How to check your ground with a DMM?

[CJL 10+ year member]

How do you check your ground with a DMM? And if i do have a bad ground would sanding it down to bare metal solve the problem?

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[dkguitarist 5,000+ posts]

run a jumper wire from your negative battery post, to the dmm, then take the other lead and probe the ground area and it should read less than .1

 

[CJL 10+ year member]

so put 1 DMM probe on a negative battery post and put the other DMM probe on the ground im checking?

 

[dkguitarist 5,000+ posts]

so put 1 DMM probe on a negative battery post and put the other DMM probe on the ground im checking?

prett much. just run a wire/lead from the battery post to the dmm (unless your dmm wires are 15+ feet long.

 

[CJL 10+ year member]

So if it reads more than .1 its a bad ground right, so would sanding it down to bare metal fix the ground?

 

[dkguitarist 5,000+ posts]

So if it reads more than .1 its a bad ground right, so would sanding it down to bare metal fix the ground?

sand down to bare metal and check it.

 

[Badfastbusa 10+ year member]

if it is still over.1 after you sand it you need to find a better gounding place in the car.

 

[dleccord 5,000+ posts]

theres this option on my dmm that when you have the two leads touching each other it has a beep. the resistance is more accurate though i believe.

 

[helotaxi 5,000+ posts]

Depends on how much power you're running. In all honesty, if you can measure the resistance with a normall DMM then the resistance is too high. That's the same resistance as 1000 ft (yes one thousand!) of 0 ga wire. Do a little V=IR math. If your amp is drawing a paltry 30A and your ground has 0.1 ohms of resistance you are losing 3v to your ground resistance and your amp will have 9-11V to try to run on. Most amps I know of will shut down under those conditions.

Like I said you ideally want your ground of low enough resistance that you can't simply measure it directly. You have to figure it based voltage drop under load.

 

[CJL 10+ year member]

UPDATE: I found and fixed the problem,it was my HU's ground.All i had to do was sand it a little with some good ol 36 grit and BAM! the noise/alt whine was fixed Thank you guys for all you help

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GOOD GROUND>ME