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How to Test Voltage Drop (by Max_Morley) (Read 62 times)
Serious Thumper
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How to Test Voltage Drop (by Max_Morley)
10/13/06 at 10:36:37
It is easy and it measures the actual voltage drop when the circuit is operational. On a digital meter it really doesn't make any difference polarity as the meter will read OK either way, it just has a minus in front of it, but the number is valid. AS you can do the same test with the low scale on an analog (swept needle type) I always teach that the + (red) lead goes to the point of highest voltage and the - to (black) to lowest. The way this test differes from an appplied voltage test is that - lead goes to the starter motor terminal rather than ground. Now when you crank the meter will show the difference in potential or voltage drop between the Bat + and the motor terminal. We usually allow 0.1 V drop per connection, set of contacts or wire, but not more than 10% or about 1.0 - 1.2 volts on either the + or ground side. You would check the - side of the circuit by putting the + lead on the starter motor ground and the - lead on the battery - again crank the engine and observe the reading. Same allowance there. For this test to work the circuit has to be complete and work and be in operation (amps flowing) for the test to be valid. As we are working with 0.01.-0.1 V that is why a digital works nice as it easy reads this small. The more expensive the meter the faster you will get a reading, that is partly waht you pay for in a more expemsive meter. Flukes are very expensive but almost instant to display the number and they have the ability to record the min and max while you are doing the cranking operation. Works on finding the problem with broken or corroded wires as they equate to resistance that the Ohmmmeter function doesn't find. think of it this way, if only one wire of the 7 in a starter ground cable are touching at a break, then the ohmmmeter will show 0 ohms or close to it. When you try and crcnk and all the 80 or so amps try to got through that one strand the get restricted and so there is less voltage left on the other side of the bad connection to try and do the work of cranking the starter at the correct speed. Another good example is those cheap grocery store aluminum wire jumper (booster) cables that tend to break the wire in the strand very quickly at the point they at crimped in the clamp. The seem to work OK at first and then each time you use them they do worse and worse job of jumping and when you go to disconnect them the clamps are warm or hot to the touch. Some of the voltage was being used at that resistance point and made heat. So a hot spot in any circuit other than the bulb or heater element is not good. End of lesson for today.
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