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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > AUDIO/VIDEO + BLUETOOTH + Electronics/Alarm/Software > Electrical issues with added amp - Need help

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      03-19-2012, 03:51 PM   #1

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Question Electrical issues with added amp - Need help

Would like to run something by you guys I'm at wits end with. I've run out of ideas. Any info or ideas you can think of would be immensely appreciated.

Sorry for the novel, but wanted to be as thorough as possible. Will probably post on bimmerfest as well.

I have a 2008 335xi sedan auto/steptronic 45k miles. Logic 7.

This weekend I decided to work on some mods I've been wanting to do. Sunglasses/ashtray convertion and the rear power install(cigarette lighter outlets) for folks in the backseat.

Before I started, about a week earlier, I installed a couple subs and amp in the trunk using the wiring harness I bought from Technic. That install went great, everything worked well. Thank goodness for that harness, really makes life a great deal easier not having to spend hours like in the past installing cable etc. FYI on the setup is an 1800w MONO amp wired to 1ohm.

So, did the rear power install first. Pretty simple really, did it just like the DIY on here instructs to do. Drove around for the day, all good.

Next, I worked on the sunglasses tray. This proved to take a little longer, because I used the LEDs that were on the ashtray lighter, for the sunglasses tray. Had to drill a couple of holes and run LEDs appropriately. Plus trying not to break any of the plastic parts, which meant figuring how to take a part, and put back together. While I was in there, I ran my NEO Car jukebox head unit to the tray. This one mod took about 4 hours or so, mainly because I wanted it to look as clean as possible so I took my time.

Because this is an auto trans, I didn't take the gear shift off. Wasn't really sure how to. However, in order to get the wood trip up enough to unscrew the ashtray, had to put the gear in neutral, which meant leaving the key in the ignition. Well, in short I drained the battery enough that I needed a jump.

Used the jump terminals under the hood. Didn't take long to get started, maybe five minutes or so. Did have to set the time, but other than that everything seemed fine.

The battery is brand new, not even a month old. OEM from dealer(glad I didn't pay for it). So I remember checking it out and seeing the green glow. Well, after jumping and driving around for a day, the battery charge indication hole is black(needs charging). So yesterday, stopped by AutoZone to test it. Interesting note: testing the charge from under the hood, said bad battery. However, testing from the trunk said good battery at 95%.

Now for my issue, which I think is related to the battery, maybe.

Everything is OPS normal, except for the added amp I put in the trunk. I did not make any changes to the added amp setup, as it was working well before the battery drain. But now, the amp doesn't seem to turn on. So I thought it was the turn on lead from the Technic harness. It's fine. So I'm thinking I've blown a fuse on the amp or the power cable to the battery, used a fuse tester and the fuses are good. I've tried another amp and same issue, not turning on or getting enough juice.

The amp is an autobahn 1800DT MONO class amp(goes from battery(top of distribution block), to inline fuse(80A) on 0 gage cable, to distro block at amp(30A fuses) to AMP. Same setup I've run in my last car for years. Upon closer inspection of the amp, it has some LED's that come on VERY dim, but the digital readout on the top shows no lights. The remote knob has no lights, and the power indicator light on the amp shows nothing as well. But, I can see very dim light coming from parts of the amp. So the fuses must be working if the amp is getting some kind of power.

Thinking it might be the fuses at the distro block close to the amp, I went to wal-mart and got a fuse kit and fuse tester. Switched out the 30A with 40A, turn on the radio to make the lead turn the amp on, and it came on and worked for about a min before it shutdown or whatever it is doing. All fuses seem to be fine at all points(except the distribution block of the battery as I have no way of knowing)

When I disconnect the lead to the amp, then reconnect it, LED's light up but quickly fade to very dim and stay very dim. This makes me think the lead cable works, but the amp isn't getting enough juice?

When the battery was drained, I still had lights and the car would try to turn over, but just couldn't. So the battery was not completely drained. Not sure if that makes a difference.

Currently there are no errors in the dash, except for the white angel eye bulbs that apparently failed or burnt out from having the lights on.


Q: Not knowing enough about the battery and the electrical system of the car, could it be the battery is still charging and won't give any power up until at full charge?

Q: Is it possible letting the battery drain would cause the fuses on the distribution block of the battery to blow? And if so wouldn't other things not work connected to the distribution block? And for that matter, I shouldn't get any power to the amp right?

Q: Any ideas on how long it will take to see the green glow back on the battery?

Q: Should I get a charger to connect to the batter? And if so, what is the best way to connect a charger, under the hood, or directly to the battery?

Q: I've read on here where people have disconnected the battery for a few min or so, then reconnected it to get systems back to normal. Is this something I should do because it was drained, or because it still had some power left no need too?

Again, sorry for the length, but I've thought through this for a day or two and can't seem to come up with any ideas on why the amp is not staying on or getting its full power.

Thanks for any help you guys can provide.


Last edited by westwing222; 03-19-2012 at 04:01 PM.
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      03-19-2012, 11:11 PM   #2
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See below
Yes for sure
I dont think it matters if you disconnect it

Car pics HERE
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      03-19-2012, 11:50 PM   #3
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You do know 1800 watts at 14.4 volts is 125 amps that is at 100% efficiency it would actually be more than that, that is assuming your amp is really 1800 watts. That said why do you have an 80 amp fuse. Also why do you have a distribution block, I'm thinking that's your problem since replacing the fuses fixed your problem temporarily.
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      03-20-2012, 12:35 AM   #4
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So thats a 2500 watt amp?

So thats a 2500 watt amp?

Last edited by ctuna; 03-20-2012 at 12:42 AM.
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      03-20-2012, 10:52 AM   #5
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Never heard of autobahn. Did you mean audiobahn? Like bmi325i said, your setup is confusing.

For a single amp install, why are you using a distribution block and only an 80 amp fuse?

You should be volt checking the system and reporting back with numbers.
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      03-20-2012, 09:11 PM   #6
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1800 watts at 1 ohm is probably drawing more current than you car can provide
I assume your 1 ohm load is 2 subs with dual 4 ohm coils in each one
So 2 and 2 wires in parallel to give a 1 ohm load
Try wiring them like this to get a 4 ohm load at amp
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amp install, audio upgrade, dead battery

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