If there’s one product category that causes more headaches and returns than any other, it’s alternators and starters (alternators primarily!). These rotating electrical components are a frequently replaced item on many older vehicles – even some newer ones. Starters are lasting much longer these days, thanks to quick-starting fuel-injected engines, but alternators are really taking a beating.
Alternators are calling it quits in big numbers these days for several reasons: higher electrical loads on the charging system, high underhood temperatures and high key-off electrical drains on the battery. Most vehicles today have dozens of specialized control modules in their electrical architecture. Turning the ignition off doesn’t cut the power to many of these modules because a small current is required to keep the module’s memory and learned settings alive. Most modules are supposed to go into a “sleep mode” within a preset time after the ignition is turned off. This time can vary from a few minutes up to 30 minutes or more depending on the system. Some modules such as the remote keyless entry system can remain active indefinitely. All of this means the battery is being constantly drained when the engine isn’t running and the charging system isn’t producing electricity.
On many late-model vehicles, these key-off parasitic loads may drain up to half of the battery’s charge in four or five days if the vehicle isn’t used or driven long enough to fully recharge the battery. This makes the alternator work harder, run hotter and eventually self-destruct – often because the diodes burn out. Shorted windings and/or brush wear can also cause a loss of charging output.
If a customer suspects their alternator has failed (charging light on and/or dead battery), several checks should be made to diagnose the cause. Charging output can be measured on the vehicle while the engine is running with a volt meter. Normal charging output should be about 13.5 to 14.5 volts at idle. No charging output would indicate some type of fault in the alternator, voltage regulator (if external), charging control circuit (in the PCM), or the wiring (loose, corroded or broken wires in the charging circuit).
A questionable alternator (or starter) also can be put on an alternator/starter test stand to verify its performance. If the unit tests bad (voltage or current not within specifications for an alternator, or excessive current drain or insufficient cranking speed for a starter), your customer needs a new or reman alternator or starter.
The battery also should be tested to determine its state of charge and condition. A fully charged battery will read about 12.6 volts. At 50 percent charge, it will read about 12.3 volts. If low, it should be recharged with a battery charger before the replacement alternator is installed.
Battery condition also should be checked with a load tester or an electronic impedance tester. If the battery tests bad, it needs to be replaced. If it tests good but is low, it only needs to be recharged.
New battery cables are recommended if the old ones are loose, damaged or corroded. Also, ground straps between the engine and body should be checked to make sure they are intact, not loose or broken.
So what happens if a customer installs a new or reman alternator and it doesn’t work? It’s possible the replacement unit was defective, but it should not be returned until it has been bench-tested to verify its condition. If the unit tests good, the problem obviously isn’t the alternator but a fault that was missed on the vehicle (such as a bad wiring connection, wiring fault, bad external voltage regulator or voltage control circuit in the PCM).