Voltage Drop Testing

   This test is the most overlooked test in the automotive service industry. Where it should be the very first test performed when an electrical problem has occurred. It is a rather simple test to do, best performed by two people.

   Since the test has to be performed on an electrical system while the device being testing is operating, great caution must be used, and it should be performed by a professional that full understands the dangers involved.

By reading past this line you are agreeing that you are or you will use a fully trained licensed mechanic to perform this test. You also agree not to hold S.V.A. Power Products liable for any damages or injuries substained while using this test. Also if you are the type of person that thinks holding a fresh cup of coffee between your legs is a good idea, please click browser back button until you are off our site.

   To perform the test you simply need a multi-meter, pen or pencil, and a pad of paper. We will use an alternators main battery line for an example of how to perform the test.

1.)Start the vehicle and turn high beams on. This will make the alternator charge at close to its full potential, at idle. It is important to have the system you are testing working under normal conditions.
Take extreme caution because you will be working around moving parts. Fan blades, belts, and other rotating parts are very dangerous. As well many items get extremely hot will operating, so burns are very possible. Therefore only professional trained mechanics should do this test.

2.)Set the tester to read DC voltage. Place the negative lead of the tester on to the main battery terminal of the alternator.

3.)Place the positive lead of the tester on the positive battery post. Yes you are reading from one postive post to another.

4.)Take a reading of the voltage on the tester.If your tester is reading “zero”, this is great. If it is reading anything above “zero”, then that is what your voltage drop is. i.e. If it is reading .5 you have a 1/2 a volt drop across the line being tested.The tester is reading the difference in voltage from one end of the line to another. More accurately, the difference in voltage between two given  points (test leads).

5.)The next step is to repeat this placing the negatve lead on the case of the alternator. Put the positive lead on the negative post of the battery.

6.)Take the reading on the volt meter. Again what ever is on the meter is the amount of voltage drop present in the path being tested. In this case the ground path of the alternator.

7.)Once completed turn off all accessories and the engine.

   If you have a reading of .2 on a postive line, .1 on a negative line, or higher, you have excessive voltage drop (Standard automotive according to Robert Bosch). The overall system should not exceed .5 volts drop. If you have a reading above either of these values. simply start tracing the line. Test the voltage at every connection until you find where the drop is occurring. It could be because of a corroded or loose connection, a corroded or frayed wire, a broken wire in the insulation, or just simply the wire is not large enough.

   Once you have established were the drop has occurred simple correct it. If it is a connection, clean it. Every electrical connection should be CLEAN, BRIGHT, and TIGHT. If it is a wire replace it, using the wire sizing formula or chart on this site as a guide. Once the repair is completed, redo the test to ensure everything is in proper order.

Some Things to Remember

   Basically all you are doing with the multimeter is checking the difference in voltage between two connections. So if you get readings that are zero or .1, don’t be alarmed, as this is a very good thing. Because what the meter is telling you is that there is only “0” or “.1” difference between the voltage at one test lead and the other test lead.

   The test should be performed on devices in their real world working environment. Testing a device that is only working at half its potential, will give you an artificially low drop figure. The same goes for testing it when it is cold, as heat causes the resistance to increase, which causes the voltage to drop further.

   This test can be performed on just about any electrical device. We do not recommend you ever poking into the insulation of a wire to do this test, this will have long term adverse effects on the wire. Also note that micro fiber can be severely damaged if poked with a metal object, i.e. your test probe.