Bad earth? Ground loop interference.

[clipper]

I have got rid of my ugly modern Bluetooth stereo and installed a "period" Blaupunkt radio/cassette which I have modified to accept a 3.5mm stereo aux / line in for my phone.

But when I connect my phone to a power supply (from the ciggy lighter) I got a terrible ground loop whine from the alternator.

I've had this before on other vehicles and normally this is a simple fix by feeding the phone power supply from the same point as the stereo power (or at least making sure that they are on the same earthing point).

But I have connected a ciggy lighter socket to the stereo switched live feed and earth and it still has the ground loop.

I think this suggests that the earth (brown wire on the BMW stereo loom) is poor. This is backed up by the fact that at tick-over, the lights on the stereo panel flicker a bit, suggesting that the power supply is not as good as it should be.

Where does the brown wire actually earth to the chassis?

Is there an alternative earth (to a different point on the chassis) I can take for the stereo and phone charger close to the centre console?

[Satan]

I usually fit a 12v to 5v unit with a couple of USB ports to a car, as you rev the car does the whine increase in frequency, sounds like EMF interference disconnect the aerial lead to see if it still happens and may need a ferrite core somewhere along the line.

[clipper]

It is definitely ground loop because it is only present when the phone is plugged into the charger. No interference when music is playing from the phone with just the audio line plugged in, and no interference on the radio signal either.

I could just put a ground loop isolator in the line, but that isn't fixing the issue, its masking it.

Where do you earth your USB 5v power adaptors in the E30?

[Satan]

The adapter just needs a live and neg, no earth, it may be worthwhile checking your main earth in the car.