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Electrical Basics
By Jim Mellema

It seems that many otherwise good mechanics ask a lot of questions about electricity. Maybe it’s hard to understand because we can’t see it? This text will give an understanding of how and why electricity works, and how to measure it, and how to use that knowledge to keep the electrical system of your car functioning.

Glossary

Voltage (E) Has been described as the “pressure in a water hose”, which is a semi good analogy. When I learned electronics in the Navy, voltage was described as a “difference in potential.” Note the word “difference.” To have voltage, 2 points must be electrically different. One of those points is often ground, the other any place where your cars battery voltage is connected. If your battery is laying on the floor and you measure it, it should read 12 volts. Note, we have a potential here, the battery is on the floor, not connected to anything but the meter. The potential exists to get some work done, and we can measure that, but nothing is happening yet.

Current (I) Current (measured in AMPS or Amperes) is what you have when electricity is actually doing work. If you have current, your electricity is doing something for you. If you have current, you have a complete circuit from the battery’s positive terminal, through a load that is performing some work, and then connected to the battery’s negative terminal to complete the circuit. Electricity cannot ever do any work without a complete circuit.

Resistance (R) Resistance is anything that “resists” the flow of electricity. These are typically the things that actually do the work for us, such as a headlight or a heater blower. These are the loads in the system. Wires have resistance, too, but a very small amount. Anything that conducts electricity has at least some resistance to doing so. A resistance, by it’s nature, will convert some of the electrical energy into action, and some into heat. Heat is always a product of using electricity. Resistance is measured in OHMs.

OHM’s law Defines how electricity behaves. Voltage equals Current times Resistance, always and without exception. If our voltage is 12, and we are moving 6 Amps of electricity, our resistance has to be 2 ohms. You can use algebra to shuffle the formula around so the one you don’t know is alone. Since E = I * R, then R = E / I and I = E / R.

Power (W) is measured in Watts, and is defined as the AMOUNT of electricity used. We can’t measure power, but we can figure it. Power = voltage times current. Again, we can use simple algebra to shuffle the formula. As an example, stay with me here. Our low beam headlight is a 55 watt bulb. 55 watts divided by 12 volts = 4.6 amps. If we apply OHMs law, we can determine the resistance of the headlight bulb. 12 volts divided by 4.6 amps = 2.6 Ohms.

Electrical Basics, Part 2, Circuits
By Jim Mellema

Ground: is confusing to many. Our cars use a 12 volt DC (direct current) negative ground electrical system. All this means is that the body and chassis of the car serve as a COMMON conductor. The cars body IS A WIRE that provides the common path for current to flow from the load back to the negative terminal of the battery. Our ground path has many connections, and we see a lot of weird electrical behavior due to excessive resistance at these connections. Remember, the entire ground path is a wire, and all it’s connections have to be good.

Open circuit: An open simply means the circuit is broken, that is, not complete. When a circuit is open, CURRENT CANNOT FLOW, but voltage can still be present and measured. A switch OPENS a circuit on purpose, to switch the circuit on and off. Too much paint where a ground wire attaches to the body can OPEN the circuit and cause it not to work. Same with a bad connection or a wire that fell off.

Short Circuit: A “short” provides an unintended parallel path for current flow. The circuit is “SHORT” because current will bypass the intended load and take the path of least resistance. The low resistance of the “short” may flow more current than the circuit is built for, and result in excessive heat, and possibly fire. Many refer to any problem in a circuit as a “short.” This is both wrong and confusing. Most of the time, the failure of something to function is due to an “open circuit.”

Fuse: A fuse is a “Calibrated Resistance” inserted in series with a circuit. Remember, a resistance has a voltage drop and generates heat. Fuses are rated in Amps, and the rating tells us how much current the fuse will pass before it’s resistance generates more heat than it can handle. At the point, the conductor in the fuse is designed to burn up and OPEN the circuit, preventing the flow of current. Each of the circuits in our cars (with a couple of exceptions) starts with a fuse. Fuses protect the devices and wiring in the circuit from overloads, and should never be disconnected or bypassed. Sometimes adding additional devices to an existing circuit will exceed it’s design current and blow the fuse. You should never replace a fuse with one of a higher rating. When a circuit is designed, the fuse rating and the size of the wire used are determined by the design load of the circuit. If you replace a 10 amp fuse with a 20, you very probably have made the WIRE used in the circuit the new fuse, and an overload may melt the wire instead of blowing the fuse.

