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  #11  
Old 06-20-2013, 05:43 AM
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DaveInDenver DaveInDenver is offline
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Quote:
Originally Posted by Rzeppa View Post
I have yet to encounter a modern DMM that isn't accurate enough for general automotive applications. Even the little $4 jobs that HFT sells. They work just fine for our automotive applications. We're not measuring cold fusion here.
Yes, exactly. My point, apparently poorly stated, was that most portable DMMs are not sufficient to measure the true resistance of a large size wire. It's just beyond their design criteria, even though a $4 ICL7106-based meter can be quite good, you are just at the limit when you are operating your ADC at the LSB range on the V x T curve with budget-minded op-amps. I also said that it does not matter for this because as you have followed proper engineering principles when you sized the wire, so you are just validating your workmanship with the meter.

The Fluke 87 I carry considers conductivity for purposes of making the little beeper beep anything up to about 100 Ω, so in the same vein no reason to be hypercritically worried about 5 decimal places and resolution. The only reason I even mentioned it was to point out limitations with DMMs at the edges of ranges. With 3-1/2 digits on a 200 or 400 Ω scale there is a lot of error for measurements lower than a few ohms and certainly so for less than 100 mΩ or so, both due to resolution and accuracy.

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  #12  
Old 06-20-2013, 08:57 AM
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Yeah i think I got it, I can't put my DMM across a big fat wire and measure its resistance. It just reads zeros.

Even a small wire for that matter.

or even a junction sometimes just measures zero....
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  #13  
Old 06-20-2013, 08:58 AM
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Resistance checks are really worthless in most automotive applications. A correct way of diagnosing a problem is using voltage drop tests. Measure from after the load on any circuit to the negative terminal of the battery. Any voltage over a couple tenths shows a possible issue on the ground side. A DMM does not load a circuit and does not provide any real valuable information.
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  #14  
Old 06-20-2013, 09:20 AM
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Times like this we need that little popcorn emoticon.
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  #15  
Old 06-22-2013, 02:04 PM
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Rzeppa Rzeppa is offline
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Quote:
Originally Posted by thefatkid View Post
Resistance checks are really worthless in most automotive applications. A correct way of diagnosing a problem is using voltage drop tests. Measure from after the load on any circuit to the negative terminal of the battery. Any voltage over a couple tenths shows a possible issue on the ground side. A DMM does not load a circuit and does not provide any real valuable information.
Excellent observation Brian!

In fact, the best way to validate design and workmanship in a high current application is to measure voltage drop across whatever is suspect, whether it is a connector or a conductor. Especially suspect is the return as you mentioned. I like to use star washers on frame and body connections so they dig into any coatings, whether oxidation or residual paint after it has been sanded.
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