by Paul Abelson, technical editor
There have been several recent revolutions in truck engineer- ing and design. Aerodynamics and electronic engines come quickly to mind. But one revolution that is just now gaining momentum is the electrical revolution. Soon, we’ll have 42-volt systems to power steer-by-wire, brake-by-wire and all the added comfort and convenience systems we’re adding to our trucks. They include communications, navigation, entertainment and even cooking systems, such as refrigerators, microwaves, coffee makers and cooktops.
A few years ago at a TMC (Technology and Maintenance Council) meeting, Mark Kachmarsky, chief cab engineer for Mack Trucks, reminded us that until recently trucks had mechanical engines, one location for electrical distribution, 85 amp alternators feeding 32 circuits (16 from the battery and 16 through the ignition switch). Today, Mark says, we have at least two distribution circuits, multiple electronic modules, no less than 64 circuits fed by 135 amp (minimum) alternators. In the foreseeable future, we’ll probably have electrically driven air conditioning, an electric water pump, electronic engine valve actuation and much more. Our trucks have become not only electronic, they are becoming more and more electrical.
Most of us understand how the mechanical parts of our trucks operate. It’s easy when you see a piston move, a shaft turn or a liquid flow. These are made real because we can see them operate. Electricity is far harder to comprehend. What happens inside a wire that makes a starter turn over or a lamp light?
In the next page or two, I’ll try to explain just what electricity is, how we define and measure it, and how we can use that knowledge to help diagnose problems in the electrical and electronic systems on our trucks. Before I do, I’d like to give credit and thanks to Bruce Purkey, TMC Silver Spark Plug winner and president of Purkey’s Fleet Electrics in Rogers, AR, for his help with this article.
Let’s start by looking at the three major properties of electricity: current, voltage and resistance. Simply stated, current is measured in amperes or amps. It is the flow of electrons in an electrical circuit. Voltage, or volts, is the force or electrical pressure pushing electrons through the circuit. Resistance, measured in ohms, is what obstructs that flow. A good and often used analogy is a garden hose. When you hook up the hose and open the valve, water should come out the other end. How much water and with what force depends on the amount of water available, water pressure and resistance in the hose.
Even if there is an infinite amount of water available, flow will vary with force and resistance. The greater the force (water pressure), the more water will flow. If pressure falls, possibly when everyone in the neighborhood wants to water their lawn at the same time, the available water supply to your faucet falls and the force at the hose diminishes. Even if force and available supply remain constant, flow may vary with resistance in the hose. Put a kink in the hose and you’ve increased resistance, reducing flow. Use a larger hose and water will flow more freely. Close a nozzle and you’ve increased resistance. The three components – force (volts), flow (amps) and resistance (ohms) – are inter-related. The relationships can be calculated using the formula (Ohm’s Law) E=IR, where E is the voltage, I is the amperage and R is resistance.
Volts times amps are watts. Watts are a measure of work being done, as in a 100-watt bulb or a 60-watt motor.
When electricity flows through a wire, a magnetic field is created around the wire. When a wire is moved through a magnetic field, current flows in the wire. This is what makes starters, electric motors, alternators and generators work. The strength of the magnetic field varies with the current in a motor. The amount of current varies with the strength of the magnetic field in an alternator.
There are two types of circuits: series and parallel. When circuits are in series, if one part fails the entire circuit fails. Series circuits are additive. If our four 12-volt truck batteries were hooked together in series, we would have a 48-volt circuit. If one battery failed, the entire circuit would be open.
Parallel circuits run side by side. Truck batteries are connected in parallel. Amperage is added together, but the system runs at 12 volts. Four 12-volt, 600 cold cranking amp (CCA) batteries in series would produce 600 CCA at 48 volts. In parallel, the same batteries produce 2,400 CCA at 12 volts. Watts would be the same: 600 x 48 = 2,400 x 12.
Headlights are another example. Two 55-watt headlights in parallel draw 110 watts, or 9.2 amps at 12 volts. If one fails, the other continues to burn, drawing 4.6 amps, still at 12 volts. If they were in series and one blew, the other would go out.
Sometimes you can have series and parallel circuits together. The headlamp circuit is protected by one fuse, even though it protects two lamps. The wiring divides into parallel circuits after current flows through the fuse wired in series.
sources of current
In order to have current flow in a circuit, the circuit must first have a source of current. Household current is generated by moving wires passing through a strong field of permanent magnets. The force to move windings of wires can be falling water, steam turbines fueled by everything from coal and oil to nuclear energy.
Automotive current is first stored and released chemically. Once the battery has powered the starter and the engine is running, current is generated by wires moving in a magnetic field inside the alternator. The alternator is driven through a belt from the engine. The chemical storage and release takes place in a battery. The generation takes place in the alternator.
A battery generates current chemically. When two dissimilar materials are immersed in a solution that can carry current (an electrolyte) such as salt water, acids or alkaline solutions, they have the potential to make electrons flow from one to the other through the electrolyte. Before this can happen, there must be a path for the electrons to flow back to replace those transferred from the material giving up electrons to the electrolyte. That path is called a circuit.
Flow occurs only if there is a complete circuit. A switch is a device for opening or breaking the circuit so electrons don’t flow.
This term has its origins in the study of lighting, where electricity generated in clouds flows to earth or to the ground. In trucks, we can complete, or ground, a circuit one of two ways: through sealed wires to the negative post of the battery, or to, then through the chassis, and through the battery’s ground cable back to the negative post.
The easiest is to ground a device – say a trailer light – directly to the closest metal. Electrons then flow through the metal, to the trailer chassis, through the fifth wheel to the tractor chassis, then to the battery. As it passes between dissimilar metals, electrolysis or corrosion may take place. The best way to ground any device is through sealed connectors to wires that take current flow directly to the battery.
We saw that with nothing more than the presence of an electrolyte, current will flow between dissimilar materials. In a typical lead-acid truck battery, the materials are lead and lead sulfate. The electrolyte is dilute sulfuric acid. As electrons flow, some of the sulfur in the acid reacts with the lead, converting it to lead sulfate and weakening the acid. That action is what runs a battery down. When current flow is reversed during the charging cycle, the current strips the sulfur from the lead, putting it back into the acid. That rebuilds the electronic force or potential of the battery, raising its voltage back to its maximum. That’s how an alternator recharges a battery.
That same action, stripping atoms from one material and combining them with another, occurs when current runs through an exposed junction of two dissimilar metals. They could be copper and steel, aluminum and steel, brass, chromium or any combination. If in a salt spray environment, or if just wet from rain and road spray, the flow of current can eat away at the materials. That’s why exposed wiring corrodes and why components seem to be eaten away by winter road chemicals.
This issue, we’ve covered the principles of what happens to, with and because of electricity in our trucks. In the next issue, we’ll review how to spot electrical problems, how to diagnose them and how we can take some preventive measures.
Paul Abelson is Land Line’s technical editor and freelances from his office in Lisle, IL.