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Hi everybody.  I am new on this site but have been reading many posts over the last month to get my system to work properly.  I have tried a lot of things to nail down any potential issues but cannot seem to find anything.  Hopefully, you guys can help me narrow down my issue.


I recently installed a LC2i, JL JX500/1D amplifier– 300W RMS at 4ohms, and a 12W3v3-4 subwoofer in a sealed box installed in the back corner of my car.  There is also a harness in between the factory speaker wiring harness connector and stock amplifier to tap into the underseat ‘subwoofers’ (but really mid-bass speakers) in my 2011 BMW M3 and connected to the LC2i speaker inputs.


The issue is the sound output from the subwoofer is not much unless I turn it up.


Things I have tried:

·         Measured for voltage drops by measuring DC voltage with car running, playing a 50Hz tone at high volume at battery (~14.4V), voltage at amplifier (~14.4V), voltage at LC2i (~14.4V), they all really fluctuate from 14.35V to 14.4V, so there’s good grounding and no voltage drop or increased resistance in my connections

·         Measured AC voltage of each underseat subwoofer at the harness and at the LC2i speaker input while playing a 50Hz tone at high volume to see if there is voltage drop due to a loose connection or incorrect wiring, but there’s no voltage drop there either.  Both were at 31.1VAC under load.

·         Replaced my RCA cables also since they seemed loose, but no change.  Checked polarity of wires coming into LC2i to make sure I did not switch + to -.

·         My logic tells me since iPad to amplifier played well, my issue is either the wiring harness, or LC2i or RCA cables.  I connected a 3.5mm headphone jack to RCA cables and played music through my iPad. At pad’s max volume, I was getting ~8VAC with the 50Hz test tone, and when I played music thru my apple music app, subwoofer was sounding better, which lead me to believe, my problem is not the amplifier.

·         Playing the same track through my USB port, I have to have the volume up ~40-50%-ish to get the same 8VAC I got with the iPad

·         I set my gains initially using the DMM method, played 50Hz tone and set LC2i until the ‘maximized’ light went off, then set the amplifier sensitivity until I got ~34.6VAC at amplifier speaker output, but I felt like bass at low end/low volume was not good at all with the gains set up to this method.

·         I increased the gains at both the LC2i and amplifier to get some more low end/low volume bass, but from reading countless threads on the BMW forums, my gains were much higher than what most people had theirs set to.  Right now, LC2i gain is set to roughly 12 o'clock using main output, and amplifier set to 9 o'clock. I've also read that to adjust gain at LC2i and keep amp gain at minimum/or slight increase on the BMW forums and that's worked pretty for most people.

·         I have also measured resistance at the subwoofer terminals to check if subwoofer was bad and got 4 ohms, I checked all wiring for continuity, and all seems good.  I checked for loose wires in the wiring harness and pushed them all in to make sure its tight.

·         Only thing I can think of is that either the wiring harness or LC2i are bad, but I cannot seem to identify what is wrong where.


I thought about getting a new harness and trying that since I bought the entire setup used on the BMW forum, but I would rather not just throw money at parts hoping for a solution.  Unfortunately, there does not seem to be many people doing audio upgrades on these cars and the Audio section of the forum is pretty quiet.


Perhaps my amplifier power is not enough since its 300W RMS at 4 ohms, but at the same time shouldn’t 300W give ‘enough’ power, at least to determine if system is working properly?


Is there any other troubleshooting or testing I can do to pinpoint the root of my issues?  Any tips or advice is appreciated!


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