I measure from a known good ground and read resistance. Because of the way the meter reads resistance, if the resistance changes between the known good ground and the test point the potential is different. Indicating the test ground is highly suspect.

Note: I've not tested this as a method of correcting groundloops.

 

http://users.adelphia.net/~sguest/gndtss/pages/s1_agnd.htm

edit:
Audio ground is the reference by which the audio is amplified. All analog audio circuits establish a zero volt DC reference used as the circuit ground. Audio ground can be established between two pieces of audio equipment in two ways.

Via the interconnected (common) AC ground.
Via an interconnected (common) shield or signal ground. The interconnection of a signal ground between units forces them to be the same "reference".

 

While this is almost a decade old, it should be noted that sage advice is not subjected to fashion or trends.

 

I'm not asking what a ground is, I'm asking for the procedure to test for good ground with a DMM. In other words, if my ground location for my amps is the floor in the rear of my truck, do I connect one DMM lead to the grounding point and the other to the negative battery terminal? In other words how do I make the connections, etc. to properly do the test?

 

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Set DMM to resistance (ohms).

You'll need some extra length of wire to reach from where your ground is to the battery.

Now, take one lead of the multimeter (doesn't matter which one) and touch it to the ground location. Take the other lead, use some sort of wire or extended leads, and attach it to the negative battery post. Let us know what reading you get. You should be getting below .5ohms. That's a good starting point. Depending on your wire, you might have some resistivity in there.

Also, make sure you touch the two leads of the multimeter together before testing so that you can see what the resistivity of the DMM itself is. For example, touch them together and you may get .1 ohms. Then, whatever your ground spot reads, take .1 ohms off of it.

 

Thank you bikinpunk, exactly what I wanted to know. By the way, I laugh everytime I see that cowbell thing at the bottom of your posts. That was the funniest thing I ever saw on SNL.

 

I was told not to use the DMM for that. The harder you press with the probe/lead, the better reading you get (and its not very repeatable). I was told that there were certain areas of the car that were pretty solid that you could go wrong (I like to use the seat belt bolt for example... designed to support hundreds of pounds if not more in a collision.

juan

 

Does anyone want to comment on the reliability of finding a good ground this way?

 

I did exactly as Erin posted.Set the DMM to resistance/ohms.Set it to the lowest number(probably 200).Subtract whatever value you get from touching probes together.I took it a step further and read the voltage directly at the battery(12.5 on mine)and then tested the voltage at my potential ground spots.Put the positive probe(attached to jumper wire)on the positive battery terminal and touch possible ground areas.Looked for the same voltage reading I got at the battery.So if the spot was reading 0-.3ohms and 12.5 volts I figured it was a good ground choice.I have zero problems with system noise.

 

It works well, with 2 caveats.

1. Don’t use a long piece of wire. Just buy the extra long set of lead wires from Fluke, or whatever company you want. They aren’t expensive, and they eliminate a lot of headaches. You’ll find plenty of uses for them, along with a full set of alligator clips and all the other accessories.

2. Only certain multimeters are actually sensitive enough to make this measurement with the required accuracy. Fluke makes a few different meters that can actually measure ultra low resistance, and those are the models you want to use. Others may work, but they may have too large of a margin of error to use reliably. Luckily, one of the most common, and affordable, models of Fluke DMM, model 187, is able to make these low resistance measurements.

Here’s some reading from Fluke about the topic. It walks you through exactly how to make the proper measurement, including compensating for the resistance of the lead wires.
Low ohms measurements you can trust

 

GOOD GOD! Those are expensive.

 

Unless directly grounded, the first ground that you want to check is the alternator to the negative battery post. OEM alternators are not directly grounded, instead they are grounded from the bolt/ engine. To test your resistance you can connect one terminal to a clean point on the case of the alternator to the battery negative terminal. Just to give you a good reference point to work with. Its also always good to run a ground between bolt/engine to the body or frame. To test an exact point use a large gauge wire connected to the negative on the battery 5ft and clip the other terminal to your desired ground point (clean/ paint removed). Also, it's not a bad idea to connect a large gauge under the car between the body and frame. When grounding its best to use a solid aluminum bolt if you can find one, otherwise, brass is good, zinc plated also works ok. Di-electric grease is also helpful to use when bolting down a ground. The grease is pretty cheap at autozone. It's the same stuff used for spark plugs. Also, star washers to "dig in" to the metal is preferable. I decided to run extra grounds from the battery with 4 AWG tinned OFC to different parts of the engine bay, block, manifold, body, etc.

