How do you test a bad earth?
27-06-2005, 03:53 PM
#1
Monte Geek
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How do you test a bad earth?
I still can't get my bloody car started, basically what happening is when cranking the engine it will turn a little bit then stop then turn a little bit then stop and so on till the battery dies.
I've tested the battery ok, I've tested the power to the starter as ok and assumed it to be another dodgy starter.
I took the starter motor off again today took it to the work shop where I bought it and had it bench tested in front of me.. working fine! The guy suggested it was a bad earth esp when I told him I noticed the power cables move slightly when I try cranking the starter.
This would make sense in a way as the battery has been relocated into the car with the method of running the power cable from the original battery position to the new location and bolting the earths to the chassis which may have not been done too well... soooo
...my question is with a multi meter how can I test an earth is ok? The Continuity reads 0.01 but I'm told thats not necessarily a good idicator of good the earth is and just means that it connected.
Any help would be greatly appreciated!!
I've tested the battery ok, I've tested the power to the starter as ok and assumed it to be another dodgy starter.
I took the starter motor off again today took it to the work shop where I bought it and had it bench tested in front of me.. working fine! The guy suggested it was a bad earth esp when I told him I noticed the power cables move slightly when I try cranking the starter.
This would make sense in a way as the battery has been relocated into the car with the method of running the power cable from the original battery position to the new location and bolting the earths to the chassis which may have not been done too well... soooo
...my question is with a multi meter how can I test an earth is ok? The Continuity reads 0.01 but I'm told thats not necessarily a good idicator of good the earth is and just means that it connected.
Any help would be greatly appreciated!!
27-06-2005, 06:24 PM
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Paul,
when testing resistance, the meter applies such a tiny current down the cables that even a crap connection can 'look' ok.. when there is a high current passing through, the picture can be very different.
With the meter set to measure voltage, connect the meter ideally between the battery negative post and the starter body - you should measure next to nothing across that connection when cranking if the connections are sound.
If you do get a voltage reading, you will have to perform the same test across each individual connection along the route to find the dodgy one..
ie - starter body to engine block, block to earth cable, earth cable to battery negative post.
when testing resistance, the meter applies such a tiny current down the cables that even a crap connection can 'look' ok.. when there is a high current passing through, the picture can be very different.
With the meter set to measure voltage, connect the meter ideally between the battery negative post and the starter body - you should measure next to nothing across that connection when cranking if the connections are sound.
If you do get a voltage reading, you will have to perform the same test across each individual connection along the route to find the dodgy one..
ie - starter body to engine block, block to earth cable, earth cable to battery negative post.
27-06-2005, 06:31 PM
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Hard to test for as voltage/continuity will appear fine. Clip leads onto starter motor (route so they wont snag anything) and check voltage as it is cranking. Prob better to do a visual check on all earth straps (inc. engine to engine bay), remove any rust etc and refit.
Shorts, justuse continuity setting on multimeter. This setting will beep when you touch both probs together. and so if you put them on seperate circuits and it beeps there is a short. Badly xplained but maybe gives you an idea.
Shorts, justuse continuity setting on multimeter. This setting will beep when you touch both probs together. and so if you put them on seperate circuits and it beeps there is a short. Badly xplained but maybe gives you an idea.
27-06-2005, 07:18 PM
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test for a volt drop between the earth you want to check and the earth at the battery...
more than 0.2 volts in a bad earth.
For shorts you need to look for resistance both to ground and also to positive.
ALex
more than 0.2 volts in a bad earth.
For shorts you need to look for resistance both to ground and also to positive.
ALex
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27-06-2005, 08:55 PM
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Originally Posted by jon@work
Wouldn't it be easier to just put a jump lead straight from the bellhousing/engine to the battery earth and see if it makes any difference, and then if necessary from the pos terminal to the starter to make sure the feed's OK ?
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