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, Timing Offset. As well as premature cap and rotor wear.
Check for a correct signal using your oscilloscope probe terminal “7” of the CDI Box. After checking, crank or start the engine and check the wave form. It should look like the one pictured directly below. The wave form must have a gradual rise and a sharp fall going below ground.
Below is an example of what a reversed Polarity trigger wave form looks like.
On a reversed polarity trigger notice that the wave form 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 wave form 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 half way 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 wave form is now correct.
This issue can affect any vehicle that use 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 backwards. It is a good practice to check these cars when they are in your shop. This is good as you can avoid potential failures buy catching the problem before it creates more damage.
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