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I experience a bunch of problems with my 2007 Mazda 3's electrical system:

The most annoying issue is that I hear [a rather loud] engine noise when I connect a phone to the AUX (3.5mm audio jack) line while it is simultaneously being charged through a cigarette lighter adapter. The noise goes away when the power line is disconnected. Somehow related to this issue: even when the charging cable is disconnected and the aux input is in use, whenever I change the [automatic] transmission into/from "Parking" mode, the connected phone panics (hangs). It seems as if a surge goes through the AUX line.

Similar threads on this forum [1, 2] suggest ground issues as the possible root cause of these kind of problems. Is there any systematic method to isolate/find the fault?

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    You might have a ground loop issue. You can buy ground loop isolators on ebay for a few pounds which might help the issue... – George Jan 2 '16 at 22:36
  • Thanks @George. I am rather looking to see if I can fix the issue altogether. Ground loop isolator sounds like a great plan B. – Mani M Jan 2 '16 at 22:58
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Your problem could be caused by two things. A shitty car charger, they are not all created equal, or a ground loop.

As far as the charger goes try several different ones, name brand ones tend to be the best (htc, iphone, samsung, etc...). If the problem goes away then it was the charger.

As far as a ground loop, what happens is that the ground potential in the cigarette lighter is slightly different than the head unit. A better way to say this is one has a better ground than the other. What happens is that the device with the worst ground will hunt for a better ground. That ground is found in the device with the better ground and a current develops in the audio cable. This current (regardless of the direction) shifts the reference point of the audio contributing to noise.

A ground loop isolator blocks DC (the bad current) and allows through the AC (the audio).

A car has a star based grounding system. There are like 6 or 7 main grounding points in the entire car. Some are under the hood and some are in the cab/trunk. If you narrow it down to a ground loop issue you can hunt for an systematically clean every ground. Unbolt the grounding point. Clean both sides of the eye and the body surface it bolts to. Apply some dielectric grease to both sides of the eye and the body to prevent corrosion. Bolt it back down.

To find these grounding points you need the vehicles service manual or access to a service manual system. These will have a list of all the ground points and pictures of their locations.

  • Thanks @vini_i. I've tried three different adapters, they all show the same issue. Your explanation on ground loop problem and how to tackle it is great. Do you think the "Shift into P" issue is also related to the ground loop creation? – Mani M Jan 3 '16 at 3:22
  • @ManiM A ground loop only exists when two separate devices are connected like the cigarette lighter and stereo. If your only connected to a single device, AUX but no charger then a ground loop cannot exist. The issue you describe is a strange one, i have no good explanation for it. – vini_i Jan 3 '16 at 12:05
  • thank you so much. I could find the electrical maintenance guide for this particular model online. I will see if I can access to the ground where the lighter is connected to easily. As the radio reception is quite poor, I suspect that the head unit might not be grounded properly as well. I will update this thread with the results. – Mani M Jan 3 '16 at 23:45
  • @ManiM Were you able to solve this issue? – Jérémie Mar 16 at 15:33
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I have the same issue with my 2007, 3 sport; It's a bad ground. I use a ground loop isolator and it works fine now when charging and plugged into the aux

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