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Author Topic: That Pesky Ground In A Car  (Read 5893 times)
W9PMZ
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« on: September 17, 2011, 05:16:48 AM »

So I have been digesting all of the comments regarding the ground in a car.

Currently I am using a pair of 4 guage wires, one goes to the positive connection on the battery, the other goes to the chassis where the battery is connected to the chassis.

So after reading many comments it seems that the chassis is the best place to ground.

So why run the negative the wire, why not just connect the negative wire to the chassis at the installation? 

This is how the mega car blasters are wired...  Actually when i went to the car stereo installation shop they were puzzled why I wanted to waste the money on the extra wire.  I really didn't have a good answer...

73,

Carl - W9PMZ
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K0BG
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« Reply #1 on: September 17, 2011, 08:50:29 AM »

You'll get as mush controversy on this subject, as any wiring scenario. The truth is, the way you have it wired is fine. Using the chassis as a return isn't all that good, and in modern vehicles can cause a ground loop to occur. This in turn can cause a sensor problem resulting in illumination of the MIL or CEL whichever yours has.

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AA4PB
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« Reply #2 on: September 17, 2011, 09:36:30 AM »

One problem with using the chassis for a negative return (ground) from the radio to the battery is that you can't always depend on a good, constant, low resistance contact. Running a heavy wire from the radio negative to the point where the battery is connected to the chassis guarentees that you always have a low resistance connection to the battery.

In spite of what the car stereo installers do, professional radio installers almost always run their own negative return wire back to the battery negative-chassis connection.
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N6AJR
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« Reply #3 on: September 17, 2011, 11:53:08 AM »

The stereo guys aer dealing with audio frequencies, and we are doing RF ,  there is a difference. 

I usually, at the minimum, ground the neg post of the battery at the chassis with a short piece of braid,  ( either store bough or stripped off some coax.) and then I also run a braid from the case or ground screw on the radio to a self tapping sheet metal screw into the floor boards  at the radio.  Then I run a ground braid from the antenna  ( shield side) to a good ground near the antenna mount.

 I had an antenna mounted on the trunk and it would picket fence on me at speeds over 50 MPH. the trunk lid was floating at speed and making intermittent contact through the hinge, ( the trunk lip sits on a rubber gasket all the way around), so a braid from the ground side of the antenna mount to the chassis side of the hinge ( under a bolt with a star washer at the car side of the hinge), got rid of the picket fencing.

 Some folks also run braid from the exhaust system to ground and so on. but RF needs short low resistance paths to ground, and the power needs to come in on low resistance ( that means use big cables) for  good voltage and current, to the radio.

  A good place to find cheap power cables is  cutting the ends off a set of $6 jumper cables or even the longer of the cables designed to go from the battery to the starter. 

They can all carry several hundred amps and are cheaper than buying #10 or #12 wire from a electric store.
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W9PMZ
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« Reply #4 on: September 18, 2011, 06:38:50 AM »

oh the dreaded ground loop...

For a moment assume that the battery and alternator as grounded to the chassis as possible. 

So you install your radio (and maybe an amplifier) and antenna.  Running properly sized conductors (one positive and one ground) back to the battery and the chassis point.

Your radio has a lug to be grounded to chassis.

Your RF system is grounded to the chassis at the ground point where the antenna is mounted.

You have created multiple ground loops have you not?
     - the loop that goes from the chassis tie point at the battery; you have the chassis and the negative wire
        both connected at the radio
     - the loop from the chassis at the antenna ground point back to the battery through the coax

Assume for a moment that you do not connect the ground lug at the radio.  Then should the DC ground lead that goes back to the battery fails, then the only way the system returns to ground is at the ground point of the antenna.

Thinking this through it seems to me that the best chance to minimize ground loops is to ensure that your bonding at the battery is good, the bonding at the alternator is good; and then to use the chassis to connect to at the radio.  This would also eliminate the possible failure mode should the long ground wire back to the battery fails.

As for the comment that this is RF and not audio; I'm not so sure if I agree with this statement unless you are referring to the ground image.  Again, assuming a good bonding of the chassis to the battery and the alternator, DC doesn't care...

73,

Carl - W9PMZ
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K0BG
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« Reply #5 on: September 18, 2011, 07:03:55 AM »

Yes, you can use the chassis, and lots of folks do, not realizing the losses through the chassis are greater than the losses in the wire running to the battery. They counter this by adding; they always done it that way, never had a problem, etc. However, every single automobile manufacturer on the planet tells you to wire directly to the battery, or as an alternative, to the jump point if so provided.

There is a valid argument to be made for connecting directly to the battery's accessory ground point on the chassis. This is in case the battery ground fails. But, if you run high power, the accessory ground wire, and any interconnection through the power train back to the starter ground, will carry the load. Whether this causes a problem is moot.

The reason for the direct battery connection recommendation has to do with creating ground loops between chassis points, where the various sensors are located. Of the highest concern, are the ABS sensors, with transmission control a close second. Suit yourself.
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M6GOM
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« Reply #6 on: September 18, 2011, 08:40:38 AM »

You'll get as mush controversy on this subject, as any wiring scenario. The truth is, the way you have it wired is fine. Using the chassis as a return isn't all that good, and in modern vehicles can cause a ground loop to occur. This in turn can cause a sensor problem resulting in illumination of the MIL or CEL whichever yours has.



Which is completely contrary to commercial radio installations and completely opposite to how 10,000's of times as many commercial mobile transceiver installations are done compared to amateur.

Connect the 0V wire to the chassis near the radio. Commercial PMR VHF/UHF transceivers typically come with very short 0V power wires.

