Auto Service World
Feature   November 1, 2004   by Jim Anderton

Alternator Basics

It spins and makes electricity, but keep a few basics in mind when it doesn't

Alternators, or alternating current generators, have been standard equipment in cars and light trucks since the early ’60’s. Alternators replaced D.C. generators because they produce better current at lower engine RPM’s, are light and compact. All technicians learn about how a spinning rotor cutting a magnetic field induces an electric current. And everyone knows that output is controlled by varying the current through the field coils to keep alternator output as independent of engine RPM as possible. You can simply hook up a tester to the charging system and get a “go-no go” test result, but what if your troubleshooting issue is more complex and you suspect that the alternator is a factor? Keep the following facts in mind when thinking your way through that weird intermittent or “impossible” fault.

Why does the Alternator produce operate at higher than the battery’s nominal voltage?

It has to. Without a potential difference between the two, current can’t flow to the battery. Nominal voltages in the 14 volt range are normal, since the battery represents both the receptacle for incoming charge, and a resistive load to the alternator.

How much of the load during steady state driving is carried by the alternator compared to the battery?

100%. If the system is healthy, the alternator supplies the small variations in current needed by intermittent electrical loads like turn signals, A/C clutch cycles, etc. This is one of the reasons why it can often make sense to replace the battery along with the alternator, giving the new unit’s regulator a chance to operate within its limits, and conclusively taking charging out of the loop if you have a complex engine control or drivability issue. When the customer talks about an intermittent electrical issue, check to see if it’s correlated with periods of high electrical demand. When test driving, turn everything on to simulate peak alternator load.

How much current passes through the field control/sense wiring?

Not much. Field wiring might carry five amps, sense wiring much less. As a result, the wiring is light gauge, and uses relatively light-duty connectors. The implications are clear: voltage drops here, say, feedback wiring to an ECU, and erratic computer decisions can result. Check every connection and look for frayed or corroded wiring, even if it’s only for the idiot light. Everyone’s seen the famous melted Ford connector, but it happens to other brands too. Replacement connectors are cheap, and well worth installing with a new alternator if they’re the slightest bit melted or corroded.

What about heat?

It’s a killer, yes, but modern units are designed to withstand the poor ventilation and tight confines of many modern vehicles. Unfortunately, they do it by using incredibly open end frames, exposing much of the good stuff to water, dirt and oil seepage. Oil is more important than you think. While it’s an insulator, the sticky residue attracts moisture-containing dust and dirt, leading to trouble. Aerosol cleaners? Even if you can find one that won’t hurt the unit’s insides, driving dirt into the alternator isn’t a good answer. A shop vac is a good tool here. And if oil is a factor, don’t forget to look at the belt.

Think about that ‘scope trace.

Modern charging system testers are easy to use, but if you’re using a ‘scope to dig deeper, check the voltage division setting and the time base at the bottom of your trace carefully. Altering the time base can flatten the telltale spike of a bad diode, or make the normal ripple of a healthy unit look like trouble. Looking at the waveform, however, is still the only way to find a single bad diode or poor output filtering. DC voltage is not enough.

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