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Old 03-26-2014, 06:19 AM   #1
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Question Fixing an A/C hum...

I installed my amp and sub day before yesterday, and everything works. However when I left this morning before dawn, I noticed an A/C hum coming from the sub.

When I arrived at work, I noticed that when I turned my headlights off, the hum went away. Turn them back on, and the hum comes back. Its not the park lights though, just the headlights.

Where would I start looking to try an alleviate the hum?

The amp is wired through the fire wall, directly to the battery, both positive and ground. The sub is in the trunk area, and the speaker wire is ran up the roll bar on the passenger side, under the zippered padding. The RCA coming from the amp to the HU is ran under the plastic molding on the passenger side, through the dash above the glove compartment, and plugged directly into the HU.

I can't think of anything, or anywhere that the headlight wiring crosses the sub wiring enough to cause that bad of a hum.

Any thoughts?

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Old 03-26-2014, 01:22 PM   #2
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Where is the amp? I know you said the sub is in the trunk. I would start by shortening the amps ground wire to be as short as possible.

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Old 03-26-2014, 01:26 PM   #3
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The amp is under the passenger seat. The ground is ran from under the passenger seat, up the side of the dash, and through the firewall through the battery.

Shortening the ground has been suggested, and I think I'm going to try that tonight. I'm currently researching how to remove the front passenger seat safely without interrupting the airbag system.
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Old 03-26-2014, 05:22 PM   #4
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I'm currently researching how to remove the front passenger seat safely without interrupting the airbag system.
If you installed the amp under the passenger seat, I don't see why it would disrupt the air bag system, if it didn't disrupt it when you installed the amp.

What I would do, as Big Sarge suggested, is shorten the ground cable. I would cut the ground and screw it into the tub under the passenger seat, as far as is safe. I wouldn't have the ground connection right next to the amp, but a foot away or even eight inches. When, you place the ground, make sure it is underneath the carpet, and clean the paint from the ground hole that you drilled for the ground connection. Also make sure that there is nothing being drilled through that would disrupt other systems. I would also check the grounding straps under the hood to make sure that your tub is properly grounded.

You might also want to trace your headlight wires, and see if they are right next to your amp power cables.

You might also want to determine if certain functions make the hum go away. For me, the GPS didn't hum, nor did my CD player. But, my Aux-in had a big hum. So, I was able to address the Aux-in as the culprit. I also noticed that my cigarette lighter cell phone charger increased the hum. I just put a noise canceller on my aux-in port and it solved the problem. It might be different for you. On other vehicles, the ignition power was the culprit, and I put a noise canceller there to solve the problem.

An amp might also magnify a slight hum that is already present. So, you might want to check your radio grounds to make sure that they are good. If your firewall and tub are not properly grounded, by the straps under the hood, then it might not be the amp that is the problem, but the radio hum, which the amp is magnifying.

Cheap components might also be the problem. In which case, a noise canceller may solve the problem.

If you are tapping into the headlight power, instead of the dedicated radio connections, this might also be part of the problem. But, I would start by eliminating all possible grounding problems.
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Old 03-26-2014, 05:31 PM   #5
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The HU is plug & play with a PAC harness, so no grounds were changed there. And I re-soldered all connections between the harness and the HU before installing this one.

With the vehicle running, I have no hum. If I turn on the park lights, I have no hum. But if the head lights come on, there's a hum. Turn the head lights off, no hum. And that goes for FM, and iPod controls.

The Jeep is two weeks old, and I haven't touched any ground straps so they should be factory fine.

I installed the amp by running the wires down the side and under the molding, under the front seat, and then connected them to the amp. When it was connected, I just slid it under the passenger seat from behind. I didn't have to remove the seat to install the amp.

Where would I place a noise canceller?
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Old 03-26-2014, 06:31 PM   #6
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Can I ground it to one of these? Aren't these ground posts under the passenger side?
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Old 03-26-2014, 07:13 PM   #7
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Ok well I took a chance. I cut 4-5 feet off the ground and reconnected it under the dash.

It helped. The hum is much much better and almost unnoticeable, but I can tell its still there when I turn the headlights on and off.
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Old 03-26-2014, 07:22 PM   #8
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Ok so we have determined that it is a ground issue for sure. Cut a few more feet out and put it closer to the amp. Long ground wires just are not good. You may be able to use a seat mounting bolt or maybe a bolt from the seatbelt. Not sure how the JKs are set up in that area. Even under the dash I would imagine you are still at around 3-4 feet of cable. Like chas said keep it around 8-12" if at all possible.

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