blocking alternator wire noise?

I have already done that. I am an installer, & have been for quite some time now. I have already done everything I can think of to find this problem, and I did. (the alternator). I have a device to measure EMI (electromagnetic interference), and have scoured my entire car with it. Setting it right above my MSD ignition coil it reads 50 (50 db? What the number represents I dont really know). The coil emits a lot, but none of the wiring around it registers any EMI. Now, scanning the amp power wire along the side of the car registers from 10 at the amp to 40 at the firewall while the care is at idle. Raising RPMs up the noise gets louder & the EMI numbers get even larger.   Scanning directly where the Alt wire connects to the fuse panel reads a 44 at idle (almost as much as the high performance ignition), but when I raise RPMs to 3k or so the number climbs to 70's & 80's. The EMI reading is the absolute strongest at the Alt wire, and when disconnected I cannot find any places in my car to register over 5. The fuse panel reads 1 or 2 with Alt wire disconnected. Power wire also reads nothing when Alt wire is unhooked, and audio system is dead silent. Reattach Alt wire & the EMI readings jump back to where they were before & the horrible noise returns.Unless it raises the hair on the back of your neck, its only noise!

I dont think the AC test is correct, as engine not running it was reading 20volts or so. I found a link saying to use a cap to filter out the DC current from the Alt & then measure for AC current (because the AC will pass right through the cap I guess). This I did not try. Something about voltage ripples emitted by the Alt due to failing diodes inside that are supposed to be converting the Alternator's AC into a DC output. Still trying to find answers about this.Unless it raises the hair on the back of your neck, its only noise!

Do you have an I-Pod or a portable MP3 device that you can run audio directly into the amp?

The 1 farad cap you tried was enough capacitance to filter out almost all AC ripple created by failing diodes.  If it helped a lot but there was still a little noise, i would say that could be your problem.  Since it made no difference at all that would lead me to look elsewhere.   When you turn the volume from 0 to 1 does your system make a popping sound?

The more I look the less I think AC ripple is the issue, especially since the the EMI readings are so damn high around the Alt. Yes, I have ran an Ipod directly to the amp and the noise is less but still there. I have also ran RCAs from my deck (in car) to an amplifier on my shops test bench & the noise is present there also. The Alt is contaminating everything.Unless it raises the hair on the back of your neck, its only noise!

No popping noises ever.Unless it raises the hair on the back of your neck, its only noise!

No other ideas other than to check grounds on the alternator itself.  Maybe pull the alternator off and get it checked.  Get another meter and recheck the AC voltage. 

Tried grounding the alt directly to the battery (even though casing had .02 ohm continuity to battery), no luck at all. I'll through another meter on it (3 other installers at work, all with different brand/quality meters). Dont think its an AC issue though.Unless it raises the hair on the back of your neck, its only noise!

Excellent Link There we go, sums it up pretty well here. Looks like I will be retesting at the Alt output wire & its case.Unless it raises the hair on the back of your neck, its only noise!