 

Unless directly grounded, ...

What does "directly grounded" mean?

 

Sorry, "directly grounded" cable from external to battery. Normally using a Lug attached by one of the alternator bolts "directly" to the negative battery terminal.

 

That makes sense now...
I was thinking you meant absolute ground.

Which I usually think of when people say clean the connection to bare metal...
(If it started out encrusted with dried mud and earth... :laugh: )

If one has a clamp amp meter then they could just measure teh voltage at the amp's - terminal and with respect to the "direct ground" and that would be enough to work it out...

 

I understand, I didnt really clarify. OEM just bolts the alternator to the engine thus grounding it through the iron (older engines or larger high power engines normally) or aluminum block to the battery negative from there. I dont recommend running a ground cable from the amp to the battery though either. Thats a great way to destroy your battery. The body, frame, subframe work as great buffers for any type of power spikes which will occur from I high power amp. Also, even though the metal of the car isnt as highly conductive as a copper or aluminum cable, given the amount of metal, there is no larger gauge than an entire car. My batteries are expensive and so is my new alternator, the last thing I want to do is put them at risk or cause premature deterioration. Theres a reason that all car amplifier manuals (at least those Ive seen) say geound to the car.

Probably the best bet is just help out the car conductivity. It'll help the engine run smoother, can increase performance, and improve an amplifier which is correctly grounded.

I added multiple 4 gauge ground points from the battery to the block, manifold, body, and frame. If you search online thers a good deal of point were you can connect different grounds. The spark plugs will love it and I noticed a difference in the engine immediatly. Also, its a good idea to run a couple of large gauge (short as posssible) cables underneath between the body and the frame of the vehicle. Of course I do mean %100 copper. Oxygen free isnt that big of a deal, but tinned will probably help given the environments that those extra grounds will be connected to.

Its minimal work and cost to improve the overall life and performance of the entire vehicle and electrical components. I did all this before my stereo system was even a thought, just to insure my engine and components were running as smoothly and proficiently as possible.

Sorry, went on a tangent, but I read about the comment on connecting the ground directly to the battery. I like my audio system but it would be pointless to me if it was causing my engine to misfire due to a current change or cause the early demise of my battery, alternator, or any other aspect of the electical systems.

On a lighter note, easieat way to tell if you have ground problems, when you open or close the door from the outside and you constantly get shocked?.

 

The body, frame, subframe work as great buffers for any type of power spikes which will occur from I high power amp.

By "power" do you mean "watts" (a function of voltage, current, resistance and time)? What exactly does "buffer" mean in this context?

• Attenuate (reduce the magnitude of the value of a "power peak" associated with a spike)?
• Delay (increase the time between the spike in power demand at the electrical component/amp and the electrical source/battery)?
• Or ...?

And whatever the intended meaning of "power" and "buffer", is the difference between the properties of a relatively small x-section highly conductive wire and a relatively large x-section less conducive chassis really meaningful in this context when the resistance of the circuits (measured in ohms) is equal?

Honest questions, enquiring (non electrical engineer) minds want to learn.

 

There's nothing wrong with grounding the amp at the battery.

 

So you are saying that 0.01 ohms through the chassis is the same as 0.01 ohms through a copper wire?

 

Grounding to the battery creates a primary groung through your amplifier. Any vehicle subsystem, including the alternator will take the "easiest" path, which could end up being your amplifier. Vehicle and audio engineers for years have always recommended grounding to the car for this reason. If you are dead set on doing this, at the very least fuse your ground to help avoid a fire. If any priority vehicle ground comes lose or corrodes it will travel through the amplifier. Just not worth the risk. Your car is a massive electrical system. Think about every electrical system you would be putting your amplifier in front of including your ignition. Its not worth putting your vehicle or life in harms way because you want to run a cable. What if that one cable takes on the ground from the entire vehicle un fused.

