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February 12, 2010
Diagnosis
Alternators and Charging Systems
Prior to the advent of alternators (a 1960’s innovation that can be attributed to Chrysler), most vehicle charging systems utilized a dC generator. Looking much like belt-driven starter motors, dC generators had weak low-rpm performance, and required fussy and unreliable mechanical voltage and current regulators to control their meagre output.
Alternators are actually AC generators, and their primary benefits are much greater output—particularly at low rpm—higher overall efficiency, and more compact construction.
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This 1999 Taurus uses an
internally regulated alternator.
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This externally regulated alternator
is on a 2000 Neon.
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Internally regulated alternator
on a 2004 Chevy Silverado.
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Two primary types
While there are different internal wiring designs out there, alternators can be generally categorized as one of two major types—internally regulated and externally regulated. Familiarity with both types will greatly aid your diagnostics, as testing procedures differ between them.
Internal affairs
Since the 1970’s, most Asian imports, and nearly all General Motors vehicles, have used internally regulated alternators. Particularly in older applications, these alternators have needed only battery voltage and key-switched power connections to operate (ground is generally provided through the housing), simplifying both wiring and testing. If power is present at both the main “B+” terminal and the “IGN” trigger terminal, key on, yet the alternator isn’t charging, the fault likely lies in the alternator.
Be aware, however, that many alternators require 12V power at a second “B+ Sense” terminal in order to function, that the IGN feed circuit may not go through the charging indicator in the dash (just because the light comes on, don’t assume that the IGN terminal is powered), and that newer alternators will often have additional wires that allow the engine computer to alter charging rates to improve fuel efficiency. Consult a wiring schematic to be certain.
Thinking outside the box
Externally regulated alternators are less common these days—unless you’re under the hood of a Chrysler product. Many Fords, up until the mid-nineties, and virtually all Chryslers (excluding rebadged Mitsubishi models) use externally regulated alternators.
Eighties and nineties Chryslers often incorporated the regulator into their fendermounted “SMEC” units, while other automakers (like Ford) used stand-alone external regulators. Virtually all newer Chryslers have the regulator built into their engine computers.
Straightforward testing
In Chryslers, diagnosis at the alternator is relatively straightforward; power at the B+, key or ASD relay-switched 12 volts at one of the two fi eld terminals, and pulsewidth modulated ground at the other fi eld terminal (other manufacturers may modulate the positive side, but the principle is the same).
Briefly grounding the negative field terminal with the engine running (“full fi elding”) should cause the alternator to charge as proof of function, but do not do it for long, as excessive system voltages—and possibly damage—will result. (It is also possible to do this test with some internally regulated alternators.)
My experience with Chrysler charging systems suggests that the computer seldom fails—engine and chassis ground issues are the most common electrical faults when the alternator itself checks out OK.
Shared faults
Both styles of alternator have common failures—mechanical, such as bearings or loose/damaged/corroded connections (watch for loose belts, too); and internal electrical, like open or shorted windings, worn brushes, and failed diodes (required to convert the AC current produced into useable DC).
Charge lamps on while running (where the lamp’s circuit is actually the trigger source), unusual noises or noise levels, or excessive AC ripple—measured by setting your multimetre to “AC Volts” (less than 0.5 VAC, measured across the battery terminals, is generally considered acceptable)—are also indicators of faulty alternators, and should result in their replacement.
Familiarity with both types will greatly aid your diagnostics, as testing procedures differ between them.
3- Internally regulated alternator on a 2004 Chevy Silverado
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Brian Early