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I recently found a fix to the loss of bass that is inherent to the bypass harness and sent my findings off to Technic for verification, below is the response I have received from Technic.
(Your findings made me contact BMWAG to confirm the ASD OEM schematic and they confirmed your conclusion: the OEM schematic is incorrect for the HiFi (676) and the Top HiFi (674) systems.
Pins #3 and #4 (white and white/black wires in the ASD bypass harness) are incorrectly labeled in the schematics. Correct polarities are Pin # 3 negative and Pin #4 positive. The schematic showed them backwards.)
So Technic was unaware that the bypass harness was wired from a faulty BMW schematic which in turn created an out of phase front speaker, luckily the fix is simple and should be very easy for harness owners to perform themselves.
Fix:
Cut (white and white/black) wires at the midpoint of the wires loop and then re-connect the two wires so that you get a (white to white/black pair of wires) (cross the streams!) You should now have a (Front left) speaker with the proper polarity, in phase, and more importantly restoring the bass.
Technic wanted me to mention that the bypass harness is now corrected and that if any current owner is uncomfortable with performing the above fix they can have their harness rectified by contacting him directly.
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