Also, Grounding straight to the battery provides an excellent means by which to add extra noise. Its not electrical engineering, that parts already been taken care of by the manufacturer, its common sense. Its also the same reason when jump starting another vehicle you are supposed to use a ground for the saftey of your own vehicle and its battery. One cable is not designed to be a ground for your entire vehicle.

 

Thanks.

FWIW I don't have any audio component > direct-to-battery ground cable(s), all my audio grounds are to the chassis near the components (installed under my rear cargo floor) with no problems; I'm not driven to change that for any reason.

Just asking to better understand your point, so thanks again for the further explanation.

 

No worries, i just dont want anyone to be in danger or leave a chance at damaging anything. I wouldnt know myself if i havent thought about it.

 

Grounding the amp at the battery is completely unnecessary and why go through the hassle of running a long thick gauge cable back through the firewall,etc.Plus it's always better to have a short ground(I keep it under 3ft).

 

It's not necessary but it won't hurt anything either.

 

Its not necessary to put fuses on anything in your car either, and honestly it wont hurt anything. I wonder we car audio installers and manufacturers do it then. Hmmm, maybe its for all the "what ifs". Just because it wont hurt anything "so you say" at the time, besides the possible horrible ground loop you just caused, doesnt mean it cant in the future. I dont really like the idea of "nothing caught on fire that time", it must be fine". We spend way to much money on our car and audio equipment just to leave it to chance when there are known ways to help avoid issues.

 

Lol..

Sent from my SM-G950U1 using Tapatalk

 

Here is a link somewhat explain what Jackal28 is talking about, starting at the 2nd paragraph--

"Battery Grounding or Negative Battery Supply Connections"

https://www.w8ji.com/negative_lead_to_battery.htm

 

Is non-tragic good enough for everyone. Your cable melts, your amp frys. By the way, the ground of every electronic in your vehicle is to the metal of the car. Pick up and look under your battery. The only thing I have ever heard of safely grounding directly with a cable to the battery, which is rarely done unless its high output, is an alternator. Given that this is the only device that provides electricity, it would make sense to prioritize that ground over all others. This is not scientific guys, it's common sense which we seem less and less of in this world.

Don't even get me started on extremely low probabilities and rare occurrences which happen every day.

 

We've got a massive thread on those things on the first page, but if there were any way that I could prevent that extremely rare occurrence of happening, then I would have done it. Wouldn't you?

I've already stated that yes you can do it, and yes, pending a probable bad ground loop, there may be a rare occurrence of tragedy. The point is that there has been a means for years by which to help avoid such an occurrence. It's one, cheaper, two, easier, and three serves the same purchase. Wow, now I'm explaining what common sense is, thanks.

 

The point is why add to the possible problems when you have a solution to at least one of them? If your vehicle is well-maintained, then you connected your electronic devices to the vehicle ground in the first place. That is a part of good maintenance. Not to simply add more risk.

 

I've said my peace.
Why should you directly ground to the battery? and please show your references

I think enough people will read this thread and go the easiest, least expensive, safest route, instead of purchasing extra cable, and drilling another firewall hole with the only gain being that they've just added more things that can go wrong in an already unstable environment.

Is it too much to ask to simply help others?

 

I am sure everything is just "good enough". As for me, I am not content until it's done the way it is supposed to be done. That's probably the reason that I won't let anyone lay a finger on my car accept me. Because that "good enough" mentality is what I have found in many service shops or installers. It's quite arrogant to spit in the face of so many Automotive and Electrical engineers.

I do believe I've already explained accountability.

 

I've read this entire thread. I'm still not sure how to test ground points and now I'm not sure about how to ground amp 😕

 

A good ground will have as little resistance as possible between it and the negative battery terminal. To find out how much resistance there is from the intended ground point back to the batter you need to set the meter to ohms, then you need to probe the spot you are testing with one lead of the meter, and probe the battery negative with the other probe. In most cases the probe will not reach that far so you use a nice thick wire to make an extension to the lead going to the battery, connect it at the negative terminal, and to the multimeter probe, then use the positive probe to check the various ground points.

 

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