iPhone ground loop: charger --> USB --> line out --> head unit --> ground
I just installed a new iPhone 4 setup in my jeep (otterbox based custom windshield mount for GPS, music and to shoot video from the windshield while offroading) and have a noise issue during ipod playback. I have a dock connector cable that splits out to usb (for charging) and an 1/8" line out (legit audio line out, not from the headphone jack). My charger is hardwired behind the dash and while trying to find the problem I discovered it stays powered on even when I completely disconnect the ground for the charger and it dies if I unplug USB going to the phone. So, the charging circuit is grounding through the iphone cable, back down the audio line and through the head unit, causing the noise. No matter where I ground the charger it wants to ground out through the audio line. I also tried a Griffin PowerJolt charger plugged into the cig lighter (on it's own circuit) and the setup still grounds through the audio line. Any ideas? This is only temporary while I put my carputer together but I'd still like it cleaned up!!