Circuit Breaker: A circuit breaker is simply an automatic fuse. It operate exactly likle a fuse, except that it does not destroy itself. When overloaded, a circuit breaker “trips” internally, then automatically resets itself when it cools off. Our cars use circuit breakers in a few high current optional circuits, including power top, power windows, and air conditioning.

Circuit: For electricity to work for us we need a “circuit.” A circuit is simply a path that allows current to flow from the positive battery lead through a load (resistance) and back to the negative battery lead. A circuit often includes a switch to turn it on or off. The switch can be either on the “hot” side or the “ground “ side. Some of the circuits in our cars include “IGNITION” “BAT” and “ACCESSORY.” The courtesy lights and wipers are examples of circuits that are switched on the ground side.

Series Circuit: A series circuit is a straight line. Only one path for current flow exists, and all the current must flow through each item in the circuit. In a series circuit, CURRENT IS CONSTANT, but the voltage drop is different for each item in the circuit, adding up to the total voltage from end to end. A simple series circuit would be from the positive battery terminal to a switch to a light bulb to ground to the negative battery terminal. When the switch is on, current is allowed to flow, and the light bulb lights. At least in theory, the switch and wires in this circuit have little resistance, and the bulb sees the full 12 volts. A more complex series circuit would be the low speed blower for the heater. In this case we insert a “resister” between the switch and the “load” The resister “drops” some of the voltage, less than 12 volts is seen by the blower, therefore it runs slower than it would if it saw the full 12 volts.

Parallel Circuit: A parallel circuit provide more than one path for current. The voltage drop across each item is the same, but the current could be different if the resistance is different. An example of a simple parallel circuit would be the courtesy or dash lights. A number of “loads” are connected in parallel, each providing it’s own path for current. You know this must be true because if one bulb burns out, the other continue to light. A more complex parallel circuit would be the tach. The tach is connected in parallel with the primary coil on the distributor. The primary coil has very low resistance, therefore it flow a great deal of current. The tachometer has very high resistance, and flows almost no current, but the same voltage is on each.

Combination Circuit: This would describe most of the circuits in our car, where a single switch (in series with the load) is used to control many parallel loads (dash lights, courtesy lights, headlight, etc.)

Measuring Electricity


An analog meter has a scale and a needle that moves. The dial selects the range and type of measurement (ohms, volts, amps. This meter cost less than $10.

A digital meter works the same as analog, except it displays numbers and is easier to read. They also cost a little more. This digital meter cost about $35.

Measuring voltage: All voltmeters have an ac/dc setting. Be sure to use DC. Some voltmeters automatically set the “scale” or range of readings, most cheaper ones are manual. Set the manual scale to ~ 20 volts. The meter “reads” voltage by allowing current to flow thru a resistance inside the meter. This resistance is “calibrated” and is very high, so very little current passes thru the meter. Voltage readings are in PARALLEL to the circuit being tested. Voltage measurements are normally referenced to GROUND, but not always. Voltage measurements are made on live circuits, with power on. Due to the large internal resistance of the meter, voltage measurements do not change the functioning of the circuit being tested.

Measuring Current: To measure current, the meter must be inserted in SERIES with the circuit being tested. When set to AMPS, the meter inserts a very low resistance into the circuit, and determines the current by measuring the voltage drop across this resistance. The meter becomes a part of the circuit, all the current flowing through the circuit must also flow through the meter. Most meters only have a 10 or 20 Amp scale, and exceeding this WILL DAMAGE THE METER. It is rarely useful to try to measure current with a meter.

Measuring resistance: To measure resistance, set the meter to Ohms, set an appropriate “scale” and connect the meter ACROSS the resistance being measured. To measure resistance, the meter puts voltage from its internal battery on the meter leads and measures the current thru the resistance being tested. To get a valid reading, the circuit being tested must be OFF, and the item being tested must be isolated. As an example, to test a fuel sender, you must disconnect it, and connect the meter directly across the sender wires. If you don’t disconnect it, the meter will also read the rest of the circuit, that is the resistance provided by the gauge, and your reading will be invalid.

Test Lights: A test light can be a valuable tool as well. It is simply a 12 volt lamp, with a probe and an alligator clip for the ground connection. It will indicate the presence of power in a circuit. For troubleshooting a battery drain, it’s much safer to use a test light than an ammeter.

To be continued…

And I thought I was good at explaining things in a laymans terms.

Wow, great info. I feel like I just finished getting a drink from a fire hose.........that is a lot of information, and I am sure for those of you who are trained it is just the tip of the iceburg.

Great info, and really not hard to understand.
Compare electrical voltage and current to a garden hose with pressure and flow, respectively.

GREAT INFO AND THATS COMING FROM A TECH ALSO!!