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Author: Subject: earth reading on multimeter
RMarine

posted on 3/7/10 at 10:57 PM Reply With Quote
earth reading on multimeter

hi all, is there a way to test how good my earth connection from the battery to the chassis is by using a multimeter?
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RichieHall

posted on 3/7/10 at 11:56 PM Reply With Quote
Disconnect the negative from the battery and measure the resistance from the battery connector to any bit of bare metal on the chassis or engine block. The lower the better!





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theconrodkid

posted on 4/7/10 at 05:38 AM Reply With Quote
you can do it with the battery connected and lights switched on,battery earth terminal to somewhere on the chassis,voltage should be around 0,higher that 1 volt means it,s not 100%





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v8kid

posted on 4/7/10 at 06:36 AM Reply With Quote
Should be less than 0.1 volts. At 1 volt its a bummer





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matt_gsxr

posted on 4/7/10 at 08:25 AM Reply With Quote
Either, everything off, and then measure resistance using Ohms setting between the battery negative and your chosen earth point. Worth checking what the resistance is when you connect the multimeter probes together, as they are not very accurate at these low impedances.

Or, everything on, and measure the voltage between the battery negative.

The only case where this will be difficult is for the starter motor, but normally the cable runs for that are not subtle.

Matt

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BenB

posted on 4/7/10 at 10:24 AM Reply With Quote
Well this is how I'd do it (as multimeters are bad at measuring low resistances)

Put one multimeter in the +ve supply as an amp-meter (in series)
Put on the headlamps including main beam and rear fog lamp (IE get some current flowing).
Put another multimeter in parallel (set as voltmeter) to the wire going from the -ve battery terminal to a known good chassis earth point and measure the voltage loss.

R= V/I

Simples

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Bob C

posted on 4/7/10 at 10:55 AM Reply With Quote
It's really only an issue for cranking, when the amps will be 10 times higfher than any other battery load. As said above yr multimeter can't be trusted below an ohm or so.
So set to measure volts. Disable the ignition somehow and measure volts between starter motor can and battery minus as the starter is cranked. Give it a second or so to get a reading. Then look between battery minus and chassis. Then between starter can and chassis. You should quickly build up a picture of where, if anywhere, you are losing volts.
I've not done this but more than a volt could indicate a problem, less probably not.
The volts reading will not be steady....

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daviep

posted on 4/7/10 at 11:28 AM Reply With Quote
As per Conrod and Bob the only correct way is to check voltage drop across the connections when cranking.

Imagine you'd used a piece of 4mm cable for your main earth. If you checked the resistance it would read 0, it would run your lights etc OK as it is making a good connection. If you tried to crank the engine it would glow red hot and melt as it is under sized.

Regards
Davie

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