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Chilton Auto Answers: Why Do Alternators Go Bad?

Cutaway alternatorModern cars are very reliable, but there are still components that will fail, most likely before the car hits the magic 10 year/100,000 mile mark, and the alternator is one of them. Your car uses a battery to turn the starter, run the electronics, and fire the spark plugs when starting, but once the motor is running, the alternator spins and generates power. Even when you have a bad battery a car can often be jump started and driven to the auto parts store because the alternator is putting out power; the battery doesn’t have to do much. If the alternator goes bad, and the car has to run solely on power from the battery, and you only have a few miles before that starts to go flat.

There are a few different failure modes of alternators and the charging system in general, but they can happen suddenly without warning when a diode burns out or a wire shorts.

The common failures and their symptoms are:

  • Undercharging – This is the most common problem.  If the headlights get dim at idle and get brighter when driving, the alternator is probably undercharging.
  • Overcharging – The alternator is by design putting out too much power, but there is a regulator that controls this by switching it on and off as needed.
  • No Output – The more extreme version of undercharging is not charging at all. The lights will dim, and in a matter of minutes the fuel pump and ignition will drain the battery
  • Mechanical Issues – Eventually everything wears out. An alternator with 250k miles on it might still charge, but the bearing can get noisy, or loose, or seize to the shaft suddenly.

Three old alternatorsUndercharging

If you measure the power with a multi-meter while the car is running. an alternator that isn’t putting out 13.8 volts at cruising RPM is in need of help. Normal wear and tear of the carbon brushes can cause this, or an internal short. But, if low output is combined with a high pitch squeal, it may be that the auxiliary drive belt is slipping. If the belt is tight and not old or shiny, low output can be due to the insulation breaking down on the internal alternator wiring. Or the carbon brushes that pass electricity to and from the windings may be worn and barely making contact. If there is a local rebuilding shop nearby you can often save money by having your unit rebuilt with new bearings, brushes and windings, when compared to the price of a new one.


An overcharging alternator will kill a battery by boiling the acid out of it, which can ruin your paint or even cause a fire. If there is an acidic smell or the battery leaks fluid after several minutes of driving at moderate RPM, the alternator is likely overcharging. Check this by measuring the voltage with a multi-meter at the battery with the car running; even with the engine reving over freeway speeds it should not go above 15 volts. The first thing to check is that all the wires to the alternator are connected and not broken, and the voltage regulator (if it is a separate unit). External regulators are usually inexpensive, and even on most alternators with a built in unit, can be changed easily enough. Many modern cars control the alternator with the computer that controls the fuel injection, ignition, and everything else; an overcharge situation in this case may require an expert.

No Output

When the alternator pats out no charge at all, there are several possibilities. First check that the auxiliary drive belt hasn’t broken or come off the pulley. On many cars the belt is held tight by a spring loaded tensioner, which can fail or get weak, allowing the belt to slip. Alternators produce alternating current (AC), and batteries needs direct current (DC), so a rectifier is there to convert it, and that can suddenly go bad, causing a no charge situation. Typically, a no charge condition means an internal break or short in the windings, bad carbon brushes, or burned out slip rings. But of course, you should check all the external wiring connections first. If a multi-meter set to DC doesn’t read 13.8 or more, but switching to AC gives you a reading, it is the rectifier.

Old junk alternatorsMechanical Issues

Like anything with moving parts, the bearings in the alternator only have so many miles before they go bad. At first they will just make noise, but if let go a bad alternator bearing will cause the auxiliary drive belt to come off which will cause overheating as well. To avoid this happening prematurely, check the drive belt and tensioner and make sure they are in good shape. An over tightened belt can cause the bearings to wear quicker, but a belt that is loose isn’t good either. On cars with a spring-loaded tensioner, inspect it every 50,000 miles, which is also when the belt should be changed. On older cars, and other with a manual tensioner, tighten it to the specification found in your Haynes Manual (a good rule of thumb is to leave it just loose enough to allow twisting it 90 degrees at teh longest run).

Keeping an alternator working well is often a matter of keeping it clean and dry. Dust and dirt getting into the alternator can over time cause shorts or wear the brushes. The wire windings have a thin coat of clear insulation on them, so avoid getting solvent on them, like oil, gasoline, or carburetor cleaner. Exhaust heat and oil leaks can also shorten the life of an alternator, so fix them as soon as possible.

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