Amp grounding point in trunk of M Roadster

[elerner61]

So, I'm troubleshooting a small amount of alternator whine in my almost finished system (JBL MS-8, Arc 805XDI in trunk). Think I've got it pinned to a cheaper RCA interconnect I used for the sub between the MS8 and Arc amp. I disconnected it and the whine disappeared. I just have to test it with another better quality one I've got l stashed away.

This did get me to thinking about my ground point for my new electronics in the trunk which is direct to the battery ground. I've done this on systems in my Miata's with no problems.

It seems that this is a bad grounding strategy in BMWs per this post http://forums.bimmerforums.com/forum....php?t=1347578 which has very good information.

So my question is, to others that have grounded electronics in a M Roadster trunk, what existing chassis bolt did you use?

[Jumbosock]

Whichever one you use, make sure you scrape off any paint or coating to get a good metal contact. I usually start off with a flat head screw driver for the scraping and then finish it off with some sand paper.

[elerner61]

Thanks. I've done sheet metal screw grounding on earlier installs but if you read the post I linked to this is frowned upon. I know many do it this way...doesn't mean its the best/proper way. I'm always willing to try and learn something and do it right. My guess is that the sheet metal screw approach is prone to loosening up over time which would give an intermittent ground and fry some electronics. I'll first hunt around the battery area for a factory bolt that'll give me ground.

[Jumbosock]

Sheet metal screws don't get anywhere near my car! I didn't even know people would do that. I was talking about an existing bolt! The size of the bolt is usually a good indicator that it is a good ground since big bolts are likely connected to a nice piece of metal. I didn't see your link because it's blocked at my work.

Originally Posted by elerner61

Thanks. I've done sheet metal screw grounding on earlier installs but if you read the post I linked to this is frowned upon. I know many do it this way...doesn't mean its the best/proper way. I'm always willing to try and learn something and do it right. My guess is that the sheet metal screw approach is prone to loosening up over time which would give an intermittent ground and fry some electronics. I'll first hunt around the battery area for a factory bolt that'll give me ground.

[elerner61]

Sorry, no harm meant. Links blocked in your office browser? Wow, I thought my firm was strict.

[Shipkiller]

See these pages: http://www.shipkiller.com/AudioSystem.html

The grounding points are there.

[RichardTS]

Originally Posted by elerner61

So, I'm troubleshooting a small amount of alternator whine in my almost finished system (JBL MS-8, Arc 805XDI in trunk). Think I've got it pinned to a cheaper RCA interconnect I used for the sub between the MS8 and Arc amp. I disconnected it and the whine disappeared. I just have to test it with another better quality one I've got l stashed away.

This did get me to thinking about my ground point for my new electronics in the trunk which is direct to the battery ground. I've done this on systems in my Miata's with no problems.

It seems that this is a bad grounding strategy in BMWs per this post http://forums.bimmerforums.com/forum....php?t=1347578 which has very good information.

So my question is, to others that have grounded electronics in a M Roadster trunk, what existing chassis bolt did you use?

There seems to be a lot of confusion concerning grounds, particularly the negative battery terminal with people implying that somehow the alternator ground is different. Not so (there can be a potential difference between differnt chassis point but that is a whole other topic). As a matter of fact if there is a potential between the alternator ground and the negative battery terminal you have other problems.

I will submit a question to all of you. We all agree that we need a ground. saying the alternator ground is different might make someone think that you could actually run something without the battery connected. The negative terminal of the battery must be connected because it is system ground. Disconnect the negative terminal and tell me what stays running. And that is my point - the negative terminal is the system ground.

The true bottom line is you should use what is called star grounding. Star grounding is grounding all pieces of the audio equipment to the same point. As long as you do this, you will be fine. If there is a whine or static after you have done this then interference is entering the circuit and it is not a ground issue.

[Shipkiller]

If you are getting a alternator whine, then you might have a ground loop and need an Isolator. I had to do this for both of my Z's when I ran the cable for my Ipods AND connected them to power on the fuse block.

The grounds in some audio equipment is different than chassis ground and at a different potential.

[RichardTS]

Originally Posted by Shipkiller

If you are getting a alternator whine, then you might have a ground loop and need an Isolator. I had to do this for both of my Z's when I ran the cable for my Ipods AND connected them to power on the fuse block.

The grounds in some audio equipment is different than chassis ground and at a different potential.

If the ground is different than the chassis ground then there must be a resistance of some type between the ground inside the audio component and the chassis. Not sure why any manufacturere would do this because it would, in effect, cut the available power from the power supply.

Do note that a multimeter can give some interesting results which may or may not be accurate. If you measure the potential between the ground at an audio component grounded to the chassis nearby which is located pretty far away from the negative battery lead, it is quite possible to measure some potential between them. When you measure that potential, you should also measure what resistance is present as well. It all must follow Ohm's law.

[Shipkiller]

Ground is not always zero (0) volts. It depends on how the power supply was designed. I have some equipment that chassis ground is -21volts and not hull ground. I have to take this into account when doing voltage measurements when troubleshooting.
CRT televisions use 70v as chassis ground, and if you don't know THAT tid bit of info, it will bite you, literately, when your hand is on the chassis and you touch earth ground..... its the reason you need an isolation transformer to work on that stuff.


Not saying this is the case on the BMW amplifiers and head units, but my alternator whine when away when I installed ground loop isolators on the audio lines. If the Ipod audio was connected and not power, then no whine...
A ground loop isolator is about $15.00.

[RichardTS]

Originally Posted by Shipkiller

Ground is not always zero (0) volts. It depends on how the power supply was designed. I have some equipment that chassis ground is -21volts and not hull ground. I have to take this into account when doing voltage measurements when troubleshooting.
CRT televisions use 70v as chassis ground, and if you don't know THAT tid bit of info, it will bite you, literately, when your hand is on the chassis and you touch earth ground..... its the reason you need an isolation transformer to work on that stuff.


Not saying this is the case on the BMW amplifiers and head units, but my alternator whine when away when I installed ground loop isolators on the audio lines. If the Ipod audio was connected and not power, then no whine...
A ground loop isolator is about $15.00.

I just remembered how right you are. I have some older fully grounded high end audio equipment connected to my surround sound receiver which is also connected to my Uverse box. Both Uverse box and receiver float. After a lightening strike I learned that under these circumstances, lightening travels right down the ground in the phone wire, out through the HDMI ground sheath to the receiver, out of the receiver on the RCA ground sheath to the fully grounded audio equipment, whereupon entering said equipment the ground traces on the circuit boards are vaporized.

Subsequent investigation revealed the ground sheath on the Uverse box is sitting at a delta 40v from the house ground. The only fix is a cheater plug on the audio gear to float it as well. I don't like that because I KNOW one day I am going to light myself up on the volume control.

So I stand corrected. I hope that doesn't start going on in cars.