green wire, disconnect it at the plug near the chain case and test for continuity between the two contacts. If you find continuity then the wire must be replaced.



You can also test it by checking the resistance between pins 7 and 31d on the 6 pin plug. Should be 600 or there abouts when connected to the dizzy.
Give the wire a good wiggle and if the reading doesn’t change the wire is good.

Replacing the green wire involves removing the distributor.

Before removing the distributor set the crankshaft pulley to TDC and check which ignition lead the rotor arm is pointing to (either cylinder #1 or #4); the rotor should now point to the notch on the distributor base.
Mark where the clamp is positioned before removing the nut and pull out the distributor.

Note that the rotor will rotate as the distributor is removed.

The green wire can now be detached.

Connect a new wire and replace the distributor checking the rotor arm points to the same ignition lead, fit and tighten the nut and connect the new green wire to the harness plug and you should be good to go.

One word of advice. It is worth checking your wiring harness before ordering a new green wire. Most SC will have the wires coming as one piece from the CDI box to the distributor with no connector in between to plug the green wire.

If you do have the plug on yours, then you can order a green wire from your OPC, reference 93060290701 which is a short green wire (128mm long).

If you do not have a plug, then you can buy this same short wire which will need to be modified (see below) or the Porsche 928 version of the green wire which is longer (reference 92860290700 – 700mm long) and onto which you can crimp a couple of electrical connectors to connect to the CDI box.




Modifying the green wire connector :

1. You will need a Timer junior connector kit or similar :

2. Cut the existing coax cable so that the total length to the distributor will be the same with the new green wire segment connected (you are replacing the last 6 inches)
3. Carefully remove about one inch of the green coax outer casing, exposing the braided shield.
4. Bend the coax at the edge of the removed outer casing point, and carefully open the braid a little, push/feed the bend of the centre green wire through it (fish it our of the braid)
5. Now you should twist the braided shield to look like a bare wire. The cable end should now look like it has 2 wires coming out of it.
6. Fit some small heat shrink over the shielded cable leaving about 1/8^th of exposed conductor at the very end. Remove about the same amount of insulation from the inner green wire you exposed.
7. Slide the new rubber boot over the cable past the wires
8. Crimp the terminals to the shield green wire
9. Use a meter to determine which wire is the green wire in the new cable at the male timer junior connector and then insert the terminals into the new female connector in the right spot. Slide on the rubber boot, connect the connectors and you are done.