joejeep92
NAXJA Forum User
- Location
- Beloit, KS
I have seen lots of people recently posting on electrical issues with the idea of I’m new at this or am I doing this right as far as how to test or how to use their meter. I teach automotive technology so I teach a fresh group all eight ASE areas including automotive electricity and electronics every year and this is the info each of them pay good money to hear so hopefully it will help someone out. It is a long read and I apologize for grammatical errors because I kinda just sat down and wrote what spilled out of my head but if you are really bored maybe it will save you some money.
First off, to do effective electrical testing you have to have a good meter. Not going to debate brands with you but you need a good high impedance multi-meter as part of your basic tool kit. Let’s start with how your meter works. There are three major things your meter can be used to look for on your car or truck, amperage, voltage, and resistance.
Voltage is electrical pressure, think of it as the force that pushes the water molecules out of the hose. It is technically the amount of force needed to move an electron from the valence ring of the atom of a conductor or electromotive force. Your meter will probably have an alternating voltage setting and a direct voltage setting. You want the direct setting, most automotive circuits have direct voltage because it can be stored in a battery. Now, when checking voltage all your meter is doing is telling you the difference between the black and red lead. So, if one lead is on battery positive which should have 12.6 Volts at rest and the other is on battery negative, or ground, which should be zero volts the difference between the two is displayed on the screen as 12.6 Volts. Ground should ALWAYS be very close to zero volts in a properly working circuit and any voltage on a ground is indicative of a problem. It is very important to use a known good ground for testing, otherwise your results are skewed. For example if one lead is on 12.6 Volts but your ground is at 6 volts your meter will display 6.6 volts which could lead you down the wrong diagnostic path. Remember, your meter only tells you the difference between the two leads so one must be at zero, or ground, to tell you source voltage. We will discuss voltage drop testing later.
Resistance is the impedance to electrical flow in a circuit often referred to as ohms. Basically it is a restriction in the hose analogy I used later. As previously stated resistance is expressed as ohms and in the “ohms” or resistance setting your meter tests the circuit by sending out voltage on the positive lead and counts/times how much or how fast it comes back on the black lead. Be careful though, the circuit must be completely off for a resistance test to be accurate. A live circuit will have its own amount of electricity on it and therefore will interfere with your meter trying to count its own voltage. Resistance testing can be useful in some cases but can lie to you. A cable or wire is composed of many tiny wires and because your meter only sends out a small amount of voltage it only needs a few strands of this wire to be perfectly comfortable flowing across. Think of it as me sending five people over a bridge to check how it will react to five-hundred people. Under the small voltage of your meter it may test well but under the full voltage of the system it will likely fail.
The other more accurate way of testing for resistance is by using voltage drop. If you remember, our meter tests for voltage by comparing the reading on the two leads. As voltage is electrical pressure, we use voltage to overcome resistance in the circuit and we can use this to basically calculate our resistance. Every resistance in a circuit takes a proportional amount of voltage to push the electrons through the resistance. If I connect a bulb by wire to the positive and negative posts of your battery it will light. If I place one lead on battery negative and one lead on the wire from battery positive to the bulb what should it show? 12.6 Volts because the difference between your leads should be 12.6-0 which is 12.6. Now what about if I move one of my leads from before the bulb to after? It should show zero volts because the bulb uses all available voltage in lighting the bulb. How do we use this knowledge? Let’s use a starting circuit. In a starting circuit you have a large cable carrying 12 volts to the starter solenoid. The vehicle doesn’t start and using your meter decide that you are not getting voltage to the solenoid. You suspect a cable but how do you prove it? Place one lead on battery positive (12.6 Volts) and the other on the solenoid terminal (ideally 12.6 volts but in this case 0 volts). The difference is 12.6 Volts and will be expressed on your screen. What does this tell you? That the voltage is being lost somewhere in the cable to the solenoid. This is a very simplified version of voltage testing but it can be used to solve many problems on the car like bad battery connections. Place one lead on the battery post and the other on the battery terminal. What should it read? Zero volts or close to it, any voltage displayed is the difference between the two points and therefore the voltage the circuit is using to overcome a resistance, in this case a bad connection. This is voltage dropping simplified but should at least get you started.
Amperage is the final measurement we will cover. Amperage is electrical current or flow and is actually the amount of electrons past a certain point in a certain amount of time. An important aspect of current is that it is the same throughout the circuit. Amperage is the actual water molecules in the hose analogy. One thing you need to know about electrons is that they are lazy, they are looking for the easiest, fastest way back to battery negative. This is apparent in amperage testing, to do so correctly the circuit must be broken and the meter placed in series with the circuit, meaning that they only path for electron flow must be through the meter otherwise your reading is not accurate. A good test for if you are accurately testing for amperage is if the circuit would function without the meter in place you are not doing it correctly. In amperage mode your meter is counting electrons moving through it, this is why it is important to have the meter as the only current path because to accurately count electrons all of them must flow through it. Amperage testing can damage your meter or at least blow a fuse inside it because excessive amperage flows through it. Electrons flowing through a conductor create a magnetic field which we use in relays and solenoids but can also be used for testing with an inductive pickup. An inductive pickup or amp clamp senses the magnetic field around the wire and sends that info to your meter so you don’t have to risk your meter by putting it inside the circuit. Most meters don’t come with them but they are a great investment. One of the more common uses for amperage testing is a parasitic draw, what makes my battery go dead overnight etc.
