By Glen Beanard
Contributing Editor

Electrical principles – voltage, amperage and resistance – can be described as cars passing along a pockmarked roadway. Voltage is represented by the vehicle’s speed, amperage is the number of vehicles on the road at the same time, and resistance is the potholes in the road’s surface.

Basically, electricity is a supply of atoms with an excess number of electrons. The flow of electricity is the actual exchange of electrons from atom to atom. Keeping with the cars-on-the-roadway analogy, we’ll also discuss diodes, DC and AC current, and define the term “short circuit.”

Think of diodes as one-way traffic cops. They act as the check valve in an electrical system, only allowing electrons to flow in one direction on one-way streets. The two ends of a diode are called an anode (+) and a cathode (-). The cathode end of the diode has an extra amount of electrons stored in its materials. The anode has a shortage of electrons.

Electrically, the anode has ‘holes’ that happily accept those extra electrons from the cathode side. The light bulb is glowing because, at the junction between the anode and cathode, the holes and the electrons meet. The electrons then fill in the holes, lower the diode’s resistance, and current then flows through the circuit.

The terms direct current (DC) and alternating current (AC) refer to the manner in which the electricity flows within the conductor. In DC, the electrons all flow the same direction, much the same as vehicles all traveling in the same direction down a single lane of traffic.

AC, however, requires a little more imagination to picture. The electrons don’t flow so much as they vibrate. They move in one direction, then change direction moving completely the opposite way, then back again. This would be one area where my illustration of cars traveling on the road is weak when explaining electricity. After all, it’s not every day you see people repeatedly slamming their cars in drive and reverse.

A short circuit resembles a driver cutting across a parking lot to avoid a traffic light. Just as that would be a wrong action for a driver, it is also an unwanted path of electrical flow in a circuit. The biggest problem with a short circuit is that the unwanted flow path often has little to no resistance to regulate flow. The electrons will take the path of least resistance. They will overcrowd (so to speak) the wiring and cause it to melt, possibly resulting in a fire.

The fuse (a circuit breaker or a fuse link) is the emergency kill switch. The fuse sacrifices itself to stop the errant flow before it melts the wiring. Fuses are there for emergency reasons and will blow only if the amperage has reached an emergency level. Under normal operating conditions, they don’t get hot, and they won’t age like a light bulb. When they do blow, they create an ‘open’ circuit where no current flows at all.

Getting a Charge Out of It
The purpose of an alternator is to supply the power needed for all electrical items on the vehicle, plus replenish the battery from the last start up. If the battery doesn’t get fully replenished, it will remain in a state of discharge and will sulfate and become inactive prematurely.

Keep in mind though, that the alternator is not a battery charger so much as it is a battery maintainer.

If the alternator has to recharge an overly discharged battery, the alternator will become overworked, which will shorten its life. This is largely due to the high amount of heat produced by the alternator during its charging process. The greater the amperage flowing through it, the higher the heat an alternator creates. So, anytime an alternator is replaced, the battery should be fully recharged with a battery charger or replaced.

An alternator transfers the mechanical energy from the belt into electrical energy. To make this possible, the alternator actually borrows a few electrons from the battery to get the process started. Inside the alternator are two strong electromagnets. One is called a “rotor,” which spins inside of another electromagnet called a “stator.” As the poles of these magnetic fields collide, an electrical current is induced into the stator.

The voltage induction grows stronger as the poles approach each other. In contrast, as the poles move further apart, the voltage induction steadily reduces. In reality, there are three magnetic fields in the stator spaced 120 degrees apart. They are created by three separate windings in the stator, which produces three times the number of waves shown in the figures. It’s like three separate alternators inside one housing.

The current induced into the stator is AC current. This is because the voltage induced into the stator changes polarity every 180 revolutions within each magnetic field due to the polar relationships between the stator and rotor being inverted. The current induced into the stator is obviously of no use to the automotive electrical system at this point. To be of any benefit, the current must pass through the diodes.

The diodes form what is called a “full wave rectifier,” which allows the positive polarity of current to pass and inverts the negative polarity current into what is desired.

The voltage has to build up to the desired 12-vdc to 14.5-vdc range. Since automotive electronics are all designed to run in that range, much of the alternator’s output is both useless and jeopardizing to the rest of those components. This is where the battery plays a very important part. Any voltage below the battery’s own base voltage is canceled out. Filling in the low places and absorbing the excessively high places in the alternator’s output is how the battery ‘silences’ electrical noise in the system.

All alternators have some sort of device for voltage regulation. Without such regulation, the alternator could fail to meet electrical demands or could be free to run away and overcharge electrical components to death.

Diagnostic Tips
An automotive charging system is pretty much the same from model to model. Yet, at the same time, it can be quite different. For some vehicles, it’s normal to see as high as 16 volts for long periods at a time. For others, it’s normal to see as low as 13 volts. For some, a constant 13.6 volts is an indication of a problem. Still, for others, it’s perfectly normal to see the alternator not charge at all intermittently. Some alternators are controlled only with an internal or external regulator. Some are controlled only by the PCM. Still others are controlled by a voltage regulator and the PCM. Not knowing what controls what, and how it is supposed to function, not only causes misdiagnosis when there is a problem, it also can cause a critical problem to be overlooked.

When confronted with a balky charging system, the best first test is a voltage test. As with any electrical device, the most common failure is the device not functioning at all. So, if an alternator has quit functioning, then only battery voltage (approximately 12 volts or less) will be seen at the battery.

