Checking Alternator Output...

It is really simple to check an alternator to see if it is producing current, all you need is a pocketknife or a flat blade screwdriver. With the engine running, touch the smooth bearing surface on the back of the alternator. If the alternator is producing current that smooth bearing surface will be magnetized. When you turn on additional accessories like headlights or a heater blower motor the magnetism should get stronger because the alternator will be producing more current to cover the additional electrical load. This works on all types of automotive alternators. No magnetism, no alternator output.

Alternators and Amp Gauges…
If you had a generator type of charging system you were used to seeing the amp gauge needle go over to the charge side quite a ways when going down the highway. When you replace a generator type charging system with an alternator type charging system you seldom see the needle on the amp gauge move much past zero…how come?

Remember –
An amp gauge tells you which direction the current is flowing between the battery and the charging system…It DOES NOT tell you if the charging system itself is working. A discharge reading means current is flowing out of the battery…a positive charge means current is flowing to the battery and current is being replaced into the battery. A neutral (+) reading (needle is just barely on the plus side of the needle) means the battery is fully charged.

You can make the assumption that the charging system is working if the amp needle is over on the positive side and the battery is being recharged. After all the current going to the battery has to come from somewhere. Just remember that the amp gauge measures current flow and direction not charging system output directly.

You also have to remember that a generator does not produce any output below 20 miles an hour, (or 1200 rpm engine speed) so when you get up to highway speeds it has the job of replacing the current you are using now… plus the current you have already used. That is why the charge rate is always high with a generator charging system...it is always behind, trying to catch up.

By contrast an alternator type charging system is able to provide a strong current output at all engine rpms so the battery is always fully charged. In addition an alternator will provide increased output as needed for any accessories… so the battery just stores the current used for starting and the alternator provides the rest of the current needed for everything else. Because the battery is always fully charged the vehicle starts much better and the battery itself will last much longer in a vehicle with an alternator type charging system.