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!
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