Tech Service bulletin
Re:1975-77 930, 1978-83 911SC, and 1978-94 930 models
Ignition Distributor Trigger Wiring.
When changing the short pigtail wire, Part number 930.602.907.01, also known as “The Green Wire,” coming out of the ignition distributor on the Porsche 930 1975-77 models, it is important that you confirm that the CDI box is receiving the trigger signal in the correct polarity. We have found that some earlier versions of the “Green Wire” have the polarity reversed. Failure to correct a reversed trigger signal polarity issue can/will result in CDI box failure, Coil Failure, and Timing Offset. As well as premature cap and rotor wear.
To check the trigger waveform, connect your oscilloscope trigger wire to terminal 7 of the CDI box and the ground clamp to the CDI box. The waveform coming out of the distributor should look like the signal below. The waveform will have a slow rise to the positive direction and a sharp fall below zero. (The CDI box should fire at the 0 crossing point).
The image below is an example of a reversed polarity waveform.
On a reversed polarity trigger, notice that the waveform is reversed. In the picture, the rise from below ground is very quick. Furthermore, the fall below ground is slow and drawn out. Because the CDI box looks for a positive going waveform to arm the box. Then it fires when the trigger signals pass the ground. This issue will result in a 20° to 25° timing offset. This means that when the spark plug fires, the rotor will be approximately halfway between two of the cap post. Due to this, the spark energy will have to jump from the rotor to the cap.
To Correct this issue, you will need to switch the connections at the wiring harness. You will do this where the green wire from the distributor plugs in. Then retest to make sure the waveform is now correct.
This issue can affect any vehicle that uses an inductive trigger, like the 1978-83 911SC and 1978-94 930 models. We have seen over the years several cases where the wiring harness has been repaired incorrectly. In these incorrect repairs, the system is wired backward. It is a good practice to check these cars when they are in your shop. This is good as you can avoid potential failures by catching the problem before it creates more damage.
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