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Thread: Fixed Ground, How to fix screech?

  1. #11
    CarFrontEnd Creator iamgnat's Avatar
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    Actually it's not that flawed of an example. The missing part is the work that it takes to get the water through the straw and the effects that has. In this case he's trying to suck a gallon of water through a 3 foot long coffee stirrer

    The wire that comes on the PSU is sufficient for the length that it is, not for the length of the run that you should be making from your battery to the PSU. If you ran that gauge the entire length, as it starts approaching it's max pull the wire will heat up and melt the casing which will allow for a short that could cause a fire.

    Think about about what happens when you pull too many amps through a small fuse (the ones you can barely see the element of). Most of the time you get some charring when it pops which means a fire, however small. All the fuse is in it's purest form is nothing but a very fine wire. All wire (no matter how thick) is essentially a fuse and if you run more amps over it than it can handle (determined by length and gauge) the same thing happens as it did to the fuse. Just on a larger scale.

    Further more, short of industrial grade wire, wire that small will not have a covering that is made it to stand up to the heat and other bad things running around the engine compartment. They usually also aren't made to withstand the rubbing that is typical in that environment either.

    You are of course free to leave it how it is and the chances are that you may never have a problem, but the cost of the damage that may occur far outweighs the cost of using the right materials.

    -dave
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  2. #12
    Newbie Red_Beard's Avatar
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    The 12 ga wire I used was 600V capable, 80 degrees celcius, pretty thick stuff in my opinion.
    So, you're thinking the PSU isn't getting enough power because I'm only pulling the amount I need from the beginning and it kinda fades? That kinda makes sense. Hm.

  3. #13
    Newbie Red_Beard's Avatar
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    I also have a 25 amp switch on the power line. You think that could induce this noise? Its a RadioShack 25 amp DC switch.

  4. #14
    CarFrontEnd Creator iamgnat's Avatar
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    No. If you were pulling too much power, you would have had a fire already.

    600v doesn't mean much, it's the amperage rating that is important. And your engine is a couple of hundred degrees.

    Like I said, the chances are that you will be ok but you are risking a fire. It would probably taking maxing the PSU out to have a problem though (e.g. running your LCD, the P5V under it's full load, and the Mini drawing a full load).

    The purpose of a fuse (or switch in your case) is that it should allow less amps to pass through than the wire it is attached to. This is to prevent exactly what i'm trying to warn you about. In your case I would not be surprised if your wiring and your fuse has a very similar failure point. It's been awhile since I installed mine, but I believe 25amps is much more than Mike recommends anyway.

    As far as your problem, I doubt this wiring has much/anything to do with it. Since you say that it doesn't happen when you connect up your iPod, it is probably a combination of 2 things (what you describe sounds almost exactly as what i've experienced in the past). You have a cheap (read poorly insulated) audio cable and the Mini/PSU still isn't grounded perfectly. The easiest thing you can do is change the routing of the audio cable to make sure it isn't anywhere near a power line (the more power it carries, the farther it should be). That's all I did to fix my issues and now I only get that noise when the Mini is off or the audio line has been disconnected from the Mini.

    -dave
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  5. #15
    Newbie Red_Beard's Avatar
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    Well, I've tried running a 12 gauge line directly to the battery negative, which didn't help.
    The best ground I found so far was a body mount in the door jamb. I can still hear a slight whine, but it nots nearly as bad as before.
    The audio cable has to be good. Its really thick, hard to bend. Its a Monster one. [LINK] I would think when I attach the ipod, that interference of the power lines should still be there no?
    I'm going to study the noise some more.
    Some more clues:
    When I pump the headunit to full volume, that noise is more enunciated and there's a uniform popping noise. The more volume, the louder it gets.
    With the ipod, nothing.

    I really appreciate your help and patience.

  6. #16
    CarFrontEnd Creator iamgnat's Avatar
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    Quote Originally Posted by Red_Beard View Post
    Well, I've tried running a 12 gauge line directly to the battery negative, which didn't help.
    Grounds should be short. Going all the way to the battery is not a good idea.

    The best ground I found so far was a body mount in the door jamb. I can still hear a slight whine, but it nots nearly as bad as before.
    That's the trick with grounding, even a half inch or so can make a big difference. The interference at a grounding point is almost impossible to guess, which makes it a lot of trial and error. Take a look at the FAQs for grounding in the FAQ forum, they have a lot of good info.

    The audio cable has to be good. Its really thick, hard to bend. Its a Monster one. [LINK] I would think when I attach the ipod, that interference of the power lines should still be there no?
    Thick doesn't mean much, it's what is making it thick that is important. I don't know about their audio cables, but I know their video cables are considered over priced and sub-par.

    As far as the iPod, it has it's own ground loop which is why the noise goes away. It is not connected to the car's ground loop so they don't interfere with one another.

    -dave
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  7. #17
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    even monster makes a cable that goes bad. if you follow my advice above you will find where the problem is coming in.

    once you know its either the computer, the cabling, or the sound system you can find the specific issue.

  8. #18
    Newbie Red_Beard's Avatar
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    Quote Originally Posted by 2k1Toaster View Post
    does it go away if you mute the music, or does it remain?
    Oh jeez. It does NOT go away when I mute the music on the Mac. It remains. Meant to say that in the beginning.

    Its not the mac's output because it works in the house no problem.

    However, when the SIRIUS is on, the screech is gone, just a faint Alternator Whine. The SIRIUS is hooked up to the Cig Lighter.

    Something having to do with the Mac. But not a faulty output.
    Also, both outputs do it on the Mini. There's one for the headphones, then something else, they both do it.. Figure I'd mention that.

  9. #19
    CarFrontEnd Creator iamgnat's Avatar
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    Definitely a ground loop problem.

    Try a new audio cable and it *might* help, but more likely than not you need to find a new grounding point for the P1900.

    -dave
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    CarFrontEnd (now it's own sub-forum!!!!)

  10. #20
    Newbie Red_Beard's Avatar
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    and using a GLI is impractical, right?

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