RMarine
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| posted on 3/7/10 at 10:57 PM |
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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
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| posted on 3/7/10 at 11:56 PM |
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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!
Rust is lighter than Carbon Fibre!
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theconrodkid
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| posted on 4/7/10 at 05:38 AM |
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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%
who cares who wins
pass the pork pies
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v8kid
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| posted on 4/7/10 at 06:36 AM |
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Should be less than 0.1 volts. At 1 volt its a bummer
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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matt_gsxr
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| posted on 4/7/10 at 08:25 AM |
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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
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| posted on 4/7/10 at 10:24 AM |
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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
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| posted on 4/7/10 at 10:55 AM |
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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
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| posted on 4/7/10 at 11:28 AM |
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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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