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 Post subject: Voltage Drop testing
 Post Posted: Fri Sep 30, 2011 6:06 am 

Joined: Thu Feb 12, 2009 7:52 pm
Posts: 348
Hello All,

I wanted to share this with you all. I found this over on yellowbullet posted by a guy called RRRAAAYYY2. It is an easy and effective way to test the quality of your circuits

"Voltage drop testing is one of the simplest tests in the world to do, but often is over looked. Here is a detailed explanation of how to perform the test.

You will need a volt meter, preferable a digital one. The help of a friend will make testing a lot easier as well. And possibly a set of booster cables if the circuit you are going to test is really long.

A few things that you should know first. When you use a volt meter it displays the difference in voltage from one lead to another. So when you place it across the battery on the + and - posts, and it reads 12.79volts, it means that the difference in voltage between the two posts is 12.79volts.

All the circuits in your car complete themselves at the battery, specifically between the + and - plates inside the battery. So any extra resistance outside the battery, any where in the circuit is a bad thing.

Industry standard for automotive production cars is a .5 total voltage drop in any system. Except maybe your starting system, as the old style starters just draw a ton of amperage. The .5 is divided up as .3 positive and .2 negative. So when we are testing the positive leg of the circuit we will want to see less than .3 of a drop, and less than .2 on the negative side. This is while the device is in use. i.e. if you are checking your lights, they need to be on while doing the check.

So here is how to do the test. Put one lead on one point of the circuit, and the other lead at another point. Say at both ends of a wire. The meter should read zero. Now turn on the device. The reading on the meter is how much the voltage drops between the two leads. If it is on both ends of a wire, then it is how much the voltage drop down that wire. If it is over .3 + or .2 - it needs to be replaced. "

Real world, as you may or may not know, I have been having issues with my electrical water pump. So I came across this test. Well initial test showed that I had .6 voltage drop from my relay center to the pump. Well looking at it I found a questionable connection,so I replaced the entire circuit and recrimped all the connections. Well the voltage drop is .1 now and I can hear that pump zinging now. I will be definitely be checking out other circuits. This test was very helpful to me and I wanted to share

This test really allows you to pinpoint where the issue is. You just check each side of the connection to want to check, if the drop is significant, you found your problem.

For those of you that have those slow cranking cars, you can also use this to determine if you have a bad ground. If your battery is in the rear like mine, you will need to extend your leads. Just put one end on the positive side of the battery, and the other on the starter positive. Have someone crank the car, excessive drop, start checking connections. You can also do this with the ground side.

Hey Ken, this should be a sticky!

74 Nova
Best 9.87@131 on the rev limitor 1 Feb 2013
1/8   6.15@110 NEW BEST 27 Nov 11

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