confused check resistance of ground

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groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 7:52 PM / IP Logged

I'm trying to figure out how to measure my resistance using a Vmeter on my ground. I understand how to set it up to 200 ohm scale and touched the leads and got .2 reading which is good. but how do i measure like it says in the sticky to have it .5 or below? I mean like where do i touch the + and - leads?

sk8ingsmurf

Copper - Posts: 198

Joined: March 18, 2004

Location: United States

Posted: February 06, 2006 at 8:09 PM / IP Logged

Touch the + lead to the end of your ground, then use a spare piece of wire (the larger the guage the better), and use it to extend the - lead to the negative side of your battery. If it is easy to rescale your meter to zero on impedance readings then it is more accurate to zero it out from one end of the extension to the other.

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 8:23 PM / IP Logged

^ah ok I do this with the - bat terminal disconnect or connected(live)?

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 8:43 PM / IP Logged

ok just tried it with the - bat term disconnected and nothing is happening it'll just read. "1 . " I made an extention from the - lead or "COM" port on the meter and connected it to the - bat term and + to end of ground on each amp and I get that reading.

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 9:05 PM / IP Logged

ok well instead of measuring resistance i tried measuring the voltage drop. I don't think I have any if I measured it right. I turned my meter to regular Vmeter again and measured the voltage from - end of bat to end of ground to both amps and got reading of 12.7 on both

stevdart

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Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: February 06, 2006 at 9:28 PM / IP Logged

groggy, it doesn't matter what resistance reading you get when you touch the meter leads together. Whatever it is, in this case you said it was .2 ohms, should be subtracted from the test reading. It's a measure of internal meter resistance and will always be included with your test readings. My meter always reads .7, for example. That's a measure of resistance that I always subtract from any resistance reading I make. And measure the resistance of any wires you use to connect front to back of vehicle... add those readings together, and subtract from your test reading. The internal resistance reading may change as the meter battery wears down, so always touch the leads for a reading before any test.

(...you might already know this, but jus' sayin' ;)

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 9:53 PM / IP Logged

ok im really confused as to what to read now lol. the way i tried to check was what sk8 said. i extended the - lead to the car battery neg terminal(disconnected) to the end of the ground wire to the amps. I didn't get any reading at all. it just said "1 . " But from what your saying i touch the + lead to the begnning of the ground and the - lead to the end of the ground wire? subtract that from the .2 reading i get and that should be .5 or less ohms right?

stevdart

Platinum - Posts: 5,816

Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: February 06, 2006 at 10:53 PM / IP Logged

What the "What is proper ground?" article states, except you can subtract the meter's internal resistance just as you can subtract the resistance in the wire extension that you use. You are measuring the resistance through the car chassis from amp ground point to battery ground point, both points on the chassis. The reading, after you make a subtraction for the added resistance load imposed by the meter and the wire, is the actual resistance of that length of chassis metal. As Rob said, 1/2 ohm or less is good.

Let's say I'm using my meter to do this. The article states that if my meter doesn't read .2 or less, I should use a better one, it seems, regardless of whether I can just subtract my meter's .7 ohm resistance or not. Hmmm. But I continue anyway because I believe in my meter ;).

I use the meter to read resistance in the extension wire first. I get a reading of 1.6 ohms. I know this is the result of the combined resistance of both the wire and the meter, so I jot that number down on paper. Then I bolt one end of the wire to the battery ground point on the chassis, tightly. The other free end of the wire, extended back to where the amp is located, is held to one of the meter probes, and the other probe is touched to the chassis at the amp ground point. It's like I'm just extending the meter probe to a spot where the meter's wire can't normally reach. I read the resistance, and subtract my 1.6 ohm equipment resistance from it. Result is the resistance through the car chassis between the two ground points. If I read 2.1 ohms, I'm good to go, because the result is .5 ohms after the subtraction.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 06, 2006 at 11:35 PM / IP Logged

^ahhh ok that makes more sense now. The thing is I have a grounding kit installed on my car. So basically my car battery has 2 ground points. both 4ga wire. I can use either ground point to measure?

groggy23

Copper - Posts: 67

Joined: January 30, 2006

Posted: February 07, 2006 at 12:02 AM / IP Logged

Wow, I must have an awesome ground. :) I got .3 across my wire extention and .4 from car bat ground point to end of amp ground point. which would give me .1 resistance :) I did this to just one of the two car bat ground points and with the - term disconnected. I think I did everything right.

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