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  1. #1
    FEP Power Member haromaster87's Avatar
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    Default Looking for some semi advanced knowledge on a persistent engine noise in the stereo

    Hey everyone,

    I'm sorry in advanced for the length of this post, I've done a lot of work on this issue so I want to explain mostly everything to hopefully present everything up front.

    I have a 1986 Mustang GT 5.0 5spd, I just completed a stereo installation in it and I've been fighting with an engine noise that comes through the stereo. The noise only changes pitch if the engine is revved, and the pitch follows the RPM's. Just to clarify, the system is Noise Free up to max volume when I'm running on battery with the car NOT running. The battery has been relocated to the back, but during troubleshooting, I disconnected my relocation wiring and reinstalled the stock wiring and put the battery back in the engine bay, this had NO effect on the noise. I used 0 gauge wiring running to the back of the car and I grounded the battery and the rear bumper frame. I also made a 0 gauge ground and installed it between the stock ground point on the engine near the power steering pump and bolted the other end to the "frame" rail right next to it in the engine bay. I did not upgrade the ground strap on the rear of the head, because I assumed my other 0 gauge cable would complete the engine to frame ground. I'm hoping I'm not totally wrong on that idea.

    I do a great deal of car stereo and car electronic work in general and I'm experienced with troubleshooting engine noise, ground loop noises, etc. I'm not mentioning that to brag, it's just to clarify that I've gone through just about any kind of basic troubleshooting that one might think of for an issue of this nature. I have also been collaborating with folks on the car audio forum I frequent and to make a long story short, any potential issue with the equipment has been ruled out.

    There are two front speakers connected to the head unit, and two rear speakers which are running from an amplifier. The noise is present in both sets of speakers, even with the devices electronically separated. Without going through everything that was done, I'll try to mention the most explanatory steps that ruled out the equipment. I moved the head unit over to a different(non fox body) vehicle and it was noise free with the car running, so that rules out a head unit issue. While the head unit and its wiring was totally removed, I used a jumper wire to turn on the amplifier and found the noise was present in the speakers connected to the amplifier. At that time, the amp was only connected to power, there was nothing plugged into its signal RCA's.

    Any issue of this nature is almost always ground related, and again without going through everything done, my troubleshooting culminated with me running temp grounds of heavily shielded cable and grounding the head unit and amp directly at the battery negative. Once again, this had literally no effect on the pitch or level of the noise. And with the head unit removed, the amp still generates the noise with its ground at battery.

    My aging 2g alternator was registering like 15.2V on the multimeter, which was a little higher than the 14.4V Max I'm used to seeing. No one could agree if this was a problem or not, so I took the opportunity to do a 3g alternator upgrade with a brand new(not refurb) alternator. The upgrade went well and it now sits at 14.4V, but once again it had no effect on the noise. I hadn't thought to do this before, but I ran the car briefly without a belt and found the noise was still present, so at this point I believe the noise is completely independent of the alternator or charging system.

    This leaves me only with the possibility of an ignition caused radio noise. I will fully admit this is where my expertise drops right off as I've not encountered this before. I've been doing google searches of anything I can think of related to this and I have found mentions that an aftermarket coil can cause radio noise. I looked at my coil and it's unbranded and just has a "made in china" stamp on the front. So I'm pretty sure it's not the OEM coil, but it's not like an MSD or Accell either, looks to probably be a car parts store replacement and it looks like it's been there for a little while.

    In all of my searching, I could not find reliable troubleshooting or related info regarding the coil situation or troubleshooting a noise issue with the ignition wiring in these cars. I don't particularly want to run out and buy a new coil to test with if I can avoid it, so I'm hoping maybe something about what I've brought up here maybe rings a bell with a previous experience for someone on here. I'm open to hear any ideas or theories anyone might have. Thanks in advanced for anything!
    Last edited by haromaster87; 04-06-2019 at 08:20 PM.

  2. #2
    FEP Senior Member Tigger's Avatar
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    Ive had the same issue on my wife’s 96 GT ragtop for a couple years now. I chased my tail for a while trying different things... coil packs, alternators etc. I finally found out it is the amp. If I disconnect the rca’s for the front channel the noise goes away. Rear channel sounds fine as does the sub on a different amp . There is something internally wrong with the front channel of my amp that it picks up the ignition noise. It’s not the wiring because the previous amp that was stolen sounded fine. In fact this amp sounded fine when I first put it in but the whine started one day. I haven’t gotten another amp yet but I will soon because the wife is getting annoyed with it.

    Have you tried just the deck and front speakers with no amp?
    67 Mustang Coupe
    96 Tangerine GT
    86 Saleen #179

  3. #3
    FEP Power Member haromaster87's Avatar
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    Ah that's a bummer. Yeah in all of my troubleshooting, I have disconnected the fuses for the amps and unplugged any RCA connections between the amps and HU.

