Alternator Diode Noise?
 

I have a 3.5 honda that makes a whining type of noise. It's louder when the AC compressor engages.

From research, it seems like a bad diode in the alternator. Is it common? I do not get any dash lights...is this something that needs to be taken care of ASAP?

Is there any way to test it in the car vs on the bench? I've saw a guy on youtube unplug the back of the alternator while running and it would go away. I don't have enough room to try that. :)

TIA!

A/C clutch puts another load on the drive belt, so I would guess the alt bearings first.
Make sure it is not too tight.
Drive belts do harden and/or lose grip over time.

If you have a multimeter- set it to A/C scale, connect the leads to the battery terminals. Any reading indicates bad diode(s)

it is pretty doubtful that a diode makes any kind of noise. If your alternator is making a noise, it is either the belt (slipping or out the pulley is out of line) or the bearing. Unplugging it would just remove the load.

this was the test i was watching:
<iframe width="560" height="315" src="https://www.youtube.com/embed/SlGJsT4DB2A?start=195" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

I have a 3.5 in my Accord, and if it were my car making that sound I would tend to agree with the video. Note that that Ridgeline he's working on is showing 159k miles I think.
How many miles on your car?

When I had a diode go bad in my SC, it caused a really bad parasitic drain on the battery.

100k miles...

Good catch on the mileage of that car...didn't notice it.

Have you had the timing belt changed?
I had mine changed even though I'm only at 50k miles. It's a 2011 so 10 yrs old. I wasn't going to take a chance. Found a shop willing to just change the belt and hyd. tensioner and NOT the water pump. Parts that came out could have passed as new IMO. Put a new serp belt on it too so should be good to go another 50k no problem, or ten years, which ever comes first.
I've read about oil leaks dripping on the alternator and ruining it but am not sure which models that actually applies to.

As noted...it is not a bad diode (electronic component that does not move), it is the bearing. That is why his replacing the entire alternator fixed it. He essentially just turned off the alternator when he unplugged it...it is the load (magnetic field) that makes it squeal.

His diagnostic equipment monitor says "diode open" because he unplugged the alternator (and the diode and any connection the diagnostic equipment had to the diode) which would make it appear/read open.

Thanks for the explanation! Going to do timing belt soon!

Also, I finally made it to 1000 posts! It only took 16 years!!