However, if only battery voltage is present on a running engine, does this mean the alternator is bad? No, it does not. That only means that the alternator is not charging. That test doesn’t reveal why it’s not charging. Therefore, it doesn’t prove a faulty alternator. Too often, the alternator is condemned by technicians who rely solely on this test. But a technician should also consider the following:

• Does the alternator have power to turn on the rotor and stator?
After consulting a wiring diagram, test the light gauge wires at the back of the alternator for battery power. If there is none, this might be due to a broken wire, an undone connector in the harness, faulty ignition switch or even a blown fuse that was caused by a shorted component that’s not even located in the charging system.

To condense the size of fuse panels, engineers have placed many unrelated circuits on the same fuse. Should a short develop in any one of those circuits, the alternator will fail to charge due to fuse “T” being blown. What’s more, if the driving conditions are just right before the vehicle comes into the shop, there may not even be a “Check Engine” light accompanying that “Battery” light. Without proper testing, that alternator may be replaced to no avail.

• Is the charging lead between the alternator and battery intact?
It’s entirely possible that the alternator may be charging without that voltage making it to the battery. If the techs are testing for charging voltage at the battery, and not finding any, the test should be repeated at the point where the charging lead meets the alternator. If charging voltage is present there, the charge lead is ‘open’ somewhere. That might be due to a blown high-amperage fuse or possibly a severely corroded cable.

If only battery voltage is present, you have not only proven that the alternator is not charging, but also that the charge lead is at least connected from the alternator to the battery.

• Is the alternator supposed to be charging at the time you are testing?
Believe it or not, some vehicles don’t leave the alternator charging at all times. Some actually turn the alternator off when not needed. Honda did this back in the 1980s on HF (High Fuel) vehicles. To help cheat a few extra miles per gallon, Honda added a module near the brake pedal that monitored system voltage. It turned off the alternator when it wasn’t needed.

Today, some vehicles still employ that tactic, but they use the PCM to control that feature. However, just because the PCM controls the alternator doesn’t mean that its PCM is programmed to completely turn off the alternator.

So, you need to be familiar with the operating strategies for the charging system. To be able to identify it as being ‘bad,’ you first must know that is ‘good.’ You must know system ‘description and operation.’

OK, what if you do have charging system voltage present at the battery? Is the alternator proven to be good? No, all you have proven at this point is that the alternator is, at least, turning on. The electricity might be flowing with enough voltage, but that doesn’t mean anything if the amperage is not there.

The next logical test is to check the alternator’s peak amperage output. There are many different amperage ratings of alternators. Find out what the peak amperage rating is for the one you are testing. It should be stamped on the side or back of the alternator. However, it may not be viewable with the alternator installed in the vehicle.

• Is the alternator capable of charging at its maximum rating?
First, raise the engine rpm to 2,000 and hold it there for the duration of the testing. Then, with a capable load tester connected to the battery cables and an ammeter clamped over the charging lead, the technician must now place a load on the system.

Watch the system voltage on the load tester while increasing the load. When the system voltage lowers to 12 volts (battery voltage), measure and record the amperage. Immediately remove the electrical load from the system. Compare the measured amperage against the rated amperage. It is acceptable for the amperage to be higher than the maximum specs, but it should not fall short of the rating by more than 10%. For example, a 100-amp alternator can peak at 130 amps, but it should not peak at less than 90 amps.

• What if the peak amperage is too low? Does that mean the alternator is bad?
A weak-charging alternator can be the result of a slipping belt (they don’t always squeal), a slipping pulley on a degraded harmonic balancer, or high resistance in the wiring (either the charge leading to the battery or the stator and rotor primary power supply).

After inspecting the belt drive components, a voltage drop test must be performed to determine the heath of the wiring. To voltage drop test the positive cable, place the negative lead of a voltmeter on the positive battery terminal (not the cable, so as to include the clamp to terminal connection). Then, place the positive lead of the voltmeter at the cable to alternator output junction on the alternator. Have a coworker help you raise the engine rpm to 2,000, and load the electrical system to the alternator’s peak output. Measure the voltage showing on the voltmeter. Preferably, this reading should be very low, no more than 2 volts.

The electricity should ‘prefer’ to travel through the cable, since the resistance of the cable is so much lower than the resistance inside the voltmeter. Repeat that same test for the negative side by testing from the alternator case to the negative battery terminal. The negative side should be less than 1.5 volts. As for the stator and rotor primary feed, you will have to consult a wiring diagram and service manual to determine what to measure. Some may require 12 volts only, while some require a duty cycling ground back to the PCM. Some might have a 5-volt duty cycle feed coming from the PCM.

• Is there anything else?
Yes. Usually, faulty diodes would have shown up as a weak alternator during the peak amps test, but not always. Most battery testers will automatically say the diode is “good” or “bad,” or some have a “ripple” meter on them to simplify this check. This check can also be performed with an oscilloscope.

If the diodes are faulty, they will pass AC current into the vehicle’s electrical system, creating a ripple effect on top of the DC voltage. The idea is to measure this ripple effect. Keep in mind that some ripple effect is normal, and it is the duty of the battery to dampen the ripple. A “failing” reading can be the result of a faulty battery. So, if you see the tester is showing that the diode is bad, repeat the test with either a new battery installed or with an emergency jump-start pack clamped to the battery cables before rendering the final verdict.

Another thing that should be checked is a scan of the PCM on vehicles where the PCM controls the alternator. Scanning for codes and observing alternator command PIDs are very important in proper diagnostics of those systems. For example, upper 1990s-and-later Ford Windstars used a voltage regulator inside the alternator that was controlled by the PCM.

If communications were lost between the PCM and alternator, the PCM would turn on the “battery” light, but the regulator would still charge the alternator at about 13.6 volts. The PCM would set a trouble code relating to that loss of communication. Imagine trying to diagnose a battery light that doesn’t appear to have any reason to be on. That is, it doesn’t appear to have a valid reason until a scanner is added to the testing.