Specifically I have a 740 sedan.

I successfully installed an amp in the trunk and a new head unit but I'm getting feedback from the alternator or something.

You can hear it whirring (sp?) sometimes.

I think I've seen something about using a device to modulate the voltage but I'd like to see if there is a good place to ground the stereo--cause I just hooked it into the ground wire that was on the DIN cable. (I bought a stereo from Crutchfield and followed their instructions.)

 

The grounding point from your old deck is perfectly fine for a new deck. It was engineered specifically with the intent to ground a radio and regulations require all aftermarket decks to comply with those engineering guildlines.

Something else to concider.....Where are your speaker/RCA lines relative to the power line on your amp? They NEED to be isolated....the speaker wire will pick up alternator feed back from the power wire if you run them together. they should run on opposite sides of your car.

If you've already done that....how do you have your amp grounded? If it's sharing a grounding point with something else or if the ground is too long you are probably ground looping. They make Ground Loop Isolators (GLI) but a GLI is a bandaid. It won't fix your problem....it'll just hide it....and your sound system will suffer for it. There's also something called an Alternator Suppression Relay (ASR) that negates the sound from your alternator at the source....most alternators have this already and if you didn't hear the interferrance with the old stereo then it's probably not that.

IMO....you either need to re-run you speaker/RCA lines or you need to find a new grounding point for your amp. Actually.....CREATING a new grounding point is the best bet.

Let us know if that doesn't fix/help and we can trouble shoot other possibilities.

 

Yeah I'm gonna try and rerun the cables later this week, I think I have one side of my speakers, if not both sides running along with the power. Not intentional but it was kinda hard to run the left side speakers along the right side.

I also ordered more cables (8 gauge) to redo my grounding--I had to splice or solder the wire for the amp ground (can't remember) since the kit stuff was so damn short.



Just to clarify--what does the alternator do after a few minutes/hours--cause the sound goes away after say an hour of driving? (cause I've had that happen numerous times)

 

Still charges the battery and powers all the electronics in your car.

Don't worry about it...you aren't the first person to do it that way. Just out of practice I'd re-run the RCA/speaker lines but it's starting to sound more like ground looping to me. Where exactaly did you run the ground line to? Because soddering it isn't the best idea. The current in a sound system travels on the outside surface of the wires as opposed to through the centers....that's why all your speaker wires, grounding straps, power lines etc. are made up of a bunch of little wires. So soddering a an 8 gauge wire will pretty much choke the ellectricity flow through the amp. Since power travels the line of least resistance...it's looping into your speaker lines.

The best thing to do for grounding an amp is to take an 8" length of grounding wire (or 10"...whatever, the point is to make it SHORT), grind away a small patch of paint from the floor of your trunk (or where ever your amp is sitting) and drive a self tapping screw through a loop connector.

 

That's what I thought the alternator does--just sometimes I don't hear the sound after a while (even with the unit on and the volume muted).

I have the amp in my trunk grounded to the bolt for the left set of brake lights (the gray plastic piece). I'm gonna get some rough sandpaper (or a block) and sand that again and replace the wiring when I get some wire.

 

That's because a ground loop doesn't always run in the same place. I'll bet grinding away the paint will help....It's also not a band idea to increase the gauge of wire for a grounding strap relative to the power wire.

This confusses me....why would you have grounded it to plastic? Plus, that seems like a long way to go for a grounding point. I STRONGLY suggest you pull up the edge of the carpet in your trunk and drive a self tapping screw into the metal. According to most install people, anything over 12" for a grounding point is bad.

 

I looked at what I did again and I have to make a correction--on the light fixture/set the cap that goes over the threading is plastic but the screw/bolt coming from the body is metal. Of course I knew that I couldn't ground the thing to plastic.

What I actually did was use a connector with an O-shape (for the end of the ground wire) and put it around a bolt right next to the light fixture.
(a bolt sticking out from the back of the trunk right next to the left light fixture, outside of the plastic box that covers it)

I definitely think I'm going to try that self-tapping screw though, I just didn't think that was practical at first.

And this is the first time I'm hearing about the 12" thing, I'll try to make sure I do that.

Thanks.

 

word...

 

So, I got some 8 gauge wire, cut it about 8 inches, sanded down my metal, made the ground...

And I'm still getting that ####ing sound.

I guess I gotta completely reroute my speaker wires. Ain't that a bitch?

 

yes it is...I hope it helps.

If it doesn't help....I'd hit up Auto Zope or Checker or whatever you've got and look into an ASR. Strange that you alternator wouldn't already have it. Then again, it could just be dead. A lot of places will call it a Radio Suppression Relay....and it may be that it was located inside the OEM deck?

 

Ideally your amp and HU should use the same ground. If you can, ground both to the negative battery terminal

 

...but...in that case I would need like a 20 ft wire running ground to the battery terminal

(right now my amp is where my spare tire goes {740 sedan} -- and I put the tire right over it)

It took quite some time but I ran all my speaker wires on the right side of the car--and that's basically all their is on that side of the bottom door panels. I'm still getting the humming, almost entirely my front right speaker (fairly minimal humming from the back ones). I know that the front right speaker has a slight tear in it so I'm going to replace it very soon but the noise from the alternator is ridiculous sometimes. If I turn down the sound it's surprisingly loud (then of course it's much harder to hear with a decent amount of volume).

I did the grounding, put the wires on one side, checked for loose wires, etc. I can't think of anything off the top of my head or with some decent (re)searching.

Is it possible that it's my amp?

My only other idea is that it may be coming from the ground wire connected to the head unit but it's such a tiny wire and I can't think of doing much more besides stripping the wires and soldering them directly. (Right now I just wrapped the wire to the u-shaped connecter and soldered it thoroughly)