Testing Grounds
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Elder User
Joined: Aug 2000
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From: Burke VA
Testing Grounds
How would I do that w/a volt meter? I read somewhere that you can check the resitance in a ground wire and that will tell you if it's bad or good but I am an electrical idiot. But I do have a volt meter so can some one help me out?
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
You actually want to do it with the ohmmeter function. Test the resistance between the negative battery post and the ground in question. You don't want more than an ohm or two resistance. This is the most direct way to test a ground. If, say, the ground you're testing is at the back of the truck or some other location that the meter can't reach the battery, I've used a jumped wire and alligator clips to make it.
The ohmmeter is the upside-down horseshoe (Greek for omega, the symbol for electrical resistance or ohms). If you have a manual ranging multimeter, you'll want to place it on the lowest scale. If a resistane check reads open-circuit on the lowest scale then you know it's probably more than 100 ohms (not good). If you have an auto-ranging meter then you just have to switch it to ohms.
The ohmmeter is the upside-down horseshoe (Greek for omega, the symbol for electrical resistance or ohms). If you have a manual ranging multimeter, you'll want to place it on the lowest scale. If a resistane check reads open-circuit on the lowest scale then you know it's probably more than 100 ohms (not good). If you have an auto-ranging meter then you just have to switch it to ohms.
Mountain Pass
Joined: Jun 2004
Posts: 241
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From: Canada
You can check for bad grounds or connections with a volt meter also. The circuit must be powered up. Put one lead on the negative of the battery, the other on the positive side of the connection but after the load. you should not get much more than a couple 10ths of a volt. For example the black lead on the negative of the battery, the postive on the starter case. You should not get any more than a tenth of a volt per connection. If the reading exessive, you would move the positve to the next connection closer to the battery until the voltage drop disappears. there must be current flow for this to work. Hope that helps a bit. Don
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Elder User
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From: Burke VA
It does, I will try that but I am also curious about the Ohm method as well. I really hate electricity. If it weren't for that fact that civilization is based on it, I'd have nothing to do with it!
MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
If you want to use the resistance check, I would put one probe on the negative terminal of the battery, and then the other probe on the bolt that holds the strap to the firewall. This will tell you the resistance from the negative battery cable, through the block and up to the firewall.
The voltage check above is perfectly fine too; I just prefer the resistance check because it's not dependent on anything.
The voltage check above is perfectly fine too; I just prefer the resistance check because it's not dependent on anything.
Posting Guru
Joined: May 2006
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From: Edmonton, AB.
If your checking with an ohm meter, the reading to ground must be less than 1 ohm. When you check a suspicious ground measure between the device (signal lamp or whatever) you should see lest than 1 ohm. A higher number idicates resistance in that path and what your seeing is another path that's parallel and through that device or another. Another way is to measure with the volt scale across the problem device. If you don't measure full voltage (battery voltage) then you have a resistive ground path. Keep in mind that the difference may be half a volt difference from the measured battery voltage. Finding bad grounds is a tuffy since many times the readings are interpretive. Sometimes it is just easier to run another ground wire parallel with the suspect ground. Good luck....
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Originally Posted by fmc400
If you want to use the resistance check, I would put one probe on the negative terminal of the battery, and then the other probe on the bolt that holds the strap to the firewall. This will tell you the resistance from the negative battery cable, through the block and up to the firewall.
Last edited by SPL Tech; Mar 10, 2007 at 04:52 PM.
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Originally Posted by oft
So the Ohm detection method wont work as my meter wont read that low? Correct?
Posting Legend
Joined: Sep 2001
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Voltage drop measurements in operation are much more accurate than resistance measurements. Resistance measurements with a meter are taken at very low current and voltage which usually have very little in common with a high current connection.
With low resistance measurements you even have to subtract out the resistance in your VOM connections. Put both leads on each endpoint and record the resistance reading between the probes. Test both ends and add the results then divide by two. Subtract that measurement from the reading you get when you measure across the connection to get about as close as you will get with a given meter to the connection resistance.
With low resistance measurements you even have to subtract out the resistance in your VOM connections. Put both leads on each endpoint and record the resistance reading between the probes. Test both ends and add the results then divide by two. Subtract that measurement from the reading you get when you measure across the connection to get about as close as you will get with a given meter to the connection resistance.
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Post Fiend
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From: "Islander"
Fluke 77 for work, Fluke 87 down to .1 ohm for automotive use at home.
Use a conductive anti-corrosion compound like by Thomas & Betts called KOPR-SHIELD.
This applied to clean terminals be it battery, starter relay, starter and all grounds you will have no surprise electrical phenomenons with resistance less than 1/10 ohm.
Be surprised how fast your starter will crank even with a hot motor.
Use a conductive anti-corrosion compound like by Thomas & Betts called KOPR-SHIELD.
This applied to clean terminals be it battery, starter relay, starter and all grounds you will have no surprise electrical phenomenons with resistance less than 1/10 ohm.
Be surprised how fast your starter will crank even with a hot motor.