Sam Memmolo
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Electricity 102 Bobit Publishing Co. Truck & SUV Performance Last article on “Electricity 101” was the first in a series of articles designed to provide you with the necessary basics of vehicle electrical systems. We began with the battery, including ratings, testing, and functions. This article deals with additional tests including batteries going dead while vehicles sit overnight or for a few days, vehicles that won’t crank over properly when hot, and other assorted problems. I would like to begin with understanding what represents a circuit, and what happens in a circuit. Electric current will only flow when voltage is applied to a complete electrical circuit. The term circuit means a circle, or unbroken path for electric current. This is usually from the battery to working devices to ground and then back to the battery. We need to define the terms “Short Circuit”, “Open Circuit”, and “High Resistance Circuit”. Again this may sound pretty basic, but every week I hear people say they have a “Short circuit that kills their battery overnight”, or that their lights don’t work due to a short circuit. To understand what the correct terminology is, we start with “Ohms Law”. E = I over R, which means the current (Amperage) in a circuit is inversely proportional to the resistance. Let’s make that easier to understand... Ohms are a measurement of resistance. The flow of electric current in amps can be compared to the flow of water in a pipe measured in gallons per minute. Voltage in volts is the electrical pressure that moves the current, and can be compared to water pressure in PSI or pounds per square inch. Resistance in ohms is the opposition to the current flow. One ohm is the resistance that will allow one volt to flow when the pressure is one volt. The key to understanding intelligent troubleshooting is a thorough knowledge of how voltage, current, and resistance work together. Ohm’s law gives you the basic relationship: Amps = volts divided by ohms. Volts = amps times ohms, and Ohms = volts dived by amps. When a short circuit occurs, that means a conductor (wire) has been unintentionally grounded, or the electrical component is shorted to ground internally. When you ground a wire or have a shorted component, you lose the resistance (Ohms) in that circuit, and that decrease in resistance allows high current flow. If the current flow exceeds the capacity of the wire or fuse, the fuse blows. Once the fuse blows, the short circuit becomes an open circuit. That’s why you would never replace a blown fuse with a higher amperage fuse. If the fuse capacity or rating exceeds the wiring capacity, Bingo you have burnt wires! If you have high resistance, you have a bad or loose connection, a wire gauge that is too small for the load, or a defective electrical component. High resistance (high ohms) means less current flow and poor component operation, if it operates at all. An “Open” circuit means that the current path is broken or open (like a bridge washed out over a creek), and there is no current flow. This is usually expressed as “Infinite resistance”. Two of the most common problems we see
include: Let’s start with a diagnosis of the current
drain on a battery: To check for a current draw, disconnect the negative or ground battery cable and connect a 12 volt test light between the battery and the battery post. With all accessories and the ignition switch off, the light should not glow. If it glows, there is a current draw. To isolate the source, start by unplugging fuses. If the light goes out when a fuse is removed, you have found the circuit that is draining the battery. Be sure the hood light is unplugged (if equipped ), and all the doors are closed. Then follow the service manual or schematic to locate which items are powered by that fuse and begin to isolate them.
If the test light continues to glow after all fuses and relays have been removed, then follow the schematic and disconnect (one at a time) all non-fused components. Be aware that there is always a parasitic draw
in today’s cars with memory radios, memory seats, and of course the on-board
computers. The current used by these components (when the car is at rest) is so
small, that an electrically sound, proper capacity battery should be able to
maintain a charge for months. Again, all of the above relates back to the battery being tested and found to have no internal problems as discussed in my previous articles. Next we will look at a common source of problems, more critical than ever with today’s electronic platforms. Bad ground connections account for a great majority of those electrical gremlins that rear their ugly heads on late model vehicles. When we think of a bad ground connection, most technicians will grab their DVOM and go right to the ohms scale. Just remember that an ohmmeter uses a very low current to check for continuity, and ohmmeter readings can lead you astray. A better way to check for an electrically poor ground connection is with a voltmeter. For an example of how to perform this test, let’s take a suspected bad ground to a PCM. Locate the ground pins in the wire connector and hook one of your voltmeter leads to that pin. Run the other lead back to the battery ground post. The important thing to remember here is that the system should be operating, so do this with the vehicle running or at least the ignition switch on. The idea is to test the voltage drop across the cable with the circuit loaded so we can dynamically measure voltage drop. Select the low volts scale or millivolts scale on the meter. Connect across the ground cable and measure the voltage drop as described above. If the meter displays more than 50 millivolts or .050 volts, the connection or ground point is bad. Whenever possible, run a ground back to the battery to obtain an electrically sound ground connection for that particular circuit. Using the voltmeter to test using this method is called “Dynamic testing” and will help you locate weak grounds that cause problems, especially under high electrical load conditions. This dynamic voltage drop test procedure will be discussed again in future articles. Remember that the circuit must be complete from the battery, through the wiring, then through the electrical component, to ground, and then back to the battery. Whew !!! We’ve covered a lot here, so let’s digest this, and next time we’ll dive into types of circuits, and more testing. Happy Motoring! Sam Memmolo
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“Parasitic Current
draw”, where the battery goes dead with the vehicle shut off for extended
periods of time.
The
most common problems are:
1. Mercury
switches on hood or trunk lights allowing lights to remain on with the lid
closed.