Alternator whine on 1991 Honda Civic DX

After 198K miles of flawless service I replaced the OEM alternator with a Bosch rebuilt unit. Now 25K later the alternator has developed a noticeable whine: the pitch is proportionate to engine speed and the loudness is proportionate to the amount of current output. The alternator is quiet when started in the morning but the whine becomes noticeable as soon as the headlights are turned on. The whine continues so long as the engine is operating. I have heard that many rebuilt alternators are unreliable so here are my questions:

1) What causes this whine? Does this suggest the bearings are failing?

2) Should I replace the alternator?

2) Are Denso rebuilt units more reliable than Bosch rebuilt units

I would say you probably have a bad bearing. I too have had rebuilt alternators go bad. When you get another one, make sure you get a decent warranty and keep your receipt. Rebuilds are always a bit of a gamble, just do your best to protect yourself.

It is also possible that you have a shorted diode in the alternator.

Whether you have a bad bearing or bad diode, the fix is the same.   Replace the alternator.

I’m on the diode crew. The pitch would change with engine speed with either a bad diode or a bad bearing, but a bad bearing noise would not change with electrical load. To verify, bring the alternator to Advance Auto Parts, either on or off the car. On the car, they will run a test that takes about ten minutes. If you bring it in off the car, they’ll run a test that takes two minutes, including hookup time, and will eliminate the possibility of a bad battery fouling up the test.

You will most likely be replacing this alternator.


Let’s hold off on replacing the alternator for a moment. First off, you can get a whine like that if the belt is overtightened. Easy to check that - twist the belt at the centerpoint between the alternator and the crankshaft pulley. You should be able to twist it 90 degrees. Any less, and it’s too tight.

If the belt is good, just wait patiently. Some of those alternators just whine. Mine does, and it’s been doing so for 5 years now, but the output is steady and there’s nothing wrong with it. If it does start to actually die, you’ll often smell a “burnt marshmallow” odor before it dies, so you’ll have some warning. and even if you don’t, the battery light will come on, and you can replace it then.

I have found with Honda and Toyota that I have the best luck with junkyard OEM alternators rather then aftermarket rebuilds. If you can’t afford a brand new one from the dealer find a junkyard and I am sure you can get a 70,000 to 100,000 mile alternator which, based on your experience, will likely last for some time. And they are cheap too!!!

Replacing an alternator can be pointless if your battery is old. The most common cause of alternator failure is a failing battery, it may have killed your old alternator and will do the same thing to your replacement … then the replacement brand of alternator gets a bad rap.

Those junkyard alternators are a good deal, as they should last until 240,000 miles before you can expect trouble. I had both a Honda and Toyota alternator quit at over 240,000 miles. A junkyard is where I find replacement brake calipers. The new one for an 83 Camry cost a lot and the uded one cost $10.