how to test for a good ground with a dmm
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Forum Name: General Discussion
Forum Discription: General Mobile Electronics Questions and Answers
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=28691
Printed Date: July 14, 2025 at 1:44 AM
Topic: how to test for a good ground with a dmm
Posted By: derrickg
Subject: how to test for a good ground with a dmm
Date Posted: March 19, 2004 at 7:59 PM
what do i set the dmm to and what do i connect both probes to
Replies:
Posted By: markcars
Date Posted: March 19, 2004 at 10:09 PM
if you do a search, you will find other posts where this topic has been quite well discussed. If you want a quick answer, then here is how you do it. Set the meter to Ohms and connect any one probe (usually black one) to the ground such as any metal part of your car (assuming yoru car is -ve ground) Touch the red probe to the metal part you are testing for a good ground. If the meter shows almost zero ohms, (less than 1 ohm) it is good ground. The reason why it is not absolutely 0 is because your contacts between the probe and the metal part is not too good. If you soldered your probe to the metal, you would see zero ohms.
Also set the meter to the lowest ohms range you can to get a better reading. If you set the meter to a high ohms range, anything will show you a reading, even if you touch the probes with your hands!
Posted By: derrickg
Date Posted: March 20, 2004 at 2:18 AM
my ground cable is getting hot and the copper wire is changing color on the inside of the insulation what would cause this the positive doesnt get hot at all
Posted By: G2tegra
Date Posted: March 20, 2004 at 9:30 AM
Sounds like not a solid ground or theres paint in between the ground and the body of the car, or even not large enough ground wire, the amps pullin to much current
Posted By: forbidden
Date Posted: March 20, 2004 at 2:47 PM
I would agree, it sounds like a bad ground but I am going to go with too small a ground wire. The ground wire should be the same size as the power wire. What amp is it hooked up to, what are the fuse sizes in the amp and what guage wire are you using.
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Top Secret, I can tell you but then my wife will kill me.
Posted By: forbidden
Date Posted: March 20, 2004 at 2:50 PM
I always test for ground return resistance by taking one probe direct to the neg battery terminal and the other probe to the potential grounding location. This will show the resistance from the battery instead of between two points. Never touch the probes as it throws the reading off. If the probes do not reach, use an extension wire to lengthen the reach. A reading of under .05 ohm is what I usually shoot for.
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Top Secret, I can tell you but then my wife will kill me.
Posted By: derrickg
Date Posted: March 20, 2004 at 3:50 PM
i am using an hifonics zeus viii amplifier 8 guage wire 100 amp fuse
Posted By: forbidden
Date Posted: March 20, 2004 at 3:53 PM
Your power and ground wire should both be 4 gauge at the smallest, the 8 gauge is not large enough.
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Top Secret, I can tell you but then my wife will kill me.
Posted By: Mad Scientists
Date Posted: March 21, 2004 at 5:43 PM
Instead of measuring ohms, measure DC voltage across that ground lead.. measuring voltage drop is much more effective (and accurate) than ohms. Is your meter accurate enough to measure below 1 ohm?.. Do you account for the resistance of the meter leads? Ohms Law.. volts equals resistance times current.. I personally own thousands of dollars of electronic test equipment.. I can't accurately measure 0.05 ohms. However, 100 amps passing through a 0.05 ohm resistance will create a voltage drop of 100*0.05=5 Volts. I can measure 5 volts easy.. That same 0.05 ohm on a starter circuit would be over a 10 volt drop, using a starter current draw in excess of 200 amps. Measuring voltage drops is the way to go for power circuits. regards, Jim
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