Alternator hum

Info: 1990 XJ 4.0 L Auto

I was recalibrating the TPS so I had the ignition on but the engine not running. I was hearing a hum from the passenger side of the engine. By using a piece of hose to my ear I tracked the hum to the alternator. What would make the alternator hum with just the ignition on ? I do not recall hearing this hum while adjusting the TPS in the past.

Thanks

DanDXJ

The field is energized whenever the ignition is on, but it's DC, so I'm surprised it would hum, and I don't think it's normal. Just to be sure, you might pull the plug on the alternator and see if it changes.

I had an older GM with the CS style alternator that would do that with the key on-- I was just getting used to it when the diode trio failed-- it may be a sign of a failing/failed diode. I'd have it checked as soon as convenient.

I crawled under to remove the plug to see if the humming would stop. As I was removing the plug I could feel the alternator vibrating. Both the humming and vibrating did stop when the plug was removed. It is currently putting out 14.4 volts when running. I have a lifetime warranty but they probably would not give me a new one just because it hums if it still is charging. Thanks for the input, I'll just have to keep an eye on it I guess.