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Registered
Join Date: May 2002
Location: Santa Barbara, CA
Posts: 4,821
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CDI stands for Capacitive Discharge Ignition. It is the silver box with the cooling fins on the electric panel in the engine bay.
It uses a DC/DC converter that runs with 3.5kHz. To check if that is the culprit simply turn the ignition to ON but do not start the engine. If the hum appears it is likely the CDI unit. If the hum only appears when the engine is running it still could be.
The fact that the hum is NOT changing pitch with engine RPM makes me think you could have a ground loop. Are you using an amplifier or only the head unit driving the speakers?
Ingo
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1974 Targa 3.6, 2001 C4 (sold), 2019 GT3RS, 2000 ML430
I repair/rebuild Bosch CDI Boxes and Porsche Motronic DMEs
Porsche "Hammer" or Porsche PST2, PIWIS III - I can help!!
How about a NoBadDays DualChip for 964 or '95 993
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