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Measuring Volt Drop

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#1 KernowCooper



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Posted 31 July 2013 - 12:29 AM

When looking for high-resistance connections using an ohm-meter (of the type available to most of us) is useless. What we have to use is a voltmeter, measuring the voltage at various points in the circuit. However the circuit must have a load as well as a supply - you need the load to draw a current and cause a volt-drop across the fault before you can measure it.


The standard multi-meter draws such a tiny current that on its own it will cause negligible volt-drop, and hence not reveal much less than a complete disconnection. A good tip is to measure the volt-drop along a circuit instead of measuring the voltage to earth at several points. We might put the +ve terminal of the meter on the +ve terminal of the battery (assuming negative earth) and the -ve terminal of the meter on the green wire on the flasher unit, for example. With ignition and turn signal on (and hazard switch off) this will reveal the total volt-drop between battery and flasher.


What your looking for in a ideal world in 0v but wiring has some resistance on a good condition circuit, so 0.5v is more likely the reading you'll see any more and you need to start looking for bad connections and earths


This method can be used on circuits and also for checking earth wires


With normal care and use bullet and spade connectors from the factory will give good connections for many years. But with PO 'repairs' using dodgy components and techniques or if the car has been abandoned in a field for years corrosion can develop that doesn't occur in normal use and both will cause electrical problems.


When adding any wiring I really don't like those blue Scotchlok connectors, I've found on a number of occasions that after a while even in the car the bifurcated blade loses tension on the copper strands and they start to cause problems. If you are near a bullet connection then substitute a 4-way for a 2-way and put a bullet on your new wire, or if there is already a 4-way with 4 wires in it I'd rather make up a couple of inch length with bullets and add a second 4-way. If you want to tap into a wire going into a multi-way connector then really there is nothing for it but to cut the wire, put two bullets on the end, and use a 4-way. Or you can if you wish solder the wire and then use shrink tube However if you are near a spade connection then piggy-back spade connectors are a good way of tapping into these.


Volt drop can help you identify problems in a circuit and pinpoint problems quite quickly without have to disconnect and check things at random

Edited by KernowCooper, 31 July 2013 - 12:37 AM.

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