yeah it's alternator. you either have a ground loop or something else is wrong. what kind of headunit?

 

Pioneer?

Most likely cause is a bad ground somewhere, either amps, HU, or DSP. If you're using the factory ground for the HU then I would suspect the amps or a DSP/Crossover/EQ first. UNLESS YOU HAVE A PIONEER HU

 

Could be 1 of many factors. The most common cause is a bad ground.

 

I recently found out that my amp is picking up the alt noise, after fighting with rcas and re-routing, re-grounding... The PG amp I put in it's place is dead quiet. I'm not sure if the amp is broken.
Consider this as the final option too, you will need another amp to substitute it for a test.

 

all amps and processors are grounded to the same point in the trunk. The HU is grounded elsewhere, its the stock HU.

Would any one piece of equipment in the trunk add to the noise even though they are all grounded to the same point? Just the alt noise, not talking about other kinds of noise.

 

I would ground each amp seperately. Processors are okay to add on same ground since they don't draw much power.

 

Grounding at seperate locations will give the greater chance for ground loops. There's no reason that the amplifiers cannot chare a common SOLID ground point.

Chad

 

Alot of possible causes. Might be using low quality material RCA. If using Pioneer HU, first to check its earthing.

 

Im using a 8ga to 4ga dist block for grounding the amps.

 

You need to check the ground on the metal is good.

Where did you ground?

Did you sand away paint and clean the surface?

 

Another easy way to test your audio ground would be to grab a length of 4 ga. wire (or larger) and tie all the grounds back to the battery ground, bypassing the chassis. This method can also be used to test different chassis ground points.

 

this ground is as solid as they get. its stripped to the bare metal and held down by the seatbelt bolt. Its a known good ground in this car. I have also tried other grounds as well.

so i assume i should ground the HU to the same point and try grounding the RCAs as well? those are the ways to eliminate alternator noise, right?

 

How are you getting signal to the amps since you have the stock HU?

 

This particular stock HU has preouts off the back.

 

Really? What kind of car? Never seen a stock HU with RCAs on it.

 

It dosent come with RCAs out the back from the factory. But it has line outs for the stock bose amps and speakers. you can remove the stock harness and replace it with a harness that gives you RCA line outs so you can bypass the bose crap. Its good, but not great. 0.5volts and a flat response.

 

Sorry to bring this back up...

I've had some bad noise in my system (E36 BMW) and since I have the car half pulled apart I want to find it and squash it. The pitch is RPM dependent and I don't hear it at idle, only when on throttle. It is a constant volume though so if I turn the system up I can't hear it.

PO installed the system. Eclipse 5425 HU, cr@ppy RCA cables to the amp(s), power and ground tied directly to the battery (battery is in trunk of car), cr@ppy zip cord speaker cable to front components.

Since I bought the car and looked at the system I've been perplex why someone who bought a BMW M3, installed a moderately expensive HU, installed a very expensive amp (PPI Pro 650), and installed good speakers would cheap out on all the cables! It looks he/they tried toget rid of the noise because there are "black boxes" on the ends of each of the RCA preouts to the amp. They are unmarked though so I can't be sure what they are...but they aren't bass or treble blockers.

Now my advice would probably be to replace the cr@ppy cables and see what it sounded like, but I really don't want to pull all the cables. What are the odds that it is the cables? I see lots of references to grounding. Is grounding directly to the negative terminal good or bad?

 

You don't have to pull the primary route, you do have to run a second set { tempoarily to rule out the possibility }.

Right over the seats would be fine, this is only a test

 

That is a good idea...I'll have to see if I have any sets of good RCAs left. I kept using the good stuff and then selling the vehicle I put them in...I'm left with nothing. What is a good brand? I used to buy Monster and Stinger. What is a good value cable on the market now a days?

What is a more plausible path for interference... RCA cable or speaker cable?

 

What is a more plausible path for interference... RCA cable or speaker cable?

RCA cable...

check the batt ground while you are at it, or even go ahead and upgrade it with some 4ga..

 

Engine RPM dependent noise can also be a problem with the ignition system. The best way to isolate ignition vs. alternator noise is to remove the belt that runs the alternator.

It is often a belt that runs many other things too. Just remove it and start the engine and test for noise. Don't run it over 60 seconds or so since the important fluids are not being pumped.

No noise when the belt is off ... alternator noise. Still have noise ... ignition system or some other more evil problem with the engine management system.

 

had a very similar problem that turned out to be ignition coil failure. noise didn't increase with volume but did with RPM's. The noise was "raspier" than an alt whine.

 

A quick check if its alt is to use a dvm and check for ac voltage. If there's some present then there's an issue with your alt.

A band-aid fix is to get a cap to filter out the ac voltage.

 

All alternators have some AC output, the rectification is not perfect because it is only the sum of many overlaid "positive direction" AC waveforms. The best test is to use a "true RMS" meter since the math used to evaluate AC voltage inside the meter should at least give you a real AC voltage on the screen if there is worrisome levels at the battery. Certainly an alternator with a bad stator winding or a open or shorted diode will put out a substantially higher level of AC than a good one. I look for over 1 volt AC at the battery terminals with a true RMS meter. In my experience bad alternators have way over that, and good ones are way under so it is a "go no-go" test.

Also, in my direct experience (limited) poorly grounded audio devices can have alternator noise even from the tiny amount of AC that comes from a properly working alternator. Hence all the properly guided advice to diagnose and solve grounding issues to address the noise in this thread. Anecdotal evidence from threads that state that the audio noise is still present after alternator replacement in several threads I've read also support this.