ECU interference on door speaker

Huw Powell audi at
Tue Dec 12 13:56:12 EST 2006

> I've just installed a new ECU in my 5000 turbo. I strated to notice
> that I'm getting a lot of interference from it on the right front
> door speaker. I actually had to route new speaker wire near the ECU
> to get to the speakers becasue the car did not originally have door
> speakers. Has anyone else had noise issues with that speaker?

Well, first, it is highly unlikely that the interference is entering the 
system at the speaker level - the low impedance high voltage line would 
not be susceptible to RFI.

It's most likely occurring in a line-level run somehow, or potentially 
through the power wiring.

> I did a little troubleshooting, but now I'm stumped... I figured if I
> disconnect that speaker from the amp, I'd be OK, but it just moved
> the noise to the rear speakers.

It was probably there all along but masked by the door speaker.

> What's also strange is the noise is
> really only audible when the headunit (and amps) are on.

Again, that's becuase it is not being induced into the speaker circuit, 
it's happening at line level and then being amplified.  So the amp has 
to be on for it to occur.

> It'll be difficult to reroute the wire the way it is under the
> carpet. Is there any shielding that might work on this? Thanks.

[insert reply about grounding]

> Thanks for the info. Unfortunately, I already have everything setup
> that way. I have a ground cable directly from the batt to the a
> splitter and then onto the amps.

What you should try, and I don't mean "if it's convenient" is a bit more 
extreme ground management.  All the components of the system should be 
well grounded to one point (like the battery terminal or a good frame 
stud).  Line level cables are vulnerable to creating ground loops and 
picking up noise, and so ideally their shields should also all be ground 
to only one point - usually the head unit, but if there is one compnent 
connected to all the others, it might be a better choice.  For this you 
need "directional" cables.  Not that the signal flows differently one 
way or another, but that the sheild is only connected to the ground of 
the plug at one end.

> The noise really got worse when I
> bolted down the ECU to its bracket near the wire grommet for the
> right front door.

I would also be concerned that the ECU might be an EFI rogue, generating 
a *lot* of noise, (or backfeeding garbage to the power circuits) or 
perhaps it isn't grounded as well as it should be.

> I will try adding a cap to the amps, but I doubt
> this is an issue.

Yeah, that's pretty much irrelevant to my mind.

> The noise comes over loud and clear even with the
> volume at 0. As long as the stereo is on, the speaker picks up the
> noise. Very strange...

It is being picked up in the head unit to amp run, obviously after the 
volume control.  You mentioned a "splitter" as well, I suppose that is 
between the head unit and two amplifiers?

You might also have a compromised line level cable - one that is not 
shielded properly, etc.

Huw Powell

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