    I'm glad you asked about that. The majority of my troubleshooting has actually been on the front speakers and head unit.. Tonight I made some interesting observations. I had known before that I could plug my head unit in while it was removed from the dash and not get the noise. However, as soon as I would bring the head unit up to the dash opening, the noise would come back and quickly rise to maximum intensity. I could vary the noise a bit by pulling the head unit back and forth.

    Just to get this out of the way, I went through a methodic process of making sure this was not the front speaker wires binding against some power wires or something of that nature and it wasn't. I can move the wires and twist them/bend them any which way and the noise will not be present unless the head unit is close to the opening in the dash. If the noise is present, any movement of those wires has no effect on it.

    I've already removed the metal head unit sleeve from the dash and that had no effect.The noise is specific to the chassis of the head unit sliding into the dash, I only have to insert like a small section of a corner of the head unit to get the noise to max intensity. There is metal dash framing that runs under the head unit area and up the sides. I pressed a wire against this metal and then jumped it to the chassis of the head unit. Regardless of where I pressed it, the noise would not happen.

    From there, I took the head unit and kind of hovered it all around the dash and floor as far as I could. The noise was present and intense any time it was anywhere under either side of the dash and when it was below the head unit slot, in front of the clock/bulb status display. As soon as I pull it towards the shifter, the noise quickly stops. I moved it all up around the AC controls and I think the noise was kind of present, although not nearly as intense as when it was below the dash.

    I ran a pair of power wires back to the battery and connected them to the power and ground connections on my head unit harness, thus eliminating the cars stock radio power harness that I plug in to. Even with the head unit running from an independent run of power, the previously made observations were all exactly the same. Same noise and same spots on the dash.

    It's as if the metal in the car is just emitting some sort of rather potent interference. I also found the amp stops making noise when I unplug the RCA's running through the car. I dug much deeper into that, but to keep the post from getting longer, I'll just say that the RCA's act as an antenna of sorts for picking up whatever is causing this. They are decent quality RCA's and well shielded, so they should be reasonably quiet without this problem.

  4. #4
    FEP Senior Member Tigger's Avatar
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    So if you unplug the RCA’s and just listen to the radio using the front speakers under the deck power Is the noise present?
    67 Mustang Coupe
    96 Tangerine GT
    86 Saleen #179

  5. #5

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    Classically, an issue like this was ignition-related, specifically the spark plug wire type. Solid (steel/copper) core wires would cause it, and a cure was to loop a knot in the coil wire, or run a resistance non-solid coil wire... or use all non-solid core resistance spark plug wires. Don't know what you're running, but you might try looping a knot in the coil-to-distributor wire if you've got enough length...
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  6. #6
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    Check to make sure you have resistor spark plugs as well.

  7. #7
    FEP Senior Member qikgts's Avatar
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    https://www.amazon.com/Copper-Foil-S...52442688&psc=1

    Wrapping the radio with the above might help but you have to make sure air can still circulate through it so it won't overheat. You might even luck out by just using a small piece over the chassis of the deck, near the location of the component on the board (trial and error on the outside of the radio) that's picking up the noise.

    Years ago when I worked at Sony and the C90 headunit was still in production, that unit along with a few others, were made with copper chassis' to keep noise out. Other units had some small copper pieces inside the chassis of the radio to do the same thing.
    '85 GT

    The other one... 2016 F-150 XLT Supercab Ecoboosted with 3.73's...

  8. #8
    FEP Power Member haromaster87's Avatar
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    Quote Originally Posted by qikgts View Post
    https://www.amazon.com/Copper-Foil-S...52442688&psc=1

    Wrapping the radio with the above might help but you have to make sure air can still circulate through it so it won't overheat. You might even luck out by just using a small piece over the chassis of the deck, near the location of the component on the board (trial and error on the outside of the radio) that's picking up the noise.

    Years ago when I worked at Sony and the C90 headunit was still in production, that unit along with a few others, were made with copper chassis' to keep noise out. Other units had some small copper pieces inside the chassis of the radio to do the same thing.
    Thanks for the link, that might be something for me to try. I ended up getting some MSD streetfire wires, some new Motorcraft plugs, and an OEM spec coil. I wanted to give the car a tune up anyway, and the streetfire wires are supposed to have good EMI suppression, but even after all of that, the noise persists just as strong as ever.

    The symptoms remain the same, the sound system is noise free as long as no components or wiring is near the underside/inside of the dash. I even broke down and tried an in line noise suppressor that's made to go on the constant 12v of the head unit. It did not attenuate or otherwise affect the noise.

    I'm convinced the noise is not wiring based, it's as if something in the dash is emitting a very strong electrical interference and the head unit acts as a big antenna to pick it up. But the noise does go with the speed of the engine. Ignition interference made sense, but I'm lost as to what else can be happening with the ignition to put off a signal like that.

  9. #9

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    Rather than using copper tape, try copper mesh as this would allow air circulation. https://en.wikipedia.org/wiki/Faraday_cage
    1985 GT owned since new

  10. #10
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    You might try talking to a ham radio operator, especially one who has mobile radio experience, or knows someone who has chased noise problems.

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