This is an old GM Technical Service Bulletin, but it should help anyone wanting to verify that their amp has a good ground:

METHOD OF VERIFYING ' GOOD ELECTRICAL SYSTEM GROUND ' #87-8-139 - (04/07/1987)
VEHICLES AFFECTED: ALL MODELS

When diagnosing electronic systems for incorrect operation, it is often necessary to verify that ground circuits are good. This article is intended to clarify what is meant by the term "good ground" and the preferred tools and methods for verifying it.

A "good ground" is a ground circuit that has a resistance of zero OHMS.

Ground circuit resistance can be measured in OHMS using a digital volt OHM meter (DVOM). When using a DVOM, it must be set on the 200 OHM scale to obtain an accurate measure of the circuit resistance. Many meters have both a 200 OHM scale and a 200 K scale. The 200 K scale will not measure zero OHMS accurately. If you are not sure how the meter is to be set for the 200 OHM scale, refer to the meter operating instructions for proper settings. If the meter is an autoranging or self-scaling meter, read the meter carefully to be sure which scale it is setting itself to.

Before measuring resistance in any circuit, the resistance of the meter should be measured by touching the leads together. A meter with a good battery and leads in good condition will read less than .2 OHMS usually zero. If the leads measure anything more, an accurate measure of the circuit resistance may not be possible.

Always remember - resistance cannot be measured accurately on a "live" circuit, All current flow through a circuit must be stopped by disconnecting its power source before measuring resistance.

Ground circuit resistance can also be checked by measuring the voltage drop across the circuit with a DVOM set on, the 2 volt scale. The voltage drop will be zero across a "good ground" circuit.

Remember, fully understand a meter's functions before using it!

To add to this, a good ground for car audio applications will have a return resistance reading of 1/2 ohm or less. I have yet to have a return reading of 0 ohms. If a ground return reading cannot be made to get below 1/2 ohm by means of the "BIG 3", then it is adviseable to ground direct to the battery. Electricity is an algebra equation, what you do to one side you must do to the other. Pay as much attention to the ground wire as you do the power wire.

The BIG 3 is a great place to start for a good ground, however it is the assumed proper method of grounding. What we are talking about here is the older and wiser 4th brother to the BIG 3 (the BIG 4).

So a proper ground wire will be as follows.

- clean of residue and paint.

- secure.

- have a resistance return of 1/2 ohm or less.

- be of adequate guage to carry the return as compared to the power wire.

To simplify the measuring of the return, use your meter as described. Disconnect the - battery terminal and disconnect the ground wire from you amp. If your dmm probes are not long enough, you will need to create a jumper extension out of some primary wire or whatever wire you have handy. Measure this wire for any resistance reading and subtract it from the total.

Many installers are not aware of this nor practice this method. It takes time and time = $ so don't get all pissy if you had a professional install done and this was not checked. A poor ground connection or high resistance reading may seem trivial under no load, but once you are pounding your nice new amp and it is drawing large amounts of current, this little reading has become a monster reading that has caused many an amp to fail for no apparent reason. It may be noticeable as a extremely hot running amplifier in a short time period, poor output levels or diminishing levels and of course a blown power supply or output section in the amplifier.

While the original article was written for the years gone by, it still is applicable to the newer generation of vehicles. A good ground is not about the amount or size of the metal in the return to the battery but about the resistance through it. Todays vehicles are a combination of metals, spot welds, glued together unibody panels and isolated chassis components. The return through these components is where the resistance reading comes into question and this is what we need people to understand, why the BIG 4 needs to be done if the BIG 3 does not solve the problem.

Thanks Rob (Forbidden) For making this up.

it talks about wire size, but it shoud also be clear that the ground wire should be the same guage as power wire, that way combined with the above process your pretty much guarenteed the best ground.

he does mention it.

And I mentioned it again here in point #4.

So a proper ground wire will be as follows.

- clean of residue and paint.

- secure.

- have a resistance return of 1/2 ohm or less.

- be of adequate guage to carry the return as compared to the power wire.

As Deva pointed out it is mentioned already. Not obviously stated though.

Seriously the best ground you can get is your battery ground terminal!! This is why MSD ignitions say always earth your MSD to Battery Neg--, The minus terminal on your battery....Obviously if your running a big amp or something that draws a lot of current use decent size cable and remember the most important thing if you don,t know what you,re doing!!! ASK AN EXPERT, not just ya mad mates....Or else you may burn your car to the ground...

OOHHH YEAH and ALWAYZ ALWAYZ use fuses!!!!!of the right amperage{not always 25amp!!!}

so what your saying is you shouldn't twist your earth wire around the nearest bolt you can find... then why do so many cars and things I work on have this fitted... I know it's because people try to fix it, when they can't then they bring it to the autoelectrician to figure out the mess they have made... 90% of the time people would save themselves alot of money if they just go to an expert first...

as 2muchofthiss? already said....

