1990 Honda Accord LX, 4D, AT, 192K miles. Alternator went bad (original alternator ~19 years old). Open it and see carbon brushes worn out. Put new brushes in and had Autozone, Kragen, and alternator builder bench check, they all say it is good. However, every time I connect wire (the wire that has 4 pins) to alternator and start engine, it has hi pitch loud squeaking noise. If I disconnect wire while engine is running, the noise is gone even I re-connect it back to alternator. However, if I stop engine and start it again with wire connected, it has noise. Both alternator and power steering belts are new with belt conditioner. The alternator guy says diode is good. The alternator has 90amp. Anyone knows what part inside alternator is bad? Diode, voltage regulator, or something else? Thanks!

 

Sounds like a loose belt.

 

Hi Diyman
So brushes had "retired"
Like Hrjrkr I think your belt is not enough tight.
When you unplug the connector, the noise stops because your alternator is no more taking energy from the engine. As soon as you fit the connector again the noise is back, because the belt is skidding.
Please do not tke off the connector too often otherwise diodes will
Re-tighten your belt. When you press on it between pulleys it must move about 1/2 inch, no more. If the belt is too tight, bearings will not like.
My car does the same noise when it is cold and lights, wipers are on. I know this and I do not care any more.
Take care!
Marc

 

Not sure I would have even attempted a rehad on that old a unit. I wouldn't discount a bearing; same as with a loose belt, when you put a load on it you might be reaching the squeeling point.

 

Thanks to all your replied! The belt tension is adjusted based on Hanyes repair manual for new belt, 3/16" - 5/16" when you apply force. I even tried to make it more tie or loose, but the problem still there. I called alternator builder and he said the roter is going bad. In addition, the voltage is only ~13.25V which is a little low. I am guessing it is the time to get a new one. I have put back and just drive a couple of times. I will give it a couple hundreds miles try. If it doesn't improve, I will get a new one. Thanks for all your helps!

 

The problem was resolved by putting more tention on belt. Battery reads 13.5V when engine is running and 12V without.