Automotive, Electrical Diagnosis

Learn How To Diagnose Using Voltage Drop Part 3

In this lesson Diagnose Using Voltage Drop Part 3 we will learn how to test circuit voltage. I will cover what the reading on the volt meter looks like when you remove the load from the circuit (a BIG NO NO).

Voltage Drop Part 3

First current must be flowing and your volt meter set to DC voltage. Place the negative meter lead on the battery ground. From this point on, you should always use battery ground whenever possible. Battery ground is the best ground source you have available, so you might as well use it. It is perfectly fine to make whatever length of jumper to battery ground for testing purposes. Positive lead from volt meter on the negative side of the load. With current flowing you should not read any more than .10 Volts DC on the ground side. Any reading more than .10 Volts DC on the ground side is a red flag. Let me explain why you should never remove the load or any connection to the load when testing for voltage drop. Let’s remove the lamp and place the negative meter lead on the negative battery post and positive meter lead at the negative side of the load. You will read 0.00 V DC on the ground side and this will make you think the ground circuit is fine when it in fact is not. Now place the positive meter lead on the positive side of the load and it will read full battery voltage or 13.15 volts DC. We know this is too high and should throw a red flag. This is why you must leave the load and all connections in place. When you remove the load current stops flowing and therefore no voltage drop can be measured. This masks the real issue if there was one. This is why it is important to use the proper methods to diagnose using voltage drop.

Let’s look at what the voltage across a closed switch looks like. This test usually throws people off. Just remember that the volt meter is designed to read a potential difference. In other words it must read a change in voltage from one lead tip to the other. Take one piece of continuous wire and apply voltage to it. Place the black meter lead on one end and the red meter lead on the other end. What do you read? The meter reads 0 volts! Why you ask? Because there is no “potential difference” in that continuous piece of wire. Now connect a switch to the wire and place the switch in the open position. What does the meter read?  With the switch open the meter now reads 12 Volts DC! Why? Because now there is a “potential Difference” between the meter lead tips. All this is just because of the way the meter is designed. Remember this important lesson.

Diagnose using voltage drop. Place the meter leads along the same path from battery to the load on either the positive side or the negative side. As an example, place the positive lead of the meter on the positive battery terminal. Place the negative lead of the meter on the positive side of the load. This method shows you the drop in voltage from the battery to the load, volts dropped. To find circuit voltage, the voltage available at the load, place the negative meter lead on the negative battery post and the positive meter lead on the positive side of the load. This shows you the actual voltage AVAILABLE to the load. Both methods are useful in determining electrical faults.

To test circuit voltage using an effective and time saving process do the following. Always place the negative meter lead on the negative battery post. Then place the positive meter lead on the positive side of the load. With the volt meter on DC volts, take the reading. You should be seeing near battery voltage. Next, place the positive lead on the negative side of the load. You should be seeing no more than .10 V DC.  Anything more than .10 V DC you know there is an issue. Start all testing using this method and the voltage drop method and you will have much greater success diagnosing electrical faults. Diagnose using voltage drop.

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