There's nothing neccessarily wrong with using a nearby bolt - it will certainly be better than running your ground all the way from the boot to the battery. The ground should be as short as possible.

As long as the bolt is connected to the chassis, clean and void from paint, and nice and tight there should be no problem. Of course the wires should be terminated with O rings or similiar, not just wrapped around a bolt..

ROCK ON
R~R

a standard ground wire is NOT designed to carry big sound system in your car so its a good idea to add some ground wire from a battery to the body

Holy Shit, that's just overkill...

Seems stupidly ineffective to me actually.

If they were all designed to ground the battery, why run them to the other side of the car? Why not next to the battery where they should be. Seeing as we all know the grounds should be as short as possible.

And that fuse seems a tad to far away as well..

Anyway

ROCK ON
R~R

reason why its on the other side because its connected to the alternator.. i dont know about the fuse but i know theres another 100A battery in the trunk where the spare wheel used to be..

yeh as rookie said, i have my amp in the boot and there is only one bolt there that is able to hold the O ring. and its not in a good position in the boot (right at the front middle) is there any other way apart from having to run the ground all the way back up to the battery?

You'll have to look under panels and carpeting, there will surely be other bolt you can use. Just be sure to scrape away any paint so you have a solid connection.

mm i've had a look, still can't find a bolt (taken off light panels, jack panel, spare tyre) i'll just run the ground underneath the carpet and pop it out beside the bolt when i get the new plywood and carpet for my boot

Again, a bolt may not necessarily be the best mounting point. This is not hard to understand at all. What is on the return path from that bolt to the battery is the problem. Measure the return to find out if it is good or not. Take that nice close bolt, scrape away the paint, use a grease to keep it from rusting, do whatever you like to it. Now what happens if that return is through a spot weld from the nut the bolt is attached to, then though a spot welded and glued together unibody panel, then to some tiny wire upfront?

Who says a ground wire has to be as short as possible? I 'd like to meet the person who first said this. If all things being equal the ground wire must be as short as possible, then so must the power wire. You do to one side like you do the other. If your ground point is the closest bolt there is and the return is a low resistance, sure go for it. DO NOT ASSUME that it is good as you will most likely be mistaken.

Resistance on a ground return is this simple to understand. You go to your favorite restaurant and pound down as large a meal as you can. Then someone sews you butthole shut, get the drift?

make your own ground..dont just rely on bolts that are already there....

grab your O0ring terminal..now grab a bolt/tek screw/whatever, that is a tiny bit smaller than the 0ring... now drill a hole in the metal of the car.. always..and i repeat ALWAYS double check to make sure you not goin to br drilling into fuel tanks.lines, brake lines, or electric cables..personnally, i dont think seat mounting bolts are good enough. then sand back the spot to bare metall..than attach teh oring in your predetermined fashion..then go over the top with a clear coat, or grease, or whatever, to stop rust.

now,fo the reason why running a single wire to the battery isnt effective....
think about this..you run the wire..fairenuff you think because its direct, its better?? noooo.. you only have the amount of wire in the cable carrying the return current..

if its mounted to teh chassis, what have you got?? an entire cars worth of metal....much more effective, much less resistance...

Which then puts you right back at square one. Instead of assuming that it is a good low resistance ground return through the chassis, meter it to find out for sure. 99% of installers do not meter the return, they assume it is good enough. A good installer does not assume, they verify first and then proceed after informing the customer and allowing the customer to make the call.

isnt that gonna defeat the purpose of scraping back the paint in the first place? Just cause its clear doesnt meen its not there.

i dont know just a though??

no. you attach the ground, and then go over the top of it....

like i said....

oh, and i know its a late reply, but in regards to what Akshunhiro and Rookie said about that groundning kit being overkill, and stupidly ineffective, ill tell you why it was done.

like i said in my pervious post, the whole chassis of teh car is essentially negative. so rather than have one wire, at one side of teh car conecting to the battery, you want multiple points for the current to return to teh battery.

and not all of those are just going to the chassi. in most of these groudning kits, there are wires grounding to the engine block, to the inlet, to the injectors..

improving the ground to all of these improves flow of electricity, and as such, improves power, and fuel economy, and engine response....

Sure - but do you really need 4ga for them? Moreso on the overkill. We all know cleaning and upgrading grounds can do wonders, but I still stand that the size of the wire used was a waste of money. If the factory decided that Xga would suffice, then I don't see the need to quadruple the size, particularly on what appears to be a stock engine with a medium sound system judging by the fuse holder.

*shrugs*

ROCK ON
R~R

your are right...

it all depends on the system its running...

no point doing it if you have a tiny 2 channel just running some splits lol

theres never overkill in auto electrics!!

best way to ground amp:

http://www.youtube.com/watch?v=xVyLDvRAzds

pretty sure most people seen it but for you noobies out there:)

so why do big name companys sell power kits that has all the fuses and wiring etc etc and the ground cable is always shorter than the power cable???