Sorry for the rambling long post but was sitting at work and thought it might help someone. If you have further questions or anything feel free to PM me.
First off, to do effective electrical testing you have to have a good meter. Not going to debate brands with you but you need a good high impedance multi-meter as part of your basic tool kit. Let’s start with how your meter works. There are three major things your meter can be used to look for on your car or truck, amperage, voltage, and resistance.
Voltage is electrical pressure, think of it as the force that pushes the water molecules out of the hose. It is technically the amount of force needed to move an electron from the valence ring of the atom of a conductor or electromotive force. Your meter will probably have an alternating voltage setting and a direct voltage setting. You want the direct setting, most automotive circuits have direct voltage because it can be stored in a battery. Now, when checking voltage all your meter is doing is telling you the difference between the black and red lead. So, if one lead is on battery positive which should have 12.6 Volts at rest and the other is on battery negative, or ground, which should be zero volts the difference between the two is displayed on the screen as 12.6 Volts. Ground should ALWAYS be very close to zero volts in a properly working circuit and any voltage on a ground is indicative of a problem. It is very important to use a known good ground for testing, otherwise your results are skewed. For example if one lead is on 12.6 Volts but your ground is at 6 volts your meter will display 6.6 volts which could lead you down the wrong diagnostic path. Remember, your meter only tells you the difference between the two leads so one must be at zero, or ground, to tell you source voltage. We will discuss voltage drop testing later.
Resistance is the impedance to electrical flow in a circuit often referred to as ohms. Basically it is a restriction in the hose analogy I used later. As previously stated resistance is expressed as ohms and in the “ohms” or resistance setting your meter tests the circuit by sending out voltage on the positive lead and counts/times how much or how fast it comes back on the black lead. Be careful though, the circuit must be completely off for a resistance test to be accurate. A live circuit will have its own amount of electricity on it and therefore will interfere with your meter trying to count its own voltage. Resistance testing can be useful in some cases but can lie to you. A cable or wire is composed of many tiny wires and because your meter only sends out a small amount of voltage it only needs a few strands of this wire to be perfectly comfortable flowing across. Think of it as me sending five people over a bridge to check how it will react to five-hundred people. Under the small voltage of your meter it may test well but under the full voltage of the system it will likely fail.
The other more accurate way of testing for resistance is by using voltage drop. If you remember, our meter tests for voltage by comparing the reading on the two leads. As voltage is electrical pressure, we use voltage to overcome resistance in the circuit and we can use this to basically calculate our resistance. Every resistance in a circuit takes a proportional amount of voltage to push the electrons through the resistance. If I connect a bulb by wire to the positive and negative posts of your battery it will light. If I place one lead on battery negative and one lead on the wire from battery positive to the bulb what should it show? 12.6 Volts because the difference between your leads should be 12.6-0 which is 12.6. Now what about if I move one of my leads from before the bulb to after? It should show zero volts because the bulb uses all available voltage in lighting the bulb. How do we use this knowledge? Let’s use a starting circuit. In a starting circuit you have a large cable carrying 12 volts to the starter solenoid. The vehicle doesn’t start and using your meter decide that you are not getting voltage to the solenoid. You suspect a cable but how do you prove it? Place one lead on battery positive (12.6 Volts) and the other on the solenoid terminal (ideally 12.6 volts but in this case 0 volts). The difference is 12.6 Volts and will be expressed on your screen. What does this tell you? That the voltage is being lost somewhere in the cable to the solenoid. This is a very simplified version of voltage testing but it can be used to solve many problems on the car like bad battery connections. Place one lead on the battery post and the other on the battery terminal. What should it read? Zero volts or close to it, any voltage displayed is the difference between the two points and therefore the voltage the circuit is using to overcome a resistance, in this case a bad connection. This is voltage dropping simplified but should at least get you started.
Amperage is the final measurement we will cover. Amperage is electrical current or flow and is actually the amount of electrons past a certain point in a certain amount of time. An important aspect of current is that it is the same throughout the circuit. Amperage is the actual water molecules in the hose analogy. One thing you need to know about electrons is that they are lazy, they are looking for the easiest, fastest way back to battery negative. This is apparent in amperage testing, to do so correctly the circuit must be broken and the meter placed in series with the circuit, meaning that they only path for electron flow must be through the meter otherwise your reading is not accurate. A good test for if you are accurately testing for amperage is if the circuit would function without the meter in place you are not doing it correctly. In amperage mode your meter is counting electrons moving through it, this is why it is important to have the meter as the only current path because to accurately count electrons all of them must flow through it. Amperage testing can damage your meter or at least blow a fuse inside it because excessive amperage flows through it. Electrons flowing through a conductor create a magnetic field which we use in relays and solenoids but can also be used for testing with an inductive pickup. An inductive pickup or amp clamp senses the magnetic field around the wire and sends that info to your meter so you don’t have to risk your meter by putting it inside the circuit. Most meters don’t come with them but they are a great investment. One of the more common uses for amperage testing is a parasitic draw, what makes my battery go dead overnight etc.
Sorry for the rambling long post but was sitting at work and thought it might help someone. If you have further questions or anything feel free to PM me.
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