I just installed the AUX kit in a 2004 Z-4 to listen to a portable Satellite radio.

It worked great for about an hour and then the AUX input on the radio display would disappear and it would switch to an FM station.

The only way to get the AUX mode back was to unplug the audio cable from the Sat radio.

This happened several times after that - sometime immediately, sometimes after 5-10 minutes.

I could recreate the issue using a Motorola Droid phone on internal battery or on vehicles 12V accessory port.

The BMW AUX cable has a small circuit card built inline. The card contains 2 ccapacitor in series with the l/r audio signal which blocks DC and allows AC (Audio) to pass. It also has a 300K ohm resistor on the Radio side of the Caps. I believe that this resistor is what allows the radio to automatically "detect" that there is an AUX input jack wired into the car.

So here is what could be going wrong - 1) There is a ground loop (grounds at slightly different voltages) between the radio and the audio device that at times is sufficient to swamp the very small voltage or current used to sense the 300K resistor. 2) The audio output of the radio/MP# players etc. are designed to drive headphones which have a impedance of 100-300 ohms ish. The AUX line in is likely a much higher impedance so there is a mismatch and the low resistance messes up the AUX sense circuit in the radio and causes it to drop hence the failures when the phone was NOT connected to vehicle power. 3) The radio is expecting a differential signal and the audio devices typically provide a signal referenced to ground and not another signal line.

So what can be done?

The best way is to "Electrically" isolate the audio device from the radio AUX input. If isolated there can be no ground loops or differential input issues.

Transformer isolation is ideal as the circuits are magnetically coupled but not electrically coupled.

I researched and purchased a product made to do this - a "Stereo" Pac-Audio SNI-1 3.5 Ground Loop Isolator. Pac-Audio.com Link The cost was $19.95 at the local car audio shop.
It comes with standard 3.5mm jack so no wiring is necessary - just plug it between the audio device and the AUX input.



So far so good. I drove around WITHOUT it in circuit for about 20 minutes until the AUX dropouts started. I plugged it in circuit and have driven for several hours with NO, NOT ONE, AUX dropout with Sat radio or phone.

There are other makes and models of "Noise" filters, ground loop isolators etc. If you get another make just be sure that it is TRANSFORMER coupled (the KEY) and is not using lower cost RC filtering. Also make sure that you get a "Stereo" version or you'll have to buy two!

Also, I couldn't detect any difference in the sound quality even when I had the convertible top fully closed and my hearing aids cranked all the way up !