ECU interference on door speaker

Huw Powell audi at
Thu Dec 14 01:52:11 EST 2006

> After all the troubleshooting I'm thinking the noise is coming into
> the system by the power leads. The alternator could be the issue.
> It's an Ultranator I bought a while back, but it only has < 5000
> miles on it, so brushes should be good. Ignition components are all
> stock and pretty new. The wires are Prenco brand wiht the loom. I
> don't know if they're noise suppressing.

Still, check the grounds (alt. mounting) for cleanliness.

> So I'm wondering if there are any beefy noise filters for the power
> cables. The ones I've seen in the store are too small... they look
> like they're designed for head units. I'd like to put a filter on it
> at the main line, before the distribution blocks so the noise isn't
> even carried into the trunk, where it might be picked up by the audio
> cables. With 3 amps in there, I think I'd need a filter to handle
> 60-80A, right? Going by the fuse ratings on the amp, the total is
> 120A. Is there a way to homebrew something like that?

I'd try to kill the noise at the source, these cars should not make 
ignition noise that gets into toys via power leads.  I mean, they don't, 
if they're working right.

I once sat on a bridge waiting for an accident to clear, and amused 
myself by "listening" to the ignition systems of cars passing the other 
way.  German cars were all dead silent.  So were most American cars. 
Strangely enough, quite a few Japanese cars were making enough RFI that 
my stereo was picking it up and amplifying it!  In spite of all the 
shielding provided by the car bodies.

Anyway, as far as current, it's highly unlikely you'd ever be drawing 
that much current (try putting in a lower value fuse on the main power 
line and cranking things up to see what size fuse blows).  Power line 
filters are not going to be more than a first or second order filter - a 
choke coil in series, and perhaps following it a capacitor to ground. 
Why not experiment with one you have lying around (oops, I guess I have 
stuff like that), I mean a cheap one, to see if it helps.  If it really 
does, then you can worry about getting one that will handle the current 
you anticipate.

There is also a noise suppression capacitor on the alternator, or should 
be.  Might want to check on that.  And don't assume the brushes, etc. 
are good.  You have to actually check all this stuff, now, now that you 
have a problem that did not exist before, to see what might be causing it.

But anyway, good luck!

(Right now I'm putting some "stuff" in an old Chevy truck.  I can't tell 
if I have ignition noise, the engine is too loud...)

Huw Powell

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