Why Does An Alternator Go Bad?

A) First, in general, why does an alternator go bad?

B) Second, how long should an alternator last? Consider: I just had a new alternator installed 2 years ago, tops, and a new battery at the same time. Recently, I’ve been forgetting to turn my lights off, and I thought I’d beaten the life out of this relatively new battery, so I bought a new battery 1 week ago. In that week, the car was still sluggish to start (as I wrote in a previous thread) and then it died with 1.5 cranks of the starter motor. My mechanic says the alternator is bad and killed the new battery.

What would cause a new alternator to die? The last one was original equipment and lasted 200k miles. Could it be a corroded cables on an old car preventing the alternator from working properly? Or, could the alternator have been used or faulty equipment as I had to take a chance on a mechanic unknown to me (who charged me for additional work I didn’t authorize).

…And, do you think a new alternator will last this time? With 250k miles, I just need another year out of this 9 yr old Honda.

Your thoughts?

You got sold a crap alternator. If you don’t want to invest in a new one, maybe you can get a rebuilt cheaper. Your last choice (but probably the cheapest) reliabilitywise is one cannibalized out of an auto carcass in a scrap yard.

The No. 1 reason a alternator goes bad is the brushes wear out. These cost about $0.25 each.

The No. 2 reason is the voltage regulator goes bad. Unlike No. 1, this greatly depends on the make of car you have. That is, it depends on the design. If the voltage regulator gets hot, it can fail sooner. Some designs run hotter than others.

Rebuilt alternators are often junk. The big rebuilding houses buy old alternators by the truckload. They separate them by make, then disassemble them, throwing the housings in one pile, the rotors in another, and the stators in a third. Then they clean them and reassemble. But, they aren’t careful to make sure they don’t have one half of the housing from a 5 year old one, the other half from a 10 year old one, a rotor from a 4 year old one and a stator from a 12 year old one.

Why does this matter? It has to do with tolerances. All of these parts are made to meet specified tolerances. But, on the new assembly line, if the housings coming from the die-casting machine are to the high side of the tolerance, they may run bearings towards the low end of the tolerance range to balance things out (or similar). That is, they can easily afford to match tolerances to have a better fitting component. They do this to make their product last longer and gain a reputation of high quality.

As I mentioned, on rebuilt units, they aren’t as careful. It generally doesn’t matter, since once a car has worn out the original alternator, it is probably near dead so the replacement alternator only has to last a few years, not a decade or more.

If you know you have the original alternator (or starter motor) for the vehicle, you will be better off having it rebuilt than buying a rebuilt one, since they aren’t likely to swap parts if you take it to a local shop for a rebuild.

Oh, one thing that will kill a brand new alternator is to install it in a vehicle with a partially charged battery. With new brushes, there can be quite a bit of arcing if the current load is high (it gets less once the brushes are worn in). This arcing can cause pitting on the contacts, causing the brushes to wear very rapidly. The high initial current load can also blow the voltage regulator. Always make sure you have a fully charged battery when you replace an alternator. New batteries are generally only about 3/4 charged, so it is a good idea to put it on a battery charger. A “good” mechanic will do this when he replaces an alternator. Some may not.

Heat and friction, wear and tear. Not that hard to understand, really.