grantman18

I've had a whine coming from the compressor/alternator area of the engine for a month or two that's most notable when idling. I left my lights on the other day, so I had to get a jump, and for the first 30 minutes or so the whine was a lot worse. I got the battery load tested and learned that it was good as dead, so I replaced it.

The new battery made a huge difference in how bright my interior and head lights are, and the whine coming from the engine isn't nearly as bad. My question is whether or not I should replace the alternator. I'm getting 14.1 volts while the truck is running, but I'm worried that running with the weak battery for so long may have compromised the alternator. Is there a rule of thumb on when to replace it?

grantman18

oh and it's a 2000 with 137k miles. I've had to do the compressor and water pump in the last year, so I'm guessing the new england weather has taken it's toll on my xj

grantman18

any thoughts? Today would be a good day to make the swap if necessary.

RTorrez1

The whine could be a bearing in the alternator going bad. I can't be 100% about that with out hearing it. Take off the belt and see if you have extra play on the alt. pully. That's where I would start.
Good luck

rrich

Tune your AM radio between stations. Turn up the volume.
If you hear the whine through the speakers, you have a bad/shorted diode in the alternator.
More than likely it caused the death of the battery. It will quickly destroy your new one too.

The alternator is a 3 phase AC generator. Diodes are used to turn the AC into DC.
It charges with pulses 6 times each revolution. If one diode is defective it disables 1 entire phase, then it charges with only 4 pulses, only 2/3 of the time.
If a diode is shorted, it drains the battery during that phase - so it charges 4 times and discharges 2 times every revolution.
That tears up the battery. It overheats and the plates warp.

You can hear a shorted diode by listening under the hood, and verify it with the AM radio. FM won't hear it as well.

Most autoparts stores that will test your alternator don't do a diode test - the operator doesn't understand what it is so he bypasses it.
A bearing is usually described as more of a growling rather than a siren type whine.