Alternator noise - 12v or ground?

Nickshu · Nickshu's Garage

Long story short I put a Bluetooth audio unit in my 996 and I get some electrical alternator noise at all but idle. I wired the unit to the unused factory phone harness. Does this kind of noise usually come from the 12 volt side or the ground? My thought is I need to change one of them or both to try and get rid of the noise. This is more of a general wiring question than a Porsche specific one, it just happens to be on a Porsche so I figured I'd post it here to see what the Brain Trust has to say . Thanks in advance!

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Hendog · Hendog's Garage

The noise could be due to a myriad of reasons. In my youth we would install a 10A noise suppressor in line with the power source close to the affected device. These filters are still available and cheap ($10-15). https://www.jaycar.com.au/10a-dc-noise-filter-improved/p/AA3074

For more information on the subject I found this. I read most of it and it's bettr than I could do to explain the issue. https://www.crutchfield.com/S-JY9PcbDJHR2/learn/learningcenter/car/noise_suppressors_installation_guide.html

I hope this helps you solve the alternator whine you're experiencing.

John Rogers · 06-10-2017, 04:52 PM

I installed a Pioneer flat screen GPS unit in my Chevy HHR and was getting the same "whizzing" noise and sometimes it would be constant, sometimes vary with engine speed. Sometimes it would not be there for weeks? I ended up doing what the second poster suggested and it worked. Sometimes it does come back and the shop I bought the unit from said to pull the head unit out and open and reclose each RCA plug, twisting it as I reassemble. That fixes it for 6 months or so.

The stereo shop I bought my unit from carried the filter.

Nickshu · Nickshu's Garage

Thanks guys. Today I tried moving the ground to a better location, but this did not help the noise. I previously had it grounded on via the unused phone connector, now it's a direct chassis ground at a factory ground point nearby, less than 6 inches away from the unit. So the ground is ruled out. Now for the power side.

The problem I am pondering with a noise suppressor on the 12v line is that this unit has two 12v inputs, one constant and one switched power. So do I need to install two noise suppressors then? (one on each line)? TIA.

Hendog · Hendog's Garage

I would start with the constant source first. This is the source for all your current draw, whereas the switched is just that, to turn on/off the unit. If it clears up then you're done.

rusnak · 06-11-2017, 10:08 PM

When you say Bluetooth audio unit, do you mean you installed a new head unit, and connected the factory phone to it for power?

Try re-routing the power to a different source, or to the fuse panel directly. Also make sure the audio cables are not twisted around a power cable. Sounds like voltage interference to me.

Nickshu · Nickshu's Garage

Quote:

Originally Posted by Hendog

I would start with the constant source first. This is the source for all your current draw, whereas the switched is just that, to turn on/off the unit. If it clears up then you're done.

I will try moving my source directly to the fuse box. I pulled the 996 wiring diagrams and saw that the phone prep connector (where I currently am taking power from) does not go directly through the fusebox but rather goes through a control module.

Quote:

Originally Posted by rusnak

When you say Bluetooth audio unit, do you mean you installed a new head unit, and connected the factory phone to it for power?

Try re-routing the power to a different source, or to the fuse panel directly. Also make sure the audio cables are not twisted around a power cable. Sounds like voltage interference to me.

It's a Dension Compact BT unit. It uses radio or aux output to allow you to sync your phone and audio streaming to the head unit. This particular year 996 has the Bose "MOST" fiber optic audio system, so changing it to a modern head unit is complicated.