In spite of what the car stereo installers do, professional radio installers almost always run their own negative return wire back to the battery negative-chassis connection.

Not any that I've met who have been trained properly. I'm quite sure you've seen some amateurs who do mobile transceiver installs for a living do that or untrained installers doing it.
« Last Edit: September 18, 2011, 08:46:05 AM by M6GOM » Logged
W9PMZ
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« Reply #7 on: September 18, 2011, 10:24:33 AM »

"suit yourself"

Well, this really isn't a matter of that.  I am trying to understand why on 40m with my amp running why RF is getting in and how to eliminate this.  This is one of the path's I am considering investigating.

I took my car to a very reputable professional auto audio installation dealer here in the Columbus, Ohio area to install the 4 gauge wiring that I have.  They were not going to install the negative lead back to the battery and thought I was wasting money to do so.  Whey they asked why I really could not think of a good reason other that to offer up the excuse that this was RF as previously mentioned.  But after much thought I don't think it really was a good excuse...

73,

Carl - W9PMZ
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M0HCN
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« Reply #8 on: September 18, 2011, 04:40:26 PM »

If you do run wire back to the battery negative, there is one important gotcha....

If you connect this to the battery negative terminal then there is a risk that should the battery negative to chassis strap come adrift and an attempt be made to start the car then the starter current will try to find its way back to the battery via the coax outer, radio and radio negative return.... Smokey at best!

In a high power installation even fusing this connection may not help as the fuse will likely need to be rated high enough that the coax braid will melt first.

UK best practice is to connect the DC return from the radio to the chassis near the point where the battery negative strap connects to the chassis, not to the negative pole of the battery, this connection is unfused. This does make the battery negative strap a common impedance, but also means that this strap coming adrift will not potentially start a fire.

The DC feed can reasonably be fitted with a common mode choke to keep RF off the supply cables (and thus out of the engine compartment), large binocular cores are good for this. 

The standard to google for is FCS1392 (a free download), which also includes the advice that vehicle manufacturers specific advice should be followed if available.

Of course US custom and practice probably does differ, as did ours (used to be the old 2 fuse thing until the standard was revised). 
You pays your money and takes your choice.

Regards, Dan.
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K1CJS
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« Reply #9 on: September 18, 2011, 09:09:49 PM »

....UK best practice is to connect the DC return from the radio to the chassis near the point where the battery negative strap connects to the chassis, not to the negative pole of the battery, this connection is unfused. This does make the battery negative strap a common impedance, but also means that this strap coming adrift will not potentially start a fire....

In a properly done installation, this lead (Negative from the radio to the battery) IS fused.  Also, contrary to your assumption, most car manufacturers recommend running the radio power cable (both leads) to the battery, with fuses on both leads.
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W9PMZ
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« Reply #10 on: September 19, 2011, 03:43:34 AM »

the only consensus is that there is no consensus...

fuse / no fuse (negative lead)
battery / chassis point for battery
ground run / no ground run

take your pick everyone seems to think that the above are all correct answers...

right now i have / no fuse / chassis point for battery / ground run /

73,

Carl - W9PMZ
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K7RBW
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« Reply #11 on: September 19, 2011, 07:13:31 AM »

In my 1-yr-old car, it seems like the car chassis is a very good conductor back to the battery (I did some testing and reported the results in another thread here). I suppose over time, this might change, but for the next several years, it'll probably be OK.

That said, for better or worse, I ran both power leads to the battery with fuses in each.

But, if the biggest reason for fusing the negative lead is to cover the event of the chassis ground failing and some high-current load using the radio wiring (or Coax) as the return path, couldn't that be solved by simply inspecting and replacing the chassis lead so that never happens? Granted on some cars, that can be a tight fit, but if you're worried about smoking your cables, an annual inspection and/or some periodic maintenance and corrosion control routine should be enough to prevent it. (This is probably a good idea whether you run a ground wire back to the battery or not).
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N5XTR
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« Reply #12 on: September 19, 2011, 09:02:44 AM »

This is how the mega car blasters are wired...  Actually when i went to the car stereo installation shop they were puzzled why I wanted to waste the money on the extra wire.  I really didn't have a good answer...

This is not correct.  I worked at Custom Sound Works for many years (one of the top car audio shops in the world, no kidding), I installed hundreds of car audio systems large and small.  Never ever did I or anyone else use a chassis ground.  We always ran  power wiring straight to the battery(s), always fused + and - at the battery and again at the amplifier. 

I think this particular car audio shop is a joke if in fact what you say is true. 
We call these type of shops "Ghetto Audio" for the fact that they has cheap speakers, chinese prefab boxes, and no MECP certification.

No, this is not how mega car blasters are wired.
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K1CJS
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« Reply #13 on: September 19, 2011, 03:36:56 PM »

Also check the literature from the US big three.  GM, Ford and Chrysler ALL recommend running any high current wiring back to the battery--both positive and negative leads.  Fusing both is also the recommended procedure.

The idea is to NOT burden the car wiring with extra current draw.  Considering that that one wire connection point from the negative battery terminal to the body carries nearly ALL the current from the car accessories and sensor/computer systems, any extra current passage may well overload that wire.

Hey, to each their own--but if you don't listen to the car manufacturer, you have only yourself to blame for anything that may happen.
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M0HCN
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« Reply #14 on: September 19, 2011, 05:55:21 PM »

Yep, where there is specific advice from the manufacturer that should be followed.

I would be very surprised to see the chassis bond run that close to overload, simple voltage drop issues make that a bad plan, but still following the manufacturers advice is probably best in the cases where it is available.

Regards, Dan.
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