Amp works, sub doesn't.

[nailPuppy]

I have a dumb car audio question, I tried googling for the answer but I figure its better to try to explain it to people who know what they're doing.

Car: 2000 Accord Coupe

Stereo: JVC KW-HDR81BT

Amp: Boss R1100M

Sub: Unknown Rockford Fosgate 12"

I recently was given the rockford fosgate 12" sub in a q-logic box by a family member. They had been given it as a gift ten years ago or so but they are hoarders so it just sat in their garage collecting dust. Since that was free I decided to spend $50 on a cheap amp to see what I could do with it. Turns out the factory headunit didn't have RCA out for the sub and I suck at wiring so I got the new stereo on Saturday.

The radio works fine. Everything with the harness seems to have gone well, front/rear left/right all seem fine, but the sub isn't doing anything at all. The amp lights up, I took checked all the fuses which are fine and a multimeter is showing it getting 12v in the trunk.

The amp is connected with this kit which seems adequate. For the speaker wiring I'm using the wiring harness on the stereo itself for the four cabin speakers like I figured you're supposed to do. Then for the sub I'm using the RCA outs. There are RCA outs for front/rear, should I have used those instead of the speaker wires from the harness for the cabin speakers? I've messed with every sub setting in the radio menu I could fine. Changing output, hpf, lpf, none of that seems to make a difference. I do have a multimeter but I'm not too sure where to test to identify potential problems.

While troubleshooting I unscrewed the little wiring port in the sub box to make sure it was all connected inside and saw that the sub is rated for 8ohm. That cheap amp lists itself as 1100@2ohms, 250@4ohms. Is it not putting any sound out because the amp just can't power the sub? I didn't see any model number on the sub itself and I can't find much about it on google. I know its old, the design on it isn't anywhere in RF's current lineup.

I don't have another amp or speaker to test it with. I might run to a local car audio shop tomorrow to see if they can just hook up another sub to it to tell me if its a problem with my wiring or what. I really should have taken an electronics class in high school or college, this stuff confuses me. I don't want to shake my neighborhood, I thought one 12" sub would sound nice.

The whole ohm thing could just be a red herring for all I know, but its the only place I see where the numbers don't match up.

edit: I guess one option is to either get a second 8ohm sub to run parallel or just get a new single sub that is 4ohm?

 

[Valet]

You'll still hear noise from the sub no matter the ohm.

I guess you made sure the sub woofer is turned on in the head unit. And that you have the RCA's coming from the sub out at the back.

You need the 12+ at the remote turn on and the power and ground. Make sure the sub isn't blow. Double check what's going to what and that it's correct. Take a test speaker, doesn't have to be a sub, and hook it up to the amp for a second. If sound comes out, then that's weird and I guess the sub just isn't going to play anything. All else fails take it to an audio shop which will charge a half out to check it out. Or they might do it for free.

 

[kn0x47]

speakers are nothing but wires in a magnet so if its getting power then you'll hear something. the fact that you're trying to power an 8 ohm sub with a amp that only rated to go to 4 ohm could be an issue, i really dont know. but otherwise, its going to be a wiring issue

 

[nailPuppy]

Thanks 6thgen folks, got it working now.

It could have been any number of things fiddling with it over an hour but I think the culprit was the rca shield ground. I had just put the ground from the stereo to the ground from the factory harness when I was doing all the harness stuff. While reading the directions putting the stereo itself I saw that if you had to use a 3rd party mounting bracket you were supposed to put the ground somewhere on the chassis/firewall instead of using the harness one but thought "hey a ground is a ground".

So yup, I'm an idiot that doesn't know how to follow directions. Turns out you aren't supposed to skip steps. Sounds great now on the inside, and you can't really hear it on the outside. Perfect.