Car owners can become extremely frustrated with intermittent alternators: if the part went bad completely, the source of the problem would be easy to trace. With a sometimes good, sometimes bad alternator, the problem is elusive, since it may not be observable the majority of the time. By understanding the problems associated with intermittent alternators, you can determine if the alternator is indeed the root culprit.
Erratic Ammeter Movement
If your car, boat or stationary generator has an ammeter, the needle will behave erratically. Sometimes it will be in the normal range, but other times it will jump around suddenly from undercharging to overcharging.
Lights dimming and becoming bright intermittently is a sign the alternator is having intermittent problems. ASE (Automotive Service Excellence) certified technicians at 2 Car Pros recommend checking all the wires first to make sure they are making good contact. If they are, the alternator is to blame.
Hesitation and stalling is a common problem with an intermittent alternator. First check out the fuel system to eliminate it as the cause. Second, check out all the other electrical systems, such as the ignition system, to eliminate them. If all the other systems are good, the problem is in the alternator.
Hot Temperature Problems
Since the alternator is in the engine compartment, it will heat up along with the engine. When the engine and the alternator are cold, everything is running fine; the alternator is putting out the correct voltage to charge the battery, with no voltage spikes or voltage drops. As the alternator heats up, problems start to occur as indicated by the ammeter. The needle in the ammeter may start to jump around wildly or go into the under- or overcharging range.
The internal components in the alternator work when cold but as they heat up, a bad component will stop working. This is because the bad component's internal resistance changes; bad internal connections also separate when hot. An alternator that works correctly when cold but stops working when hot is a common sign of an intermittent alternator problem.