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Ground/earth testing

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Old Feb 13th, 2023, 12:29   #1
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Default Ground/earth testing

Hi,I want to check out the earthing on one of my headlights.Which ohms setting is best to use on multi meter, high or low. Should I do this with the light switched on or off?
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Old Feb 13th, 2023, 14:19   #2
Ron Kwas
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Short of it: Measure V! ...Long explanation follows:

Handheld DVMs typically run a minuscule current with which to make the Resistance measurements, so the low but significant resistance associated with poor chassis connections will likely not show up as a measurable number on one of those.

I would instead suggest using the same (but less costly) technique as a "Four-Wire Resistance" or Kelvin Measurement test instrument, which runs a high test current through the low R in question, and measures the resulting voltage. Simply turn ON the Load (in this case Headlights), and with Neg probe of DVM tied direcly to Batt Neg, probe what is supposed to be a good chassis connection. If any more than a few tenths of a volt are measured, a VDrop is occurring, decreasing the V available to the load...which in this case would result in a dim/yellow Headlight...locate and minimize this and any other VDrops on connections!

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You asked about checking for a poor chassis connection...of course, this same technique can be used to check for a poor power connection! Simply reconnect Pos meter probe to Batt Pos, and check power connection of Load in question with Neg probe...if more than a few tenths of Volts are measured, there is a significant R is series with the power path, which needs to be located and minimized. Remember, the source (Batt) V should only be measured at the Batt and Loads where it serves to drive current through the Load to do the work...any VDrop elsewhere High Side OR Low Side means Load is getting less than it should, so not able to work its best.

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