blocking alternator wire noise?

Swapped engines in my car, and while doing so put in a new alternator.

After chasing down some noise in my car with a Monster cable noise sniffer, I found the wire from my alt is the culprit. Car running, alt cable attached, noise is horrible loud.
Car running, alt wire disconnected, noise it gone.

Pic of the wiring.


Power wire runs from connector to visible circuit breaker, drops straight down & runs along firewall to drivers side & enters firewall.
Alt wire you can see leaving the fuse panel, crosses the ground wire & then hugs the tranny & engine to the alternator, roughly 12" away from power wire at its closest point (except at battery of course).
The blue wire running from batt terminal to the left side of battery is the main power for the car & enters the fuse panel right next to the alt wire.

The noise is most definitely coming from the alternator , and I think it is infecting everything else.

How can I filter the noise from the alternator? It is driving me crazy and I have been driving the last couple months with the radio off just because of it.

Need ideas.......

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Unless it raises the hair on the back of your neck, its only noise!

On a scale of 1 to 10 with ten being the worst engine noise you have ever heard, what would you rate this noise?   If it is in the 7 to 10 range, do you have a pioneer deck?

It would be probably be a 7. At low/mid volume the whine is almost as loud as the vocals, as I crank the volume it gets less noticable but is still there. It doesnt change with volume, its just so loud that the volume has to be quite high to completely cover it up.

I have a JVC KD-AVX33. The noise is coming from the Alt. Deck is powered & grounded at the battery, amp ground is almost perfect, and with a noise sniffer the power wire goes through the roof, but if I unhook the alternator the noise sniffer reads almost nothing on the power wire.

I think I need something to clean the power coming from the alt, because it IS the source of the noise.

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Unless it raises the hair on the back of your neck, its only noise!

Sounds a lot to me like your RCA cables have lost their groun referrence, but since you insist that the alternator IS the source of the problem, we probably should not waste any time checking this.

Already checked RCA cables, so rechecking would be a waste of time.

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Unless it raises the hair on the back of your neck, its only noise!

I have tried pretty much everything there is to trace down this noise.

I found THIS link for a capacitor by MSD to filter their ignition systems incase of added noise. I assume this is a polarized capacitor, opposed to the traditional non-polarized caps.


Anyone think this could do the trick?
I cant find any specs on it :(

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Unless it raises the hair on the back of your neck, its only noise!

Just for fun, how about you toss a meter on there while the car is running. At the battery, you should see 14.4V DC with the car running. There should be almost no AC voltage when you flip the meter over to AC.

Tell us what you measure.

Oh, and how is the ground for the engine block? The accessories bracket is grounded too, right? Block grounded back to where the battery ground is?

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Wherever I go, that is where I end up......

AC reading between the battery ground terminal & the Alternator wire.


Battery has 0/1 ga ground wire to strut tower (oem location). Engine ground 1 is 4ga from tranny housing to lower chassis (oem locations), and 4ga from same lower chassis location to battery ground terminal. Engine ground 2 is 4ga wire from aluminum head to drivers side upper chassis, oem locations.
I tested ground points all over the car & everywhere reads equal to or lower than .2 ohms to the battery ground terminal.

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Unless it raises the hair on the back of your neck, its only noise!

I can't make out the markings on your meter. If it is on an AC Millivolt scale, 31.3 millivolts should not be a problem. However if it is not a millivolt scale, 31.3 volts of AC would cause more problems than a lot of alternator noise. Try putting the black meter lead in the common jack in the middle of the meter. I know technically it is connected to that lead via the shunt of the ammeter of the meter. But just to make sure there is no problem with the ammeter shunt, take the reading again with the lead in the common jack.

No that is volts, not milli-volts.

Switched ground wire to COM port & the reading didn't change.


I put a 1farad cap under the hood today (temporarily for testing) to see if a normal cap might take the noise out.
I tried multiple wiring configurations & nothing worked at all. I also put the alt wire directly to the battery (I read it on an aviation thread) and that also didnt remove the noise.
Also ground the alt directly to the battery & that did nothing either.

Alternator is under 1yr old.


Nobody has ever had this problem before?

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Unless it raises the hair on the back of your neck, its only 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.

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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.

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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.

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Unless it raises the hair on the back of your neck, its only noise!

No popping noises ever.

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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.

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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.

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Unless it raises the hair on the back of your neck, its only noise!

Let